McElhaney1,2 1 Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada;2Protein Engineering Network of Centers of Excellence, University of Alberta, Edmonton, Albert
Trang 1The effects of ring-size analogs of the antimicrobial peptide
gramicidin S on phospholipid bilayer model membranes
Monika Kiricsi1, Elmar J Prenner1,2, Masood Jelokhani-Niaraki1,2,*, Ruthven N A H Lewis1,
Robert S Hodges1,2,†and Ronald N McElhaney1,2
1
Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada;2Protein Engineering Network of Centers of Excellence, University of Alberta, Edmonton, Alberta, Canada
We have examined the effects of three ring-size analogs of the
cyclic b-sheet antimicrobial peptide gramicidin S (GS) on the
thermotropic phase behavior and permeability of
phos-pholipid model membranes and on the growth of the cell
wall-less Gram-positive bacteria Acholeplasma laidlawii B
These three analogs have ring sizes of 10 (GS10), 12 (GS12)
or 14 (GS14) amino acids, respectively Our high-sensitivity
differential scanning calorimetric studies indicate that all
three of these GS analogs perturb the gel/liquid-crystalline
phase transition of zwitterionic phosphatidylcholine
(PtdCho) vesicles to a greater extent than of zwitterionic
phosphatidylethanolamine (PtdEtn) or of anionic
phos-phatidylglycerol (PtdGro) vesicles, in contrast to GS itself,
which interacts more strongly with PtdGro than with
Ptd-Cho and PtdEtn bilayers However, the relative potency of
the perturbation of phospholipid phase behavior varies
markedly between the three peptides, generally decreasing in
the order GS14 > GS10 > GS12 Similarly, these three
GS ring-size analogs also differ considerably in their ability
to cause fluorescence dye leakage from phospholipid
vesi-cles, with the potency of permeabilization also generally
decreasing in the order GS14 > GS10 > GS12 Finally,
these GS ring-size analogs also differentially inhibit the
growth of A laidlawii with growth inhibition also decreasing
in the order GS14 > GS10 > GS12 These results indicate
that the relative potencies of GS and its ring-size analogs in perturbing the organization and increasing the permeability
of phospholipid bilayer model membranes, and of inhibiting the growth of A laidlawii Bcells, are at least qualitatively correlated, and provide further support for the hypothesis that the primary target of these antimicrobial peptides is the lipid bilayer of the bacterial membrane The very high anti-microbial activity of GS14 against the cell wall-less bacteria
A laidlawii as compared to various conventional bacteria confirms our earlier suggestion that the avid binding of this peptide to the bacterial cell wall is primarily responsible for its reduced antimicrobial activity against such organisms The relative magnitude of the effects of GS itself, and of the three ring-size GS analogs, on phospholipid bilayer organi-zation and cell growth correlate relatively well with the effective hydrophobicities and amphiphilicities of these peptides but less well with their relative charge density, intrinsic hydrophobicities or conformational flexibilities Nevertheless, all of these parameters, as well as others, may influence the antimicrobial potency and hemolytic activity of
GS analogs
Keywords: antimicrobial peptides; gramicidin S; phospholi-pid bilayers; membranes
Gramicidin S (GS) is a cyclic decapeptide of primary
structure [cyclo-(Val-Orn-Leu-D-Phe-Pro)2] first isolated
from Bacillus brevis [1] and is one of a series of
antimicrobial peptides produced by this microorganism
[2,3] GS exhibits potent antibiotic activity against a broad spectrum of both Gram-negative and Gram-positive bacteria, as well as against several pathogenic fungi [4– 7] Unfortunately, GS is rather nonspecific in its actions
Correspondence to R N McElhaney, Department of Biochemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2H7.
Fax: +1 780 4920095, E-mail: rmcelhan@gpu.srv.ualberta.ca
Abbreviations: GS, gramicidin S; Myr 2 Gro-PCho, dimyristoylglycerophosphocholine; Myr 2 Gro-PEtn, dimyristoylglycerophosphoethanolamine; Myr 2 Gro-PGro, dimyristoylglycerophosphoglycerol; DSC, differential scanning calorimetry; L a , lamellar liquid-crystalline phase; L b or Lb¢, lamellar gel phase with untilted or tilted hydrocarbon chains, respectively; L C or L C¢ , lamellar crystalline phase with untilted or tilted hydrocarbon chains, respectively; P b¢ , lamellar rippled gel phase with tilted hydrocarbon chains; PtdCho, phosphatidylcholine; PtdEtn, phosphatidylethanol-amine; PtdGro, phosphatidylglycerol; PamOleGro-PCho, 2-oleoyl-glycerophosphocholine; PamOleGro-PEtn, 1-palmitoyl-2-oleoyl-glycerophospholamine; PamOleGro-PGro, 1-palmitoyl-2-oleoyl-glycerophosphoglycerol.
*Present address: Department of Chemistry, Wilfred Laurier University, Waterloo, Ontario, Canada N2L 3C5.
Present address: Department of Biochemistry and Molecular Genetics, University of Colorado, Health Sciences Center, 4200 East Ninth Avenue, Denver, CO 80262, USA.
(Received 9 August 2002, revised 9 October 2002, accepted 15 October 2002)
Trang 2and exhibits appreciable hemolytic as well as antimicrobial
activity, thus restricting the potential use of GS as an
antibiotic to topical applications at present However,
recent work has shown that structural analogs of GS
can be designed with markedly reduced hemolytic activity
and enhanced antimicrobial activity (see below),
suggest-ing the possibility that appropriate GS derivatives may
be used as potent oral or injectable broad-spectrum
antibiotics [4–7]
GS has been extensively studied by a wide range of
physical techniques [2,3] and its 3D structure is well
determined In this minimum energy conformation, the
two tripeptide sequences Val-Orn-Leu form an antiparallel
b-sheet terminated on each side by a type II¢ b-turn formed
by the two D-Phe-Pro sequences Four intramolecular
hydrogen bonds, involving the amide protons and carbonyl
groups of the two Leu and two Val residues, stabilize this
rather rigid structure The GS molecule is amphiphilic, with
the two somewhat polar and positively charged Orn
sidechains and the two D-Phe rings projecting from one
side of this molecule, and the four hydrophobic Leu and Val
sidechains projecting from the other side Moreover, the
amphiphilic nature of GS is required for its antimicrobial
activity [2,3] A number of studies have shown that this
conformation of the GS molecule is maintained in water, in
protic and aprotic organic solvents of widely varying
polarity, and in detergent micelles and phospholipid
bilay-ers, even at high temperatures and in the presence of agents
which often alter protein conformation
There is good evidence from studies of the interaction of
GS and its analogs with bacterial cells that the destruction
of the integrity of the lipid bilayer of the inner membrane
is the primary mode of antimicrobial action of this peptide
[8] In support of this hypothesis, GS has been found to
interact strongly with phospholipid model membranes,
perturbing their organization and increasing their
per-meability In recent years we have extended these studies
of GS/phospholipid bilayer interactions considerably
Specifically, we have investigated the strength and nature
of the interactions of GS with phospholipid bilayers by
utilizing differential scanning calorimetry (DSC) to
mon-itor the effect on this peptide on phospholipid
thermo-tropic phase behavior [9] We demonstrated by31P-NMR
spectroscopy [10] and X-ray diffraction [11] that GS can
induce inverted nonlamellar cubic phases in various
phospholipid vesides by producing negative monolayer
curvature stress and that GS causes thinning of
phosphol-ipid bilayers We showed by densitometry and sound
velocimetry that GS binding to PtdCho bilayers decreases
the temperature and cooperativity of their
gel/liquid-crystalline phase transition and increases the volume
compressibility and decreases the density of the host
bilayer [12], indicating that GS increases the motional
freedom of the lipid hydrocarbon chains We also showed
that cholesterol decreases the effect of GS on phospholipid
bilayers, at least primarily by reducing the penetration of
the peptide into the phospholipid model membrane [13]
We demonstrated by Fourier transform infrared
spectros-copy that GS is located at the polar/apolar interfacial
region of phospholipid bilayers near the glycerol backbone
region below the polar headgroups and above the fatty
acyl chains, and that GS penetrates more deeply into
anionic and more fluid phospholipid bilayers [14] Finally,
using solid-state19F-NMR spectroscopy and a19F-labeled
GS analog, we showed that GS is aligned with its cyclic b-sheet ring lying flat in the plane of the bilayer, consistent with its amphiphilic character, although the peptide molecules rotate rapidly and wobble in liquid-crystalline PtdCho bilayers [15]
We have recently shown that there are several structural variations of the GS molecule which can lead to a dissociation of antimicrobial and hemolytic activities [4–7], including variations in ring size [5] The secondary structures of these ring-size analogs exhibit a definite periodicity in b-sheet structure, with rings containing six,
10 and 14 residues having the conventional antiparallel b-sheet structure of GS, and those containing eight or 12 residues having largely distorted structures [5,16] Although GS analogs containing fewer than 10 residues exhibit no significant antimicrobial or hemolytic activities, the 12-residue peptide (GS12) retains appreciable activity against Gram-negative bacteria and fungi, exhibits consid-erably reduced activity against Gram-positive bacteria, but most importantly also displays a significantly reduced hemolytic activity, resulting in a significant improvement
in microbial specificity (therapeutic index) for Gram-negative bacteria In contrast, the 14-residue analog (GS14) shows markedly reduced antimicrobial activity against both Gram-positive and Gram-negative bacteria and increased hemolytic activity as compared to GS itself and thus a much lower therapeutic index [5] These results are important because they establish that it is possible to dissociate the antimicrobial and hemolytic activities of GS
by ring-size alterations
In this paper, we present our initial results dealing with the interactions of the three biologically active ring-size analogs of GS (GS10, GS12 and GS14) with phospholipid bilayer model membranes We first investi-gated the effects of these GS ring-size analogs on the thermotropic phase behavior of LMVs composed of dimyristoylglycerophosphocholine (Myr2Gro-PCho), dimy-ristoylglycerophosphoethanolamine (Myr2Gro-PEtn) and dimyristoylglycerophosphoglycerol (Myr2Gro-PGro) by DSC, in order to determine their effects on phospholipid bilayer organization in the gel and liquid-crystalline states
We then studied the effect of these analogs on the permeability of LUVs composed of 2-oleoyl-glycerophosphocholine (PamOleGro-PCho), 1-palmitoyl-2-oleoylglycerophosphoethanolamine (PamOleGro-PEtn) and 1-palmitoyl-2-oleoylglycerophosphoglycerol (PamOle-Gro-PGro), in order to assess their relative abilities to disrupt lipid membranes We chose to study the zwitter-ionic, bilayer-preferring phospholipid PtdCho as it is abundant in the outer monolayer of the lipid bilayers of mammalian plasma membranes [17], while the zwitter-ionic, nonbilayer phase-preferring PtdEtn and the anzwitter-ionic, bilayer phase-preferring PtdGro are thought to be common components of the outer monolayer of the lipid bilayer of bacterial membranes [18,19] Finally, we investigated the relative abilities of GS10, GS12 and GS14 to inhibit the growth of A laidlawii B, a cell wall-less Gram-positive bacteria The goal of this work is to understand the relationship between the structure of these GS ring-size analogs, their interactions with phospholipid bilayer model membranes, and their anti-microbial and hemolytic activities
Trang 3M A T E R I A L S A N D M E T H O D S
The three ring-size analogs of GS studied here were
synthesized and purified as described previously [4–6] The
phospholipids utilized in this study were purchased from
Avanti Polar Lipids (Alabaster, AL, USA) and the calcein
from Molecular Probes (Eugene, OR, USA) and all were
used without further purification All of the other chemicals
utilized here were highest purity reagent grade purchased
from BDH Inc (Toronto, ON, Canada) and were used as
received
The lipid/peptide mixtures for the DSC studies were
prepared by mixing the appropriate amounts of
phospho-lipid and peptide dissolved in methanol and ethanol,
respectively, removing the solvent under a stream of N2,
and exposing the resultant lipid/peptide films to high
vacuum overnight to remove any traces of solvent Fully
hydrated peptide-containing MLVs were then prepared by
vortexing in excess aqueous buffer (10 mM Tris/HCl,
100 mM NaCl, 2 mM EDTA, pH 7.4) at a temperature
above the main phase transition temperature of the
phospholipid component This procedure, and the rationale
for utilizing it, have been described previously [9]
The calorimetry was performed on a Nano-DSC
Calori-meter (Calorimetry Sciences Corp., Spanish Fork, UT,
USA) utilizing a scan rate of 10CÆh)1 Sample runs were
repeated at least three times to insure reproducibility Data
acquisition and analysis was carried out using MICROCAL
DA-2 (MicroCal LLC, Northampton, MA, USA) and
ORIGIN software (OriginLab Corporation, Northampton,
MA, USA) Samples containing the GS ring-size analogs
alone, dissolved in buffer at a peptide concentration
corresponding to that present in the phospholipid-peptide
mixtures, exhibit no detectable thermal events over the
temperature range 0–90C This indicates that these
peptides do not undergo any cooperative thermal
denatur-ation over this temperature range and thus that the
endothermic events observed by DSC arise exclusively from
phase transitions of the phospholipid
The calcein leakage experiments were performed
essen-tially as previously described [20] Briefly, the phospholipid
vesicles were prepared by drying chloroform solutions of
PamOleGro-PCho or
PamOleGro-PEtn/PamOleGro-PGro (7 : 3 molar ratio) under N2 in a round-bottomed
flask and removing traces of the solvent by overnight
vacuum The dry lipid film was then hydrated by the same
buffer used for the DSC experiments, but in this case also
containing a high concentration (70 mM) of calcein, by
vortexing at room temperature The resulting MLVs were
then freeze-thawed several times and extruded through a
100-mm filter using a LipoFast apparatus (Avestin Inc.,
Ottawa, ON, Canada) The resulting LUVs were then
passed through a Sephadex G-50 column to remove calcein
not trapped inside the phospholipid vesicles The
peptide-induced leakage of the self-quenched calcein from the LUVs
was then monitored by measuring the fluorescence of
calcein released into the aqueous buffer as a function of time
at 25C The fluorescence intensity was measured with a
Perkin-Elmer LS50Bspectrophotometer (Beaconsfield,
UK) utilizing slit widths for both excitation and emission
of 3 nm and quartz cells of 1-cm path length; the excitation
and emission were recorded at wavelengths of 496 and
516 nm, respectively
The A laidlawii Bcells were grown and cell growth was monitored turbidometriedly, all as previously described [21] The effect of the ring-size analogs studied here on cell growth was monitored by adding various concentrations of these peptides to the culture medium just prior to the addition of a 10% by volume inoculation with cells in the mid log phase of growth
R E S U L T S
Structure and biological activities of GS ring-size analogs
The amino acid sequences and the 3D structures of the three ring-size analogs of GS studied here are presented in Fig 1 These three peptides are all based on the structure of GS itself except that theD-Phe residue in each of the two type II¢ b-turns has been replaced by aD-Tyr residue and the Orn residues have been replaced by Lys residues The former replacement was made to increase the water solubility of these compounds and the latter to decrease the cost of chemical synthesis [4–7] Note that these conservative amino acid substitutions do not by themselves significantly alter the conformation or biological activity of these peptides, as shown by the fact that the structure in aqueous solution, and antimicrobial and hemolytic activities, of GS and GS10 are similar, although GS is slightly more active against both Gram-positive and Gram-negative bacteria than is GS10 [5]
Perturbation of phospholipid thermotropic phase behavior by GS ring-size analogs
We studied the effects of concentrations of these GS ring-size analogs ranging from 1 to 4 mole percent on the thermotropic phase behavior of aqueous dispersions of two zwitterionic phospholipids and one anionic phospholipid by DSC In each case the result of progressively increasing the
Fig 1 The amino acid sequence, structure and conformation of the ring-size analogs of GS studied here (GS10, GS12 and GS14) in aqueous solution.
Trang 4peptide concentration was simply to progressively increase
the magnitude of the characteristic effects of each particular
analog on the thermotropic phase behavior of each
phospholipid system examined We have thus chosen to
present DSC thermograms for each GS ring-size analog and
each phospholipid system at only the highest peptide
concentration tested, as the characteristic differences in
their effects are most clear under these circumstances We
also point out that a peptide concentration of 4 mole
percent (phospholipid/peptide molar ratio of 25 : 1) is well
within the physiologically relevant concentration range for
GS itself [2,3,8]
The initial DSC heating thermograms, illustrating the
thermotropic phase behavior of large MLVs composed of
Myr2Gro-PCho alone and of Myr2Gro-PCho/peptide
mixtures, are presented in Fig 2 The MLVs composed of
Myr2Gro-PCho alone exhibit two transitions on heating, a
less enthalpic, less cooperative pretransition centered at
14C and a more enthalpic, more cooperative main phase
transition centered at 24C The pretransition corresponds
to the conversion of a planar lamellar gel phase with tilted
hydrocarbon chains (the Lb¢phase) to the rippled gel phase
with tilted hydrocarbon chains (the Pb¢phase) and the main
phase transition to the conversion of the Pb¢phase to the
lamellar liquid-crystalline (La)phase A subtransition
cen-tered at 16C is not observed here because this sample was
not extensively annealed at low temperatures For a more
complete description of the thermotropic phase behavior of
Myr2Gro-PCho and other members of the homologous
series of linear disaturated PCs, the reader is referred to
Lewis et al [22]
The effect of the incorporation of 4 mole percent
(peptide/phospholipid molar ratio 1 : 25) of the three GS
ring-size analogs studied here on the thermotropic phase
behavior of the host Myr2Gro-PCho bilayer varies greatly,
as illustrated in Fig 2 For GS12, a single DSC endotherm
is observed whose temperature and enthalpy are essentially
unchanged from that of Myr2Gro-PCho alone and whose
cooperativity is only moderately reduced In contrast, the incorporation of both GS10 and GS14 produce much broader, lower enthalpy DSC endotherms, particularly in the case of the latter peptide In fact in both instances, these two peptides produce two-component DSC traces consist-ing of a relatively more cooperative, higher enthalpy component centered at a lower temperature than the main phase transition temperature of Myr2Gro-PCho alone, and
a much less cooperative, more enthalpic component centered at a higher temperature (see Fig 3) Moreover, the magnitude in the downward shift in the temperature of the sharp component, and of the upward shift in the temperature of the broad component, is greater for GS14 than for GS10 Also, the relative enthalpy of the higher temperature DSC component is considerably greater and the cooperativity of this component is considerably less in the GS14/Myr2Gro-PCho than in the GS10/Myr2 Gro-PCho MLVs According to our prior studies of GS/ Myr2Gro-PCho mixtures, we interpret the sharp and broad components of the two-component DSC endotherms as the chain-melting phase transition of poor and peptide-enriched phospholipid domains, respectively [9] Note also that the incorporation of GS10 and GS14 abolish the pretransition of Myr2Gro-PCho whereas the incorporation
of GS12 does not These results suggest that GS12 perturbs the thermotropic phase behavior of Myr2Gro-PCho bilay-ers to a much lesser extent than does GS10 and GS14, and that GS14 is more potent in this regard than is GS10
Fig 2 Initial high-sensitivity DSC heating scans illustrating the effect of
the addition of 4mole percent GS10, GS 12 or GS14on the thermotropic
phase behavior of Myr Gro-PCho MLVs.
Fig 3 A DSC heating thermogram of Myr 2 Gro-PCho MLVs con-taining 4mole percent GS14(– –—) and its deconvolution into sharp and broad components (- - - -).
Trang 5Interestingly, GS14 and GS10 alter the phase behavior of
Myr2Gro-PCho MLVs to a greater extent than does GS
itself [9]
DSC heating scans of MLVs composed of Myr2
Gro-PEtn alone, or of Myr2Gro-PEtn containing 4 mole percent
of one of the three ring-size analogs of GS, are presented in
Fig 4 Aqueous dispersions of Myr2Gro-PEtn alone, which
have not been extensively incubated at low temperatures
prior to calorimetric analysis, exhibit a single fairly
cooper-ative, relatively energetic Lb/La phase transition centered
near 50C (see [23] for a more complete description of the
thermotropic phase behavior of Myr2Gro-PEtn and other
members of the homologous series of linear saturated
PtdEtns) If peptide-containing Myr2Gro-PEtn MLVs are
exposed to temperatures above but near to the Lb/Laphase
transition of Myr2Gro-PEtn, the presence of these peptides
has only a very small effect on the main phase transition,
causing a slight reduction in the phase transition
tempera-ture and a modest decrease in the cooperativity of the phase
transition with no detectable change in the overall transition
enthalpy (see Fig 4A) However, if these peptide-containing
Myr2Gro-PEtn vesicles are exposed to temperatures well
above the Lb/Laphase transition temperature, then
subse-quent DSC heating scans reveal an additional decrease in
the temperature and cooperativity, but still little change in
the enthalpy, of the chain-melting phase transition (see
Fig 4B) Interestingly, however, GS14 and GS10 again
exhibit larger effects on the thermotropic phase behavior of
Myr2Gro-PEtn than does GS12 However, in all cases the
presence of these peptides produces only a slight
destabi-lization of the Lbphase relative to the Laphase of Myr2
Gro-PEtn bilayers, as also observed previously with GS itself [9]
Also, repeated recycling through the phase transition
temperature actually increases the magnitude of the effect
of these peptides on Myr2Gro-PEtn phase behavior, in
contrast to the situation with Myr2Gro-PCho MLVs This
effect, which was also observed to a lesser extent with GS
itself [9], suggests that repeated exposure to high tempera-tures facilitates peptide incorporation into Myr2Gro-PEtn bilayers
The initial DSC heating scans of MLVs of Myr2 Gro-PGro alone, or Myr2Gro-PGro MLVs containing 4 mole percent of one of the three GS ring-size analogs studied here, are shown in Fig 5 Aqueous dispersions of Myr2 Gro-PGro alone, which have not been extensively annealed at low temperatures, exhibit two endothermic events upon heating, a less energetic pretransition near 14C and a more energetic main transition near 24C Again, a subtransition (LC¢/Laphase transition) centered near 25 or 40C is not observed under these conditions The pretransition arises form a conversion of the (Lb¢) to the (Pb¢) phase and the main transition from the conversion of the Pb¢to the La phase For a more detailed discussion of the thermotropic phase behavior of Myr2Gro-PGro and other members of the homologous series of linear saturated PGs, see Zhang
et al.[24]
The addition of 4 mol percent of one of the three GS ring-size analogs of GS studied has a relatively modest effect
on the thermotropic phase behavior of Myr2Gro-PGro MLVs In all cases the presence of peptide decreases the cooperativity of the main transition Also, each peptide induces the presence of a second, less enthalpic, broad component of the DSC endotherm which occurs at a higher temperature than does the more enthalpic sharp compo-nent As well, in the case of GS14 only, additional endothermic events are noted at temperatures near 31 and
39C As before, the magnitude of the effect of these peptides on the cooperativity of the main phase transition decreases in the order GS14 > GS10 > GS12 Moreover, GS14 decreases the enthalpy of the main phase transition of Myr2Gro-PGro substantially whereas GS12 and GS10 actually appear to slightly increase the total enthalpy of the two-component main phase transition We note that GS itself, however, has a greater effect on the thermotropic phase of MyrGro-PGro MLVs than do any of the three
Fig 4 Initial high-sensitivity DSC heating scans illustrating the effect of
the addition of 4mol percent GS10, GS12 or GS14on the thermotropic
phase behavior of Myr 2 Gro-PEtn MLVs (A) Myr 2 Gro-PEtn MLVs
not exposed to high temperatures (i.e temperatures above 65–70 C).
(B) Myr 2 Gro-PEtn MLVs exposed to high temperatures (i.e
temper-atures of 75 C or higher).
Fig 5 Initial high-sensitivity DSC heating scans illustrating the effect of the addition of 4mole percent GS10, GS12 or GS14on the thermotropic phase behavior of Myr 2 Gro-PGro MLVs.
Trang 6ring-size analogs studied here [9] Also, as noted previously
for GS, recycling through the phase transition temperature
has little effect on phospholipid phase behavior in contrast
to the situation with Myr2Gro-PCho and Myr2Gro-PEtn
MLVs, suggesting that these peptides readily incorporate
into Myr2Gro-PGro bilayers in the liquid-crystalline state
and remain incorporated in the gel state
Permeabilization of phospholipid bilayers by GS
ring-size analogs
In order to determine the relative abilities of these three
ring-size analogs of GS to permeabilize phospholipid bilayers, we
determined the amount of entrapped calcein dye released by
the addition of 4 mole percent peptide to LUVs composed
of either PCho or a mixture of
PEtn/PGro (7 : 3 molar ratio)
PamOleGro-PCho was selected to mimic the phospholipid composition
of the outer monolayer of eukaryotic plasma membranes
[17] and the PamOleGro-PEtn/PamOleGro-PGro mixture
tomimicthephospholipidcompositionoftheEscherichia coli
inner membrane [18] Although we intended to study vesicles
composed of PamOleGro-PEtn or PamOleGro-PGro
alone, the former did not form well defined LUVs under
our experimental conditions [25] and PamOleGro-PGro
formed only small unilamellar vesicles [26]; as antimicrobial
peptide binding can be influenced by the degree of curvature
strain in phospholipid [27], such a size difference would have
made comparisons between the three individual
phosphol-ipids difficult However, the
PamOleGro-PEtn/PamOle-Gro-PGro mixture formed well-behaved LUVs
As illustrated in Fig 6, the addition of each of the three
ring-size analogs of GS cause considerable fluorescence dye
leakage when added to PamOleGro-PCho LUVs at a final
peptide/phospholipid molar ratio of 1 : 25, with the extent
of dye leakage decreasing in the order GS14 >
GS10 > GS12 [28] Moreover, at lower peptide
concentra-tions, GS14 is perhaps 10-fold more potent at releasing
calcein than is GS10 or GS12 [28] Interestingly, the
addition of the same amount of these three peptides to PamOleGro-PEtn/PamOleGro-PGro LUVs is generally less effective at releasing entrapped calcein, particularly in the case of GS10, and the relative effectiveness of the three peptides now decreases in the order GS14 > GS12 > GS10 Moreover, in this vesicle system GS14 does not exhibit a relatively much greater potency at lower concen-trations than do the other peptides GS14 was thus the most effective peptide in both phospholipid vesicle systems with GS10 exhibiting the greatest phospholipid compositional selectivity, much more strongly affecting the PamOleGro-PCho system in comparison to the PamOleGro-PEtn/ PamOleGro-PGro mixed system
Inhibition of growth ofA laidlawii B by GS ring-size analogs
In order to extend the above studies to a living microbial system, we investigated the effect of these three ring-size analogs of GS on the growth of A laidlawii B, a cell wall-less Gram positive bacteria (Mollicute) The absence of a lipopolysaccharide-containing cell wall or outer membrane
or a lipopeptidoglycan outer layer is a major advantage of utilizing this organism for such studies, as the antimicrobial peptides added should have free physical access to the surface of the limiting membrane and extracellular structures should not compete with the membrane lipid bilayer for peptide binding Moreover, the membrane lipid composition [29], and the organization and dynamics of the membrane lipid bilayer [30] of this organism, have been extensively studied by ourselves and others, potentially facilitating a molecular interpretation of any results obtained
We present in Fig 7 growth curves for A laidlawii in the presence or absence of various concentrations of the three
GS ring-size analogs studied here It is clear from these curves that a considerable difference in the growth inhib-itory potency of these three peptides exists For example GS10 is a fairly potent antimicrobial agent, inhibiting
A laidlawiiBgrowth slightly at the lowest concentration tested (0.25 lM), strongly at the next highest peptide
Fig 6 A bar graph illustrating the percentage of entrapped calcein dye
leakage at equilibrium from LUVs composed of either
PamOleGro-PCho (white bar) or PamOleGro-PEtn/PamOleGro-PGro (7 : 3 molar
ratio) (hatched bar) upon the addition of 4mol percent GS10, GS12 or
GS14.
Fig 7 Growth curves at 37 °C of A laidlawii B in the absence or presence of various concentrations of GS10, GS12 or GS14 The sym-bols utilized are: (·), absence of peptide, and (h), (s), (n), (,), and (e), peptide concentrations of 0.25, 0.50, 1.0, 2.0 and 4.0 l M , respectively, in the growth medium.
Trang 7concentrated (0.50 lM), and completely suppressingly
growth at concentrations of 1.0 lM and higher In
con-trast, GS12 is a much weaker growth inhibitory agent,
with significant inhibition of growth being observed only
at peptide concentrations of 1.0–2.0 lM with complete
growth inhibition occurring only at the highest peptide
concentration tested (4.0 lM) On the other hand, GS14 is a
very potent inhibitor of the growth of this organism, with
significant growth suppression being observed at the lowest
peptide concentration tested (0.25 lM) and the complete
inhibition of growth at all higher peptide concentrations
Thus the potency of growth inhibition of these peptides
decreases in the order GS14 > GS10 > GS12 We note
also that when sufficient peptide was added to these
A laidlawii Bcultures to completely inhibit cell growth,
the initial turbidity of the 10% (v/v) innoculum of mid-log
phase cells added to fresh culture media was reduced to
blank values This result indicates that these GS ring-size
analogs of GS exert a cidal rather than a static effect on
A laidlawiicells, presumably by causing cell lysis to occur,
as has also been reported to be the case for GS itself in
mycoplasma and bacterial systems [8] Interestingly, we find
that GS itself is slightly less effective at inhibiting the growth
of A laidlawii than is GS10 (data not presented), in contrast
to results with most species of conventional bacteria, where
GS is slightly more effective than GS10 [5]
D I S C U S S I O N
We showed previously that the effect of GS on the
thermotropic phase behavior of phospholipid bilayers
depends markedly on both the structure and charge of the
lipid polar headgroup [9,10] Specifically, the presence of GS
has only a very small effect on the thermotropic phase
behavior of Myr2Gro-PEtn bilayers, even at very high
peptide concentrations and after multiple cycling through
the gel/liquid-crystalline phase transition Only upon
expo-sure of the Myr2Gro-PEtn bilayers to high temperature is a
small decrease in the temperature, enthalpy and
coopera-tivity of the main phase transition and the induction of a
minor lower temperature shoulder on this endotherm
observed The addition of similar amounts of GS to
Myr2Gro-PCho bilayers results in a somewhat greater but
still rather small decreases in the temperature, enthalpy and
cooperativity of the main phase transition and induces a
new broad component of the DSC endotherm centered at a
slightly higher temperature In contrast, the addition of GS
to Myr2Gro-PGro bilayer produces a considerably larger
decrease in the temperature, enthalpy and cooperativity of
the main phase transition and induces the presence of a
second transition at a considerably higher temperature, but
whose temperature decreases more rapidly than that of the
main phase transition with increasing peptide
concentra-tion We thus concluded that GS interacts more strongly
with anionic phospholipids such as PtdGro than with
zwitterionic phospholipids, and more strongly with more
fluid zwitterionic phospholipids like PtdCho than with less
fluid zwitterionic phospholipids like PtdEtn However, the
three ring-size analogs of GS studied here thus exhibit a
somewhat different phospholipid polar head group
specif-icity, as discussed below
The overall phospholipid specificity of the three GS
ring-size analogs studied here is broadly similar to that of GS
itself in that the degree of perturbation of phospholipid thermotropic phase behavior increases in the order Myr2Gro-PEtn < Myr2Gro-PCho < Myr2Gro-PGro Moreover, the magnitude of the decrease in temperature, enthalpy and cooperativity of the main phase transition of the three phospholipids studied here generally decreases in the order GS14 > GS10 > GS12, as does the temperature and the relative magnitude of the new endotherm or endotherms induced by the addition of the peptide However, the magnitude of the effects of GS [9] and its three ring-size analogs on the thermotropic phase behavior
of the phospholipids studied here depends on the specific phospholipid vesicle system being studied Specifically, the order of decreasing perturbation of phospholipid phase behavior by GS itself, and by the three ring-sized analogs studied here, is GS14 > GS10 > GS > GS12 in Myr2Gro-PCho MLVs, GS14@ GS10 > GS @ GS12 in Myr2Gro-PEtn MLVs, and GS > GS14 >GS10 > GS12
in MLVs of Myr2Gro-PGro Thus, although GS12 has the weakest effect in all three vesicles systems, the relative order
of effectiveness varies with polar headgroup structure for the other three peptides, with GS14 and GS10 exhibiting a greater effect than GS itself in the two zwitterionic phospholipid bilayers studied here but a smaller effect in the anionic phospholipid bilayer system
The potency of the three ring-size analogs of GS in inducing the leakage of calcein dye entrapped in PamOle-Gro-PCho LUVs also decreases in the order GS14 > GS10 > GS12, which is the same decreasing order as exhibited by these three peptides in perturbing the thermo-tropic phase behavior of Myr2Gro-PCho MLVs Interest-ingly, the three GS ring-size analogs are generally less potent
at releasing entrapped calcein from PamOleGro-PEtn/ PamOleGro-PGro than from PamOleGro-PCho LUVs, particularly in the case of GS10, so that the order of decreasing potency of vesicle permeation in PamOleGro-PEtn/PamOleGro-PGro LUVs is GS14 > GS12 > GS10 Thus, although GS14 is the most potent ring-size analog in both perturbing the thermotropic phase behavior and permeabilizing PtdCho vesicles and GS10 is the second most potent analog, the behavior of GS12 is somewhat anomalous in that its ability to induce dye leakage in PamOleGro-PEtn/PamOleGro-PGro vesides, but not in PamOleGro-PCho vesicles, is much greater than predicted
by its smaller effect on the thermotropic phase behavior of all three phospholipids examined Interestingly, GS itself is less potent than GS14 but more potent than GS10 and GS12 in permeabilizing PamOleGro-PCho LUVs [31] However, in PamOleGro-PEtn/PamOleGro-PGro LUVs,
GS is less potent than both GS14 and GS12, but remains more potent than GS10 [31]
The effectiveness of the three ring-size analogs of GS studied here on the growth inhibition and killing of
A laidlawii Bcells also decreases in the order GS14 > GS10 > GS12, again paralleling the decreasing relative potency of these peptides in perturbing the thermotropic phase behavior of the three phospholipid MLVs studied and
of the permeabilization of PamOleGro-PCho LUVs As well, the relative antimicrobial potency of this series of peptides also mirrors the order of the decreasing extent of dye leakage from PamOleGro-PEtn/PamOleGro-PGro LUVs, except that the order of GS10 and GS12 are reversed in this system Nevertheless, there is generally a
Trang 8good correlation overall between the relative perturbation
of host bilayer organization as measured by DSC, the
permeabilization of phospholipid vesicles as measured by
calcein leakage, and the inhibition of the growth of
A laidlawii B We note also that GS is less potent at
inhibiting the growth of this organism than is GS14 but
more potent than GS10 and especially GS12 (data not
presented) Overall, then, these results indicate that studies
of the interactions of other analogs of GS with phospholipid
vesicles may be useful for predicting the antimicrobial
potency of these analogs and possibly also for
understand-ing the molecular basis for their differential antimicrobial
potencies again different classes and species of bacteria,
many of which may differ considerably in the lipid
compositions of their membranes [18,19]
It is instructive to compare the relative antimicrobial
potencies of these three GS ring-size analogs against various
Gram-positive and Gram-negative bacteria and against
A laidlawii, as the former types of bacteria possess either a
lipopeptidoglycan outer barrier or a
lipopolysaccharide-containing cell wall or outer membrane, respectively, which
is lacking in the latter organism Against conventional
bacteria, the order of decreasing antimicrobial potency is
GS10 > GS12 > GS14 [5], whereas against A laidlawii B
the order is GS14 > GS10 > GS12 This result would
appear to confirm our previous suggestion that the low
effective antimicrobial activity of GS14, particularly against
Gram-negative bacteria, is due to its strong binding to the
lipopolysaccharide component of the bacterial cell wall [5],
which effectively competes for the binding of available
peptide with the lipids of the inner membrane [5] Thus, when
an outer cell wall is absent, as in A laidlawii B , such
competition is not observed and GS14 is then able to exert its
intrinsically high antimicrobial activity However, the
aggre-gation of GS14 in solution may also reduce its ability to
penetrate the cell wall of Gram-negative bacteria, which
could also reduce its effective antimicrobial activity
What-ever the reason for its low activity against conventional
bacteria, the high antimicrobial potency of GS14 against
A laidlawiisuggests that this peptide might potentially be
clinically useful in treating the many serious diseases of man
and animals caused by various Mollicutes [32,33] It is also
interesting to note that with these particular ring-size analogs
of GS, their relative potencies at perturbing the organization
and increasing the permeability of phospholipid bilayer
model membranes, and of causing the lysis of A laidlawii B
and human erythrocytes, are at least qualitatively correlated
As discussed earlier, GS10 and GS14 both exist in
aqueous solution as antiparallel b-sheet structures separated
by type II¢ turns, as does GS itself, although GS14 has a
somewhat less rigid structure as compared to GS10,
presumably due to its expanded ring; in contrast, GS12
exists in a distorted b-sheet and b-turn structure and is
conformationally much more flexible [5,16,28] Although
the intrinsic hydrophobicities of these three peptides are
predicted to decrease in the order GS12 > GS14 > GS10,
based on the ratios of the number of charged polar Lys
residues to hydrophobic Val and Leu residues (4 : 4, 4 : 6
and 2 : 4, respectively), the actual measured solubilities in
water decrease in the order GS12 > GS10 > GS14 The
lower solubility of GS14 as compared to GS10 in water
appears to be related to its slightly greater amphiphilicity
and to its significantly greater exposed hydrophobic surface
area, which results in GS14 forming aggregates in aqueous solution above a concentration of about 50–60 mM, whereas GS10 and GS12 remain monomeric even at much higher concentrations [22] Note also that GS14 and GS10 are considerably more amphiphilic than is GS12, as in the former two peptides the more polar charged Lys and the less polar Val and Leu residues project on opposite sides of the ring, whereas this is not the case for GS12
We can ask whether or not the observed order of biophysical or biological potencies of GS itself and of the three ring-size analogs of GS studied here, namely GS
@ GS14 > GS10 > GS12, correlates well with any of the physical properties of these peptides which we have previ-ously measured In terms of the relative conformational flexibility of this series of peptides, there is not a particularly good correlation with the observed results, as conforma-tional rigidity decreases in the order GS > GS10 > GS14 > GS12 Similarly, an even poorer correlation is observed between the intrinsic hydrophobicities of these peptides, which decrease in the order GS > GS10 > GS14 > GS12, and their biophysical and biological activ-ities or with the ratio of positively charged Orn or Leu residues to the total number of residues (GS12 > GS14 > GS10¼ GS) However, a reasonably good correlation is observed between the effective hydrophobicities of these peptides, as assessed by their decreasing solubilities in water, and their decreasing amphiphilicities, as measured by their retention times on reversed-phase high-performance liquid chromatographic columns, which are both related to the accessible nonpolar surface areas of these peptides [5] Both effective hydrophobicity and amphiphilicity decrease in the order GS > GS14 > GS10 > GS12, which correlates well with their decreasing ability to perturb the organization
of lipid bilayer model and A laidlawii membranes These results are in only partial agreement with previous studies of
GS analogs with 10-membered rings, which indicated that a high b-sheet content, as well as a high effective hydrophob-icity and amphiphilhydrophob-icity, are correlated with a high antibac-terial activity [2,3,8] However, the present results may not be surprising, as the requirement for an ordered b-sheet structure was rationalized previously by assuming that in disordered GS analogs, the absence of the interstrand hydrogen bonds present in the b-sheet ring results in solvation of the amide NH and CO groups, in turn causing
a decrease of partitioning into the lipid bilayer and reducing their effectiveness [5] However, because the three ring-size analogs of GS studied here have fairly markedly different intrinsic hydrophobicities due to their variable ratios of Lys
to Val and Leu residues, these amino acid compositional differences may dominate the phospholipid/water partition-ing process, thus overshadowpartition-ing any smaller changes arispartition-ing from conformational effects However, the generally positive correlation between the effective hydrophobicity and amphiphilicity of the GS ring-size analogs and the magnitude
of their perturbation of phospholipid bilayer membranes and the growth of A laidlawii, generally agrees well with the results of previous studies of the antimicrobial activity of these [5] and other [4–7] ring-size analogs of GS This finding
is important in that it adds further support to the hypothesis that GS and its analogs kill bacteria primarily through their disruption of the lipid bilayer of the cell membrane
In the absence of complications arising from the differ-ential interactions of GS and its ring-size analogs with the
Trang 9outer membrane or cell wall of conventional bacteria, we
can identify at least three factors which can determine the
degree to which a particular antimicrobial peptide will
perturb the organization and integrity of phospholipid
bilayer membranes and inhibit the growth of A laidlawii B
These are the phospholipid bilayer/water partition
coeffi-cient, the localization and orientation of the peptide within
the phospholipid bilayer, and the degree to which the
peptide disturbs phospholipid packing once inserted into
the bilayer The fact that the relative order of both
decreasing effectiveness in perturbing the thermotropic
phase behavior and compromising the integrity of
phosp-holipid model membranes, as well as inhibiting the growth
of A laidlawii B(GS@ GS14 > GS10 > GS12) correlates
well with the increasing water solubility of these peptides,
can be explained in part by the fact that the phospholipid
bilayer/water coefficient should also decrease in the above
order, so that the effective concentration of peptide in the
target membrane also progressively decreases Similarly, the
good correlation observed between the biophysical and
biological effects of GS and its ring-size analogs with their
degree of amphiphilicity may be related to the fact that the
degree of amphiphilic character may determine the
good-ness of the characteristic interfacial location of these and
some other antimicrobial peptides, which is thought to be at
the polar/apolar region of the phospholipid bilayer near the
glycerol backbone, where the polar, positively charged Orn
or Lys residues can interact with the negatively charged
phosphate polar headgroups of the lipid bilayer and the
nonpolar Val and Leu sidechains with the upper portions of
phospholipid hydrocarbon chains [9,14,15] Although we
have no independent information at present about the
intrinsic perturbing effects of these peptides on
phosphol-ipid bilayers (i.e the degree perturbation per peptide
molecule actually present in the bilayer), we might expect
that this would be related to the asymmetry of shape and
possibility also to the size of the peptide molecule A crude
estimate of these two parameters, based simply on the
structure and conformation of these peptides in water as
illustrated in Figs 1 and 2, might suggest that the intrinsic
perturbation of these peptides would decrease in the order
GS14 > GS > GS10 > GS12 Experiments are currently
underway to actually determine the phospholipid bilayer/
water partition coefficient, the localization and orientation
in the phospholipid bilayer, and the effects of the presence
of these peptides on phospholipid organization and
pack-ing The results of these experiments should allow us to
quantitate the above parameters and gain additional insight
into the molecular basis of the structure/activity correlations
reported here
A C K N O W L E D G E M E N T S
This work was supported by operating grants from the Protein
Engineering Network of Centers of Excellence and the Canadian
Institutes of Health Research, and by major equipment grants from the
Alberta Heritage Foundation for Medical Research MK was
sup-ported in part by a Hungarian Eo¨tvo¨s Fellowship.
R E F E R E N C E S
1 Gause, G.G & Brazhnikova, M.G (1944) Gramicidin S and its
use in the treatment of infected wounds Nature 154, 703.
2 Izumiya, N., Kato, T., Aoyaga, H., Waki, M & Kondo, M (1979) Synthetic Aspects of Biologically Active Cyclic Peptides: Gramici-din S and TyrociGramici-dines Halsted Press, New York.
3 Waki, M & Izumiya, N (1990) Recent advances in the bio-technology of b-lactams and microbial bioactive peptides In Biochemistry of Peptide Antibiotics (Kleinhaug, H & van Doren, H., eds), pp 205–244 Walter de Gruyter Company, Berlin.
4 Kondejewski, L.H., Farmer, S.W., Wishart, D., Kay, C.M., Hancock, R.E.W & Hodges, R.S (1996) Gramicidin S is active against both gram-positive and gram-negative bacteria Int.
J Pept Protein Res 47, 460–466.
5 Kondejewski, L.H., Farmer, S.W., Wishart, D., Kay, C.M., Hancock, R.E.W & Hodges, R.S (1996) Modulation of structure and antibacterial and hemolytic activity by ring size in cyclic gramicidin S analogs J Biol Chem 271, 25261–25268.
6 Kondejewski, L.H., Jelokhani-Niaraki, M., Farmer, S.W., Lix, B , Kay, C.M., Sykes, B.D., Hancock, R.E.W & Hodges, R.S (1999) Dissociation of antimicrobial and hemolytic activitities in cyclic peptide diasteeomers by systematic alteration in amphipathicity.
J Biol Chem 274, 13181–13192.
7 Jelokhani-Niaraki, M., Kondejewski, L.H., Farmer, S.W., Han-cock, R.E.W., Kay, C.M & Hodges, R.S (2000) Diaster-eoisomeric analogues of gramicidin S: structure, biological activity and interaction with lipid bilayers Biochem J 349, 747–755.
8 Prenner, E.J., Lewis, R.N.A.H & McElhaney, R.N (1999) The interaction of the antibacterial peptide gramicidin S with lipid bilayer model and biological membranes Biochim Biophys Acta
1462, 201–221.
9 Prenner, E.J., Lewis, R.N.A.H., Kondejewski, L.H., Hodges, R.S.
& McElhaney, R.N (1999) Differential scanning calorimetric study of the effect of the antimicrobial peptide gramicidin S on the thermotropic phase behavior of phosphatidylcholine, phosphati-dylethanolamine and phosphatidylglycerol lipid bilayer mem-branes Biochim Biophys Acta 1417, 211–223.
10 Prenner, E.J., Lewis, R.N.A.H., Newman, K.C., Gruner, S.M., Kondejewski, L.H., Hodges, R.S & McElhaney, R.N (1997) Nonlamellar phases induced by the interaction of gramicidin S with lipid bilayers A possible relationship to membrane disrupting activity Biochemistry 36, 7906–7916.
11 Staudegger, E., Prenner, E.J., Kriechbaum, M., Degovics, G., Lewis, R.N.A.H., McElhaney, R.N & Lohner, K (2000) X-ray studies of the interaction of Gramicidin S with microbial lipid extracts: Evidence for cubic phase formation Biochim Biophys Acta 1468, 213–230.
12 Krivanek, R., Rybar, P., Prenner, E.J., McElhaney, R.N & Hianik, T (2001) Interaction of the antimicrobial peptide grami-cidin S with dimyristoyl-phosphatidylcholine bilayer membranes:
a densitometry and sound velocimetry study Biochim Biophys Acta 1510, 452–463.
13 Prenner, E.J., Lewis, R.N.A.H., Jelokhani-Niaraki, M., Hodges, R.S & McElhaney, R.N (2001) Cholesterol attenuates the inter-action of the antimicrobial peptide gramicidin S with phospholipid bilayer membranes Biochim Biophys Acta 1510, 83–92.
14 Lewis, R.N.A.H., Prenner, E.J., Kondejewski, L., Flach, C.R., Mendelsohn, R., Hodges, R.S & McElhaney, R.N (1999) Fourier transform infrared spectroscopic studies of the interaction of the antimicrobial peptide gramicidin S with lipid micelles and with lipid monolayer and bilayer membranes Biochemistry 38, 15193–15203.
15 Salgado, J., Grage, S.L., Kondejewski, L.H., Hodges, R.S., McElhaney, R.N & Ulrich, J (2001) Membrane-bound structure and alignment of the antimicrobial b-sheet peptide gramicidin S derived from angular and distance constraints by solid-state 19F-NMR J Biomol NMR 21, 191–208.
16 Gibbs, A.G., Kondejewski, L.H., Gronwald, W., Nip, A.M., Hodges, R.S & Wishart, D.S (1998) Unusual b-sheet periodicity
in small cyclic peptides Nat Struct Biol 5, 284–288.
Trang 1017 Verkleij, A.J., Zwaal, R.F.A., Roelofsen, B., Comfurius, P.,
Kastelijn, D & van Deenen, L.L.M (1973) Asymmetric
dis-tribution of phospholipids in the human red cell membrane A
combined study using phospholipases and freeze-etch electron
microscopy Biochim Biophys Acta 323, 178–193.
18 Wilkinson, S.G (1988) Gram-negative bacteria In Microbial
Lipids (Ratledge, C & Wilkinson, S.G., eds), Vol 1, pp 299–488.
Academic Press, London.
19 O’Leary, W.M & Wilkinson, S.G (1988) Gram-positive bacteria.
In Microbial Lipids (Ratledge, C & Wilkinson, S.G., eds), Vol 1,
pp 117–201 Academic Press, London.
20 New, R.R.C (1990) Characterization of liposomes In Liposomes:
a Practical Approach (New, R.R.C., ed.), pp 105–161 IRL Press,
Oxford.
21 Silvius, J.R & McElhaney, R.N (1978) Lipid compositional
manipulation in Acholeplasma laidlawii B Effect of exogenous
fatty acids on fatty acid composition and cell growth when
endogenous fatty acid production is inhibited Can J Biochem.
56, 462–469.
22 Lewis, R.N.A.H., Mak, N & McElhaney, R.N (1987) A
differ-ential scanning calorimetric study of the thermotropic phase
behavior of model membranes composed of phosphatidylcholines
containing linear saturated acyl chains Biochemistry 26, 6118–
6126.
23 Lewis, R.N.A.H & McElhaney, R.N (1993) Calorimetric and
spectroscopic studies of the polymorphic phase behavior of the
homologous series of N-saturated 1,2-diacyl
phosphatidyletha-nolamines Biophys J 64, 1081–1096.
24 Zhang, Y.-P., Lewis, R.N.A.H & McElhaney, R.N (1997)
Calorimetric and spectroscopic studies of the thermotropic phase
behavior of the n-saturated 1,2-diacylphosphatidylglycerols
Bio-phys J 72, 779–793.
25 Allen, T.M., Hong, K & Papahadjopoulos, D (1990) Membrane
contact, fusion and hexagonal (H II ) transitions in
phosphatidyl-ethanolamine liposomes Biochemistry 29, 2976–2985.
26 MacDonald, R.C., MacDonald, R.I., Menco, B.Ph, Takeshita, K., Subbarao, N.K & Hu, L.-R (1991) Small-volume extrusion apparatus for preparation of large unilamellar vesicles Biochim Biophys Acta 1061, 297–303.
27 Beschiaschvili, G & Seelig, J (1992) Peptide binding to lipid bilayers Nonclassical hydrophobic effect and membrane-induced
pK shifts Biochemistry 31, 10044–10053.
28 Jelokhani-Niaraki, M., Prenner, E.J., Kondejewski, L.H., Kay, C.M., McElhaney, R.N & Hodges, R.S (2001) Conformation and other biophysical properties of cyclic antimicrobial peptides in aqueous solution J Peptide Res 58, 293–306.
29 Smith, P.F (1992) Membrane lipid and lipopolysaccharide structures In Mycoplasmas: Molecular Biology and Pathogenesi (Maniloff, J., McElhaney, R.N., Finch, L.R & Baseman, J.B., eds), pp 79–91 American Society for Microbiology, Washington, D.C.
30 McElhaney, R.N (1992) Membrane structure In Mycoplasmas: Molecular Biology and Pathogenesis (Maniloff, J., McElhaney, R.N., Finch, L.R & Baseman, J.B., eds), pp 113–155 American Society for Microbiology, Washington, D.C.
31 Jelokhani-Niaraki, M., Prenner, E.J., Kay, C.M., McElhaney, R.N & Hodges, R.S (2002) Conformation and interaction of the cyclic cationic antimicrobial peptides in lipid bilayers J Peptide Res 60, 23–36.
32 Simecka, J.W., Davis, J.K., Davidson, M.K., Ross, S.E., Stadt-lander, C.T.K.-H & Cassell, G.H (1992) Mycoplasma diseases of animals In Mycoplasmas: Molecular Biology and Pathogenesis (Maniloff, J., McElhaney, R.N., Finch, L.R & Baseman, J.B., eds), pp 391–415 American Society for Microbiology, Wash-ington, D.C.
33 Krause, D.C & Taylor-Robinson, D (1992) Mycoplasmas which infect humans In Mycoplasmas: Molecular Biology and Patho-genesis (Maniloff, J., McElhaney, R.N., Finch, L.R & Baseman, J.B., eds), pp 417–444 American Society for Microbiology, Washington, D.C.