Dynamical spin chirality and magnetoelectric effect of α glycine

Chinese edition available online at www.whxb.pku.edu.cn ARTICLE Dynamical Spin Chirality and Magnetoelectric Effect of α-Glycine Xinchun Shen1, Wenqing Wang1,*, Yan Gong2, Yan Zhang

Volume 24, Issue 12, December 2008

Online English edition of the Chinese language journal

Cite this article as: Acta Phys -Chim Sin., 2008, 24(12): 2153−2158

Received: July 7, 2008; Revised: September 30, 2008

*Corresponding author Email: wangwqchem@pku.edu.cn; Tel: +8610-62752457

The project was supported by the Special Program for Key Basic Research of the Ministry of Science and Technology of China (2004-973-36) and the National Natural Science Foundation of China (20452002)

Copyright © 2008, Chinese Chemical Society and College of Chemistry and Molecular Engineering, Peking University Published by Elsevier BV All rights reserved Chinese edition available online at www.whxb.pku.edu.cn

ARTICLE

Dynamical Spin Chirality and Magnetoelectric Effect of

α-Glycine

Xinchun Shen1, Wenqing Wang1,*, Yan Gong2, Yan Zhang3

1Beijing National Laboratory for Molecular Sciences, Department of Applied Chemistry, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P R China;

2School of Medicine, Tsinghua University, Beijing 100084, P R China;

3

School of Physics, Peking University, Beijing 100871, P R China

Abstract: Dynamical spin chirality of α-glycine crystal at 301−302 K was investigated by DC (direct current)-magnetic

susceptibility measurement at temperatures ranging from 2 to 315 K under the external magnetic fields (H=±1 T) parallel to the b

axis The α -glycine crystallizes in space group P21/n with four molecules in a cell, which has centrosymmetric charge distribution

The bifurcated hydrogen bonds N + (3)−H(8)···O(1) and N +(3)−H(8)···O(2) are stacked along the b axis with different bond intensities

and angles, which form anti-parallel double layers Atomic force spectroscopy result at 303 K indicated that the surface molecular structures of α -glycine formed a regular flexuous framework in the b axis direction The strong temperature dependence is related to

the reorientation of NH 3+ group and the electron spin flip-flop of (N + H) mode Under the opposite external magnetic field of 1 T and

−1 T, the electron spins of N + (3)−H(8)···O(1) and N + (3)−H(8)···O(2) flip-flop at 301−302 K These results suggested a mechanism of the magnetoelectric effect based on the dynamical spin chirality of (N + H), which induced the electric polarization to produce the onset of pyroelectricity of α-glycine around 304 K.

Key Words: α-Glycine; Dynamical spin chirality; Magnetoelectric effect; Pyroelectricity; DC-magnetic susceptibility; Atomic

force spectroscopy

Most natural proteins are comprised of 19 L-amino acids

and glycine, which is achiral Up to date, it remains a puzzle

in the origin of biochirality Crystalline glycine exists in three

modifications, viz α with point group C 2h, β with point group

C2, and γ with C3 symmetry α-Glycine crystals are

centro-symmetric and do not exhibit piezoeffect, whereas β- and γ-

glycine have polar symmetry groups, i.e., pyroelectrics and

ferroelectrics[1−5]

In 1999, Chilcott et al.[6] discovered the onset of

pyroelec-tricity in α-glycine around 304 K This unusual electric

be-havior was not explained readily by the conduction

mecha-nism Pyroelectricity arises only in non-symmetric materials

The onset of pyroelectricity was speculated to accompany

with a change from the centrosymmetric space group P21/n to

a non-centrosymmetric space group

Langan et al.[7] speculated on the anomalous electrical be-havior as the possible correlation with structural phase transi-tion Neutron diffraction measurement did not show any evi-dence of change in the space group symmetry with tempera-ture However, the thermal expansion was found to be very anisotropic in the unit-cell parameters The most striking

fea-ture is the large increase in b axis with increasing temperafea-ture The relative change in b is far greater than the changes in the other lattice parameters a and c The significant structural

change is the bifurcated hydrogen bonds N+(3)−H(8)···O(1) and N+(3)−H(8)···O(2) that link molecular layers stacked in

the b axis direction The glycine molecule itself possesses a relatively large dipole moment lying approximately to the c

axis The anomalous electronic properties of α-glycine most

likely arise from libration-driven changes in stacking

interac-tions between anti-ferroelectric molecular dipole layers, which

can have large effects on the dielectric properties of crystals

Dawson et al.[8] studied the effect of high pressure on the

crystal structure of α-glycine and also found that the variation

of b-axis length reflected the increase of the stacking distance

between the layers

Murli et al.[9,10] performed Raman scattering study on

α-glycine crystal in the temperature range of 83−360 K and

high-pressure behavior from 0.76 GPa up to 23 GPa They

found that the N+H stretch frequency (3145 cm−1)

corre-sponding to the interlayer hydrogen bond N+(3)−H(8)···O(1)

shows rather large pressure induced blue shift, 3.8 cm−1·GPa−1

However, as this hydrogen bond is a bent bifurcated hydrogen

bond, with N+(3)−H(8)···O(1) angle being 154°, the

correla-tion of pressure induced changes in N−H···O distance is not

straightforward[11] They speculated that the shift of N+(3)−

H(8)···O(1) may be owing to the dipole nature of molecule[12]

Alternatively, the intralayer hydrogen bonds N(3)−H(6)···O(1)

and N(3)−H(7)···O(2) were found to stiffen at pressures above

3 GPa

To account for the above studies, the conduction

mecha-nism remains unclear yet For a crystal to be ferroelectric, it is

necessary for the centers of gravity of the positive and

nega-tive electric charges to be distinct and the crystal has no center

of symmetry In α-glycine, the distribution of the electric

charges and the magnitude of the individual electric dipoles

(NH3-CO2) are sensitive to a change of temperature On

heat-ing, the individual dipoles (NH3) are oriented in one direction

The permanent electrical polarization can appear during

varia-tion of the temperature to produce ferroelectricity and the

crystal has undergone an anti-ferroelectric/ferroelectric

transi-tion

The interplay between the magnetism and ferroelectricity is

a phenomena of magnetoelectric (ME) effect in which the

magnetization is induced by the electric field or the electric

polarization is induced by the magnetic field[13] Li et al.[14]

found that the energy barriers for internal rotation of the NH3

and CO2− groups in glycine were 14.4 and 255 kJ·mol−1,

re-spectively The internal rotation barriers indicate that the CO2

group is no rotation in agreement with the solid structure of

double layers of molecule held together by hydrogen bonds

The dynamics of NH3+ group provides most of the

contribu-tion[15] In this article, we study the ME effect and spin

flip-flop transition of N+(3)−H(8) mode in NH3+ group of

α-glycine by DC-magnetic susceptibility measurement from 2

to 315 K under the external magnetic field strength of ±1 T

parallel to the b axis

1

1 Experimental

1.1 Sample recrystallization and characterization

α-Glycine (Sigma Corporation, minimum 99% TLC) was

recrystallized from thrice distilled water by slow evaporation

at 277 K Optically clear seed crystals were obtained after a period of 7 days[16,17] The obtained crystals were thoroughly

dried under vacuum and stored under moisture-free condition

Powder XRD pattern of α-glycine was performed using X-ray

diffractometer (Rigaku D/Max-3B, Japan) with Cu K α

radia-tion of λ=0.15406 nm The sample was scanned in the 2 θ

val-ues ranging from 10° to 50° at a rate of 4 (°)·min−1 The XRD

result was shown to be the monoclinic α-polymorph only, without characteristic peak of the γ-glycine[18,19], Fig.1

1.2 N−H··· O bond length, angle, and direction

The unit cell parameters of α-glycine were measured by

X-ray diffraction as follows: a=0.5107(2) nm, b=1.2040(2)

nm, c=0.5460(2) nm, β=111.82(2)° [20]

stable modification at ambient conditions, existing as zwit-terionic form (NH3+CH2CO2−) in monoclinic structure (space

group symmetry P21/n) The unit cell contains four

symmetri-cally related molecules, which are hydrogen bonded pairwise, A−B and C−D, around the centers of symmetry[21] The mo-lecular pairs are linked together by means of a two-dimen-sional network of the hydrogen bonds forming an anti-parallel

double layer of molecules perpendicular to the monoclinic b

axis, with the intra-layer linkage of two relatively short hy-drogen bonds N(3)−H(6)···O(1) (length of 0.2771 nm) with H(6)···O(1) (length of 0.1729 nm) and N(3)−H(7)···O(2) (length of 0.2847 nm) with H(7)···O(2) (length of 0.1820 nm)

In sub-layer, the molecules are related by simple translation A

two-fold screw axis perpendicular to the layer (i.e., parallel to

the b axis) transforms one (A−B) of the two molecular pairs in

a unit cell to the other one (C−D) belonging to the adjacent double layer These layers are connected by interlayer longer bifurcated hydrogen bonds N+(3)−H(8)···O(1) (length of 0.2950 nm) with H(8)···O(1) (length of 0.2362 nm) and bond angle of 154.26° and N+(3)−H(8)···O(2) (length of 0.3065 nm) with H(8)···O(2) (length of 0.2101 nm) and bond angle 114.91° to form anti-parallel double layers The different dou-ble layers are joined by weak C(5)−H(9)···O bonds with H(9)···O(1) (length of 0.2446 nm) and H(9)···O(2) (length of 0.2378 nm) hydrogen bonds Neutron diffraction has shown

the structure of α-glycinewith atomic numbering (Fig.2(a)) The direction of N(3)−H(6)···O, N(3)−H(7)···O, and N+(3)−

Fig.1 Powder XRD pattern of α-glycine at room temperature

H(8)···O bonds was viewed down the c axis (Fig.2(b)) The

interlayer hydrogen bonds N+(3)−H(8)···O(1) and N+(3)−

H(8)···O(2) formed anti-parallel double layers as shown in

Fig.2(c)[7,22,23]

The DC-magnetic susceptibility was measured on α-glycine

using SQUID magnetometer ranging from 2 to 315 K[24] A

transparent small crystal of α-glycine was selected as seed

under triple recrystallization for obtaining a large crystal The

crystal face and b-axis were ascertained by XRD diffraction

The quantum design SQUID XL-5 magnetometer was used to

measure the DC-magnetic susceptibility of the α-glycine

crys-tals (0.08357 g) from 2 to 315 K The external magnetic field

was implied to provide a certain preferred atomic direction of

electron spin in the molecule Measurements were taken by

the applied magnetic field strength (H=100 Oe, ±10 kOe)

par-allel to the b axis The magnetic moments (M) were measured

by scanning three times and the mass susceptibility values

were calculated from χ ρ =M/(H×m), where, M is the magnetic

moment, H is the magnetic field strength, and m is the sample

mass

2

2 Results and discussion

2.1 DC-magnetic susceptibility of α-glycine

α-Glycine molecules in crystals exist as parallel chains of

hydrogen bonded zwitterions (NH3−CO2) that form magnetic dipoles The quasi-metallic hydrogen N+(3)−H(8) has a mag-netic moment μB (μB=1 Bohr magneton=0.927×10−23 A·m2),

which runs along the b axis The orientational potential energy

when the dipole is anti-parallel to the field So the energy that must be supplied to turn the dipole is 2μBB

2μBB=2×0.927×10−23×1≈1.85×10−23 J=1.16×10−4 eV Although this energy is small, the dipole moment cannot turn unless the energy is supplied At low magnetic field

Fig.2 (a) Structure of α-glycine with the atomic numbering (bond length in nm); (b) hydrogen bonded double layers of α-glycine (NH 3+CH 2 CO 2−) viewed down the c axis, N+(3)−H(8) along the b axis, N(3)−H(6) along the c axis, N(3)−H(7) approximately along the

a axis, α=γ=90°, β=111.697°; (c) hydrogen bonded double layers of α-glycine (NH 3+CH 2 CO 2−) viewed down the a axis, the interlayer

hy-drogen bonds N + (3)−H(8)···O(1) and N + (3)−H(8)···O(2) formed anti-parallel double layers

strength of 100 Oe with H//b axis, there is no peak appearing

in Fig.3

An external magnetic field strength was applied with

mag-nitude H=1 T=10 kOe=1 J·A−1·m−2 The potential energy of

the field is required to turn the magnetic dipole anti-parallel to

the field In the case of H=±10 kOe, the spin-flop peaks of

α-glycine appeared at 301−302 K (Figs.4a, 5a)

When T=302 K, kT=2.6×10−2

eV

The assumption μBB <<kT is valid at ordinary temperature

and fields, μBB being about 0.2% of kT We have seen that

eV at H=10 kOe, which is a very small energy shift

compared to the Fermi energy, εF ≈1 eV, hence, the number of

electrons with parallel moments is only slightly larger than

those with anti-parallel moments Because the randomizing

thermal effect dominated over μBB, the mass susceptibility

should have a small value Conversely, if the dipole is

origi-nally aligned anti-parallel to the field, it cannot turn to align

itself parallel to the field unless it can release the same amount

of energy[25]

Since N+(3)−H(8)···O(1) and N+(3)−H(8)···O(2) are

bifur-cated hydrogen bonds connected the interlayer of α-glycine

The corresponding H(8)···O(1) distance of 0.2362 nm is

longer than H(8)···O(2) of 0.2101 nm The dipole of N+(3)−

H(8)···O(1) is parallel to the field A spin-flop peak of

N+(3)−H(8)···O(1) was observed at 301−302 K under H=10

kOe (Fig.4a) The N+(3)−H(8)···O(2) was anti-parallel to the

field, therefore, the spin-flop peak of N+(3)−H(8)···O(2) was

observed at H=−10 kOe (Fig.5a) The spin flip-flop peaks in

the plot of dχ ρ /dT versus T at 301−302 K (Figs.4b, 5b)

indi-cate the dynamical spin chirality and spin anisotropy along the

b axis It can be concluded that the dynamical spin chirality of

N+(3)−H(8)···O(1) and N+(3)−H(8)···O(2) of α-glycine is a

property of the ensemble rather than a molecular

characteris-tic[26]

2

2.2 Surface structure of α-glycine crystal by atomic force

microscopy

Nanoscope IIIa produced by Digital Instruments Company

was used for direct observation of the surface structure of

α-glycine crystal at 303 K The image was obtained by

re-cording the Z coordinate of the tip as it scans the surface in

contact mode with deflection set point from −2 to −3 V, scan rate 20.35 Hz, and scan size 4.22 nm[27] The surface

molecu-Fig.3 Temperature-dependent susceptibility χ ρ of α-glycine

Fig.4 (a) Temperature-dependent susceptibility χ ρ and

(b) dχ ρ/dT versus T of α-glycine

m=0.08357 g; warming; H=10 kOe, H//b axis

Fig.5 (a) Temperature-dependent susceptibility χ ρ and

(b) dχ ρ/dT versus T of α-glycine

m=0.08357g; warming; H=−10 kOe, H//b axis

lar structures of α-glycine were shown in both the lateral and

longitudinal dimensions in Fig.6 In α-glycine, the lateral

hy-drogen bonds of N(3)−H(6)···O and N(3)−H(7)···O are

stronger than the hydrogen bonds of N+(3)−H(8)···O These

chains are packed together by the lateral hydrogen bonds,

forming a three-dimensional network of the hydrogen bonds,

which provides the evidence of the ferroelectricity in

α-glycine crystal

The dominating surface feature of the intermolecular

pack-ing is bifurcated hydrogen bonds N+(3)−H(8)···O(1) and

N+(3)−H(8)···O(2), which link the molecules into right- and

left-handed helices around the threefold screw axes It helps to

solve the puzzle of how glycine can play an important role in

the critical folding of functional protein occurring near room

temperature[28]

3

3 Conclusions

Temperature-dependent measurements of DC-magnetic

susceptibility of single-crystal α-glycine demonstrate the spin

flip-flop transition of N+(3)−H(8)···O(1) and N+(3)−H(8)···O(2)

hydrogen bonds The crystals undergo an anti-ferroelectric/

ferroelectric transition at 301−302 K Proton seems like a

ba-ton and transfers along the intra-layer hydrogen bond chains

below 301 K

Drebushchak et al.[29] proposed that NH3 tails of zwitterions

stick out of the layers uniformly either up (↑) or down (↓)

bonding with oxygen in a neighboring layer, which are paired

(↑↓↑↓↑↓) in α-glycine and unpaired (↓↓↓↓↓↓) in β-glycine

Katsura et al.[13] proposed the ME effect based on the spin

current in terms of a microscopic electronic model for

noncol-linear magnets The spin current is induced between the two

spins with generic nonparallel configurations[30] We propose a

mechanism of the ME effect based on the intrinsic dynamical

spin chirality, which causes charge separation in glycine and a

net spontaneous polarization Current generated by small

changes in temperature below the critical temperature of

py-roelectric effect causes a dramatic increase in conductance It

elucidates macroscopically the anomalous electrical

conduc-tance of α-glycine near room temperature

Acknowledgments

The authors are indebted to Mr Xiu-Teng Wang and Professor

Song Gao for DC-magnetic susceptibility measurements with MPMS XL-5 system The authors thank Professors Dong-Xia Shi and Hong-Jun Gao for surface structure measurement with Nanoscope IIIa AFM instrument

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