Organic Chemicals in the Environment pdf

At constant volume and temperature the work of surface formation is equal to the Helmholtz free energy: dA = ydo since dA < 0 is a spontaneous change surfaces tend to contract... Lipids

Organic Chemicals in the

Environment

Where are they found?

What are the effects do they have?

The air-water interface

The surface tension of water is 0.073 N/m lons can increase this value somewhat to 0.075 N/m However, organic

compounds Tend to lower the value The surface tension of organic liquids (e.g octane, benzene etc.) range from

0.020 to 0.050 N/m

There are two types of surface adlayers on water

1 Organic thin films such as oil spills (prior to oxidative

weathering) [These compounds are both insoluble and

less dense than water

2 Surfactants are amphipathic molecules There are dry

surfactants such as detergents (mostly hydrophobic) and wet surfactants such as proteins (mostly hydrophilic)

Surface tension

Liquids tend to adopt shapes that minimize their surface area This places the maximum number of molecules in the bulk

Droplets of liquids tend to be spherical because a sphere is

the shape with the smallest surface-to-volume ratio The work needed to change the area by do is:

dw = ydo

The coefficient y is called the surface tension It has

dimensions of energy per unit area (J/m2) At constant

volume and temperature the work of surface formation is

equal to the Helmholtz free energy:

dA = ydo since dA < 0 is a spontaneous change surfaces tend to contract

Lipids and Surfactants

Both lipids and surfactants consist of a hydrophobic tail

region and a hydrophilic head group (amphipathic)

Both lipids and surfactants will orient with their head

groups pointed towards the water interface and their

tails sequestered from water

Lipids have the ability to form a bilayer This property

makes these molecules the constituents of biological

membranes Bilayers can be gel-like or crystalline They

can have a planar phase or form hexagonal phase

Surfactants can tend to form micelles Micelles are spherical The hydrophobic tails form the interior and the charged head groups are on the surface

Biochemistry of Lipids, Lipoproteins

and Membranes, Vance & Vance, Elsevier 1996

Lipid structure

Representative lipids are

shown in Figure to the

right There are two acyl

Ethanolamine (Phosphatidylethanolamine)

Serine (Phosphatidylserine)

Glycerol

(Phosphatidylglycerol)

Glycerol (Diphosphatidylglycerol)

Myo-inositol (Phosphatidylinositol)

The fluid mosaic model of membranes

Biochemistry of Lipids, Lipoproteins

and Membranes, Vance & Vance, Elsevier 1996

Membrane asymmetry

The inner and outer leaflets of membrane bilayers have

different compositions Erythrocytes are the most studied The cytosolic side is composed of PE and PS The PE

distribution is ca 80% in the inner membrane and 20% in the outer membrane PS is negatively charged and PE Is

moderately negative in charge The inner membrane is thus largely negative

The outer membrane consists of PC, sohingomyelin, and glycolipids Cholesterol is also important and associates

with the membrane to provide added fluidity Plasma

membranes have equimolar quantities of cholesterol By

contrast, the endoplasmic reticulum and mitchondrial

membranes have small amounts of cholesterol.

Detergents and

Lysophospholipids

—o ⁄

molecules that can have charged

or uncharged head groups and

single hydrophobic tails If one

of the acyl chains is cleaved from

a lipid then a lysophospholipid is

created

Other associating molecules

include aromatic or fused ring

compounds (dyes, purines,

pyrimidines) and alicyclic fused

ring compounds (bile salts,

Transmembrane Potential

Membranes are essential for life They provide a compartment for

chemistry to occur separate from the environment Living organisms have also evolved to use membranes for generation and storage

of energy using photosynthesis This is done by pumping protons

across a membrane to generate a transmembrane potential

The transmembrane electrical potential is represented as the

voltage difference of the inside with respect to outside We

express the potential as V (unit is the volt) In chemistry, the Nernst equation is used to describe the dependence of the oxidation

potential on concentration The symbol is E (or E°) and the unit is

also the volt.

The Nernst equation

The Nernst equation describes the potential for each half-reaction

a(Ox) + ne’ «— b(Red) The standard potential (i.e potential at 1 molar concentration) is E°

The electrode potential is given by:

_ po RT, | [Real”

E=E°- “In Em

where R is the gas constant and F is the Faraday (96,450 J/volt)

In an electrochemical cell where two half-reactions are combined

to make a redox reaction, the electromotive force Is:

emf = E(+) - E(-) The free energy is G = -nFE Therefore, the standard free energy

change for a redox process is AG° = -nF(E°,, - E° 4) = -nFAE?® red

Application of the Nernst equation

to membrane potential The free energy per mole of solute moved across the membrane

AG pone = -RTIN(C,/C;,) where C, is the concentration outside the

membrane and CG, is the concentration inside the membrane The

difference in charge concentration results in a free energy contribution

from the voltage difference AG, = -FAE (assuming n=1) The

balance of forces at equilibrium requires that AG, = AG so that

the trans-membrane potential is obtained as follows

Natural foams from wax

Natural foams can occur from tree sap, fallen leaves and zooplankton For example, zooplankton are composed of wax esters Sometimes as much as /5% of the organism

is attributable these compounds An example of a wax

ester is bee’s wax, which is Palmitic acid (C16:0) esterified

by a C30 chain, Known as triacontanol (or melissyl alcohol)

O

|

CH;(CH;);¿—C—O—CH;—(CH;);ạ—CH;

Palmitic acid Triacontanol

The word "wax" is derived from the old english "weax" for the honeycomb of the bee-hive Thus, bee wax can be

considered as the reference wax

Zooplankton wax

Polar zooplankton species are known for the storage of wax esters as natural energy reserves Thus, in the antarctic

Euphausiid Thysanoessa macrura the wax deposits reach

up to /0% of the total body lipids and contain high levels of 18:1(n-9) and 18:1(n-7) alcohols Carnivorous zooplankton species are characterized by the presence of shorter-chain alcohols (14:0, 16:0) while herbivorous species, as the

calanoid copepods, contain mainly long-chain alcohols

(20:1, 22:1)

Kattner G et al., Mar Ecol Prog Ser 1996, 134, 295

Cutin is a wax found on leaves

Monoacylglycerols are important in the constitution of

cutin polymer Cutin is the structural component of the plant cuticle, the outermost layer of leaves and other aerial organs Waxes embedded in the cutin make the cuticle an efficient

barrier against desiccation, gas exchange and pathogen attack Cutin polyester is typically composed of esterified hydroxy-

fatty acids with 16, 18 and 22 carbon-chains and one terminal hydroxyl (w-position) and other hydroxyl groups in secondary positions The cutin polymer has been found to be based on

the inter-esterification of hydroxyacids (head-to-tail in a linear form or cross-linked) and of glycerol esterified with various

hydroxy-fatty acids

Graca J et al., Phytochemistry 2002, 61, 205

Cutin structure

The cuticle is part of the epidermis

cuticle

upper epidermis palisade layer

loss When moving plants from sun vascular bundle

spongy mesophyll

to shade the amount of cutin required

lower epidermis

auard cells stomate

The formation of a polymeric

cutin is shown on the right

Monomer composition in cutin

Table I Stem cutin monomer amount (ug/dm* +s.d.) and percent cutin monomers (% +s.d.)

Columbia attl-1 Cutin monomer Amount Percent Amount Percent

9-Hydroxy pentadecanoic acid 1.3+0.4 2.6+0.3 0.7+0.1 4.3+1.0

10(9)-Hydroxy heptadecanoic acid 1.9+1.1 3.9+2.3 0.7+0.3 4.3+1.9

16-Hydroxy hexadecanoic acid 1.6+0.3 3.2+0.2 1.1+0.3 6.4+1.6 10,16(9,16)-Dihydroxy hexadecanoic acid 8.1+2.6 16.4+3.5 3.5+1.1 21.3+5.3 Hexadecane-1,16-dioic acid 24.1+3.5 49.3-+ 1.2 5.4+0.1 33.6+2.6 7(8)-Hydroxy hexadecane-1,16-dioic 5.5+0.7 11.3+0.6 2.2+0.3 13.7+2.7

Octadecane-1,18-dioic acid §.8+1.1 11.8+0.3 1.8+0.1 11.1+1.5

Octadecadien-1,18-dioic acid 0.7+0.1 1.4+0.1 0.8+0.1 5.3+0.7 Total 49.1+8.2 16.1+1.1

Cuticle was enzymatically isolated from the first three internodes of the main stem of 35-day-old plants (three replicates) Each replicate includes 12-14 individual plants pooled together Isomers are listed in parentheses Position of the two double bonds in octadecadien-1,18-dioic

acid was not determined

10,16-dihydroxypalmitic acid In the other 32%, these

hydroxyls are replaced by

carboxy or aldehyde groups

Biochemistry of wax synthesis

Acetyl-CoA

| FA syn PAAASASY COOH (16:0)

|

O (09 15k-Co9) oxidoreductase

) ứa elongation sys ớ^^+~~*#ề<®~~«x.-:—————_ >

NADP* | CoA SH (often Caa & Ca+)

Acetyl co-A

Acetyl-CoA is an important molecule in metabolism Its

main use Is to convey the carbon atoms within the acetyl

group to the Krebs Cycle to be oxidized for energy production

In chemical structure, acetyl-CoA is the thioester between

coenzyme A (a thiol) and acetic acid (an acyl group carrier)

It is also a carrier of two carbon units in fatty acid synthesis

acetyl f-mercapto- pantothenic acid ’

°Pantothenic acid group ethylamine ¬"-

|

°3', 5-adenosine diphosphate 3°, S-ADP

Acetyl coenzyme A, showing its constituents

Suberin is found in bark

Waxes are also found in suberin, which is a lipidic polyester present in tree barks, tuber skins and abscisic tissue of

falling leaves It is also formed in plant after wounding Upon depolymerization, cork suberin releases a mixture of

monomers and oligomers, including monoacylglycerols of monoacid (C22), of w-hydroxyacids (C16, 18, 22, or 24), and

of a,@-diacids (C16, 18, or 22) Glycerol is a major

compound of this polyester, constituting up to 20% by weight

of suberin in oak, cotton and potato The current model

describes a monoglyceride containing 26-hydroxy-26:0 fatty acid has been isolated from the root bark of Pentaclethra

eetveldeana, used in Congo as a traditional medicine for the treatment of hemorrhoids, malaria and epilepsy

Graca J et al., Chem Phys Lipids 2006, 144, 96 Graca J et al., Agric Food Chem 2000, 48, 5476 Byla B et al., Phytochemistry, 1996, 42, 507

Soil carbohydrates

Soil contains organic matter only near the surface The

organic compounds arise mostly from the action of micro- organisms Most of the organic matter is trapped in colloids and is not in equilibrium

Soil contains a number of carbohydrates and sugars

comprising about 10% of the organic matter Most of it is polymeric although some monomeric sugars exist,

glucose, galactose, fructose etc

As much as 50% of soil phosphorous is esterified as

inositol hexaphosphate (see page 64 in Larson and Weber)

I IP5 OP(OH);„ IPG OP(O), OP(OH),

Inositol hexaphosphate

Phytic acid (inositol hexaphosphate lở 5 id

many plant tissues, especially in

the grass family (wheat, rice, rye,

barley etc) and beans Phosphorus

O

humans lack the digestive enzyme,

phytase, required to separate

phosphorus from the phytate molecule

Phytic acid binds to important minerals such as calcium,

magnesium, iron and zinc and can therefore contribute to

mineral deficiencies, as the minerals are not released from the phytate and are thus unavailable to the body

Good and bad effects of phytate

For people with a particularly low intake of essential minerals, especially young children and those in developing countries, this effect can be undesirable

A common way in developing countries to increase the bio- availability of minerals from grains and beans is using

fermentation Many bacteria possess phytase activity and by fermenting grains or beans by lactic acid bacteria the phytate

is destroyed and the bioavailability of the minerals is

increased

Phytic acid recently has been studied for its potential anti-

carcinogenic properties Recent studies have indicated that phytic acid may have some preventive effect in prostate,

breast, pancreatic and colon cancer The mechanism,

however, is not yet understood

Breakdown of lignin

Lignin is a biopolymer composed of 4-alkylcatechol units The breakdown of lignin in the soil leads to five common phenolic acids

Humus

The chemistry of humus is one of the most complex in nature Humus consists of a oxidation and polymerization products of Polysaccharides, phenols and amino acids found in the soil Humic matter is anionic and therefore it binds metal ions well Extraction of humus is performed in aqueous alkall

(0.5% NaOH) The base soluble fraction is called humic acid The fraction which is solubilized by acid is called fulvic acid

It is not Known whether these two fractions are significantly different (Humus is discussed on pages 68-83 of Larson and Weber)

Solubility and partitioning

Solubility Hydrophobic Effect Linear Free Energy Relationships

Octanol/Water Partitioning

Water as a solvent for

organic molecules

The maximum solubility of carbonaceous matter in water

occurs at around C, This is an optimum size for water

molecules to act as a clathrate (cage) around the solute

organic solutes tend to be hydrophobic and so the

organization of solvent water around these solutes in an

ice-like structure increases the entropy of the solution This unfavorable interaction tends to cause carbon materials to aggregate