BIOCHEMISTRY - Textbook - Ostapchenko L. I. - 2012

Chapter 13. STRUCTURE AND PROPERTIES OF LIPIDS

13.1. Structural Components of Lipids

The common Structural components of lipid molecules are Higher Fatty acids, higher aliphatic alcohols, and aldehydes.

Higher fatty acids (HFAs) are incorporated into lipid molecules as simple or complex esters or amides. Free, non-esterified Fatty acids are also present, accounting for approximately 3 % of the neutral lipid fraction. Over 200 naturally occurring fatty acids are known; their molecules consist of a hydrophilic carboxyl group and hydrophobic hydrocarbon chains that vary in carbon atom count, presence of hydroxy and oxo groups, and degree of unsaturation. Natural fatty acids typically contain an even number of carbon atoms (most frequently 16 and 18). The aliphatic chains of fatty acids can be fully saturated or contain one or more double bonds, and occasionally triple bonds.

Saturated fatty acids are the Main Components of fats, oils, Waxes, and other substances. Most commonly, these are aliphatic carboxylic acids containing from 12 to 24 carbon atoms, although Branched-Chain Fatty Acids also occur in nature.

Short-chain fatty acids (butyric, caproic, capric, etc.) are found primarily in milk fat and butter, whereas higher saturated fatty acids (palmitic, stearic, arachidic) are components of animal adipose tissue Lipids.

Unsaturated higher fatty acids occur in the lipids of animal and plant Tissues twice as frequently as saturated ones. Depending on the number of double bonds in their molecules, Unsaturated fatty acids are classified into mono-, di-, and trienoic (polyenoic). Double bonds can be:

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The most common in lipids are cis-isomers of unsaturated HFAs containing isolated or methylene-interrupted double bonds. The presence of double bonds, regardless of their type, causes spatial isomerism. Unsaturated higher fatty acids differ significantly in spatial configuration from saturated ones. In saturated HFAs, alkyl radicals possess considerable flexibility due to the free rotation of atoms around single bonds and thus can exist in various Conformations. The hydrocarbon chains of unsaturated HFAs, due to the restricted rotation of atoms around the double bond,

feature a rigid bend. In cis-isomers of natural unsaturated fatty acids, the bending angle of the acyl chain is approximately 30°, whereas their trans-isomers hardly differ from the configuration of saturated hydrocarbon chains (Fig. 13.1). It should be noted that cis-isomers of unsaturated HFAs are less stable than the corresponding trans-isomers, which is of great importance for biomembrane function.

Fig. 13.1. Geometric representation of acyl chains and valence angles in higher fatty acid molecules

Fatty Acid Nomenclature. Natural fatty acids have trivial and systematic names (Table 13.1). Systematic names of fatty acids are derived from the name of the corresponding hydrocarbon with The addition of the suffix -anoic. For instance, the C₁₆ HFA is called hexadecanoic acid, corresponding to the C₁₆ hydrocarbon, hexadecane. The C₁₆ unsaturated fatty acid with one double bond is termed hexadecenoic acid. The C₁₈ unsaturated fatty acid with two double bonds is called octadecadienoic acid, and with three bonds, octadecatrienoic acid.

THE POSITION OF the double bond on the alkyl chain of a fatty acid is indicated by the symbol Δ (delta, Greek) with a number. For example, the symbol cis-Δ⁹ denotes the presence of a double bond in the cis-configuration between the 9th and 10th carbon atoms. The numbering of carbon atoms in a fatty acid molecule begins from the carboxyl carbon; the 2nd and 3rd carbon atoms are designated as α- and β-, respectively, and the carbon of the terminal methyl group as ω-C:

Table 13.1

Some natural fatty acids of animal origin

The PHYSICOCHEMICAL PROPERTIES OF HFAs are determined by the features of their chemical Structure. Thus, the melting point of fatty acids depends on their molecular weight as well as the number, localization, and configuration of their double bonds. As a rule, the melting point increases with a growing number of carbon atoms in the chain. At room Temperature, saturated fatty acids with 1 to 8 carbon atoms are liquids, whereas those with 10 or more are solids. Unsaturated fatty acids always melt at significantly lower temperatures than their saturated analogues: for instance, stearic acid (C₁₈) has a melting point of 70 °C, whereas for oleic acid (C₁₈, Δ⁹) it is 14 °C.

Most fatty acids, with the exception of branched-chain and hydroxy acids, lack asymmetric carbon atoms in their molecular structures and are therefore optically inactive.

Lower fatty acids with short chains are soluble in Water (formic, acetic, propionic). With increasing chain length and molecular weight, the solubility of fatty acids in polar Solvents decreases, while in non-polar solvents it increases. For example, lauric acid (C₁₂) and its higher molecular weight homologues are insoluble in water but completely soluble in non-polar solvents (chloroform, benzene, ethers, etc.).

Fatty acids dissociate weakly in water (pK ~ 5) and are therefore classified as weak acids.

Higher fatty acids are capable of forming a film on the water surface, which represents a monomolecular layer in which hydrophilic carboxyl groups interact with water, while hydrophobic hydrocarbon radicals are oriented perpendicularly to the water surface.

In dilute aqueous solutions of NaOH or KOH, fatty acids can form micelles that turn into "soaps" — Na⁺ and K⁺ salts of fatty acids. Na⁺ or K⁺ soaps are amphipathic compounds possessing an ionized carboxyl group as a polar "HEAD" and a non-polar "tail". They act as natural fat emulsifiers. Their hydrophobic radicals immerse into oil droplets, while the hydrophilic heads interact with water, forming a hydrophilic shell around the fat droplets and thereby creating emulsions, i.e., finely dispersed mixtures of fat particles in water. Ca²⁺ and Mg²⁺ salts are sparingly soluble in water and thus incapable of forming emulsions.

To isolate higher fatty acids from biological Materials, Methods based on their differential solubility in polar and nonpolar solvents, as well as their ability to form complexes with urea and thiourea, are employed. The Separation of HFAs is carried out using adsorption, partition, and Gas-Liquid Chromatography.

Higher alcohols and aldehydes. Higher alcohols occur as constituents of various lipids in the form of simple esters. These are primarily residues of high-molecular-weight alcohols with an even number of carbon atoms and long, unbranched carbon chains, which can be either saturated, CH₃-(CH₂)_n-CH₂OH, where n = 6-30, or unsaturated, CH₃-(CH₂)_n-CH = CH-(CH₂)_m-CH₂OH, as well as branched higher fatty alcohols derived from isoprene:

along with residues of polyene primary alcohols (terpenols):

where n = 2-13.

With higher fatty acids, higher monohydric fatty alcohols form esters—waxes:

where m = 12-34, n = 16-22.

In PLANT AND ANIMAL organisms, waxes perform water-repellent Functions for the outer coverings of leaves, stems, fruits, Skin, fur, and feathers in animals, as well as an energy-storage function in planktonic organisms.

Higher fatty aldehydes are components of plasmalogens. Their proportion in Lipid Composition is minor, but the hydrocarbon chains of aldehydes exhibit considerable structural diversity and varying degrees of saturation. The general formula for higher saturated aldehydes is CH₃(CH₂)_nCHO, where n = 6-20, while for higher unsaturated fatty aldehydes it is

Higher unsaturated isoprenoid aldehydes are components of plant fragrances and insect pheromones, such as geranial:

Polyhydric alcohols in natural lipids are predominantly represented by diols (Ethylene glycol, 1,2- and 1,3-propanediol), glycerol, myo-Inositol, and higher amino alcohols: