The role of amino acids in the structure and properties of plant proteins

Peptide bond and the structural basis of α-amino acids

All plant proteins are constructed from amino acids — universal structural units capable of polymerizing and forming stable molecular chains. The basis of this process is the reaction between an amino group (–NH2) and a carboxyl group (–COOH) with the elimination of a water molecule. As a result, a strong peptide (amide) bond is formed, creating the backbone of the protein polymer.

The molecule of any amino acid is built around a central carbon atom, to which four elements are attached: a hydrogen atom (–H), a carboxyl group (–COOH), an amino group (–NH2), and a side chain (R radical). It is the R radical that determines the individual chemical properties of the compound. Depending on the degree of distance between the amino group and the carboxyl group, several types of structures are distinguished:

• α-amino acids — the amino group is located at the nearest (α) carbon atom (for example, alanine with a methyl group –CH3 or glycine with a hydrogen atom –H);
• β-amino acids — the amino group is located at the second (β) carbon atom (β-aminopropionic acid);
• γ-amino acids — the amino group is at the third (γ) carbon atom (γ-aminobutyric acid);
• δ-amino acids — the amino group is at the fourth (δ) carbon atom (δ-aminovaleric acid).

Chirality and spatial isomerism of amino acids

With the exception of glycine, all α-amino acids possess the property of chirality (dissymmetry). Their molecules contain an asymmetric atom (chiral center) connected to four different functional groups. This center is most often a carbon atom, but it can also be silicon, nitrogen, phosphorus, or magnesium atoms. This phenomenon was discovered in 1848.

Chiral compounds exist in the form of two mirror images (enantiomers) — D and L. If, in a projection image, the amino group is located to the right of the COOH–R axis, the compound is classified as the D-form, and if to the left — as the L-form. Enantiomers have identical chemical and physical properties, but differ in optical activity — the direction of rotation of the plane of polarized light.

• Standard substance concentration — 1 g/ml
• Measurement layer thickness — 1 dm
• Light wavelength (sodium D-line) — 546 nm

The form that rotates the polarization plane clockwise is denoted by the "+" sign (dextrorotatory), and counter-clockwise — by the "–" sign (levorotatory). Membership in the D- or L-series indicates exclusively the spatial arrangement of atoms around the α-carbon and may not coincide with the direction of beam rotation. Therefore, there are L(+), L(–), D(+), and D(–) isomers, while the direction of rotation itself depends on the solvent and the pH of the medium.

A mixture of equal amounts of right- and left-handed enantiomers forms a racemate, which possesses no optical activity. In a plant organism, biochemical processes are strictly selective: only L-amino acids participate in the construction of functional proteins, which must be taken into account when assessing the biological activity of agrochemicals.

Plants respond differently to D- and L-forms of amino acids; while L-forms are easily incorporated into various metabolic processes, D-forms are not assimilated by plants and sometimes even inhibit metabolic processes. This is because the enzymatic systems of organisms are specifically adapted to L-amino acids. Most D-series amino acids have a sweet taste, while natural L-forms are bitter or tasteless. Chemical, microbiological, and enzymatic methods are used to separate amino acids into optical antipodes. Synthetic amino acids are racemates, i.e., mixtures of D- and L-forms.

In appearance, amino acids are white crystalline powders, most of which are highly soluble in water at normal temperatures. Aqueous solutions of amino acids are stable and can be autoclaved at a temperature of 100–120°C.

As already noted, amino acids contain both a basic amine group and an acidic carboxyl group simultaneously. Therefore, like other amphoteric compounds, they can dissociate to form H+ and OH– ions.

All α-amino acids exist in the aqueous environment of living cells predominantly in the form of bipolar ions or zwitterions* with a dissociated carboxyl group and a protonated amino group:

NH3 2 NH2 + H+

Therefore, most amino acids, being monoaminomonocarboxylic, do not possess any significant charge near neutral pH values.

The bipolarity of amino acid molecular structures results in a number of their properties, in particular, the high solubility of most amino acids in water and relatively low solubility in organic solvents, large dipole moments of their molecules, and high values of dielectric constants and melting points. Depending on the pH of the medium, amino acids can be in the form of anions, cations, electrically neutral bipolar ions, or as a mixture of these forms with one of them dominating. In highly acidic solutions, amino ac-

A zwitterion is a neutral molecule that contains spatially separated opposite charges simultaneously.

lots are present in the form of positive ions, and in alkaline conditions – in the form of negative ions, i.e., amino acids are amphoteric electrolytes:

R –– С ОН  R –– С 2O

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R –– С С