Biochemistry

The location of the substituents around the chiral carbon atom distinguishes between L- and D-stereoisomers, which can be considered as an object and its mirror reflection. In order to determine the amino acid isomer, the configuration of its chiral center is compared with the stereoisomers of glycerol aldehyde. If the amino group is located to the right of the OH - R axis, then it is a D-amino acid; if to the left of this axis, it is an L-form. CHO I HO-C-H I CH 2 OH L-glyceraldehyde CHO 1 HO-C-H I CH 2 OH 1 L-glyceraldehyde CHO I HO-C-H I CH 2 OH 1 L-glyceraldehyde CHO I HO-C-H I CH 2 OH 1 L-glyceraldehyde This dividing of amino acids is of biological importance. Proteins include only L-amino acids. Data on the presence of D-amino acids in proteins have not yet been obtained. However, the D-forms of amino acids are isolated from various organisms (including humans) in a free state or as part of peptides. They are included in the mucopeptides of the cell walls of certain microorganisms, are components of peptide antibiotics (gramicidin, actinomycin). L-amino acids are necessary for the body to construct proteins, while D-amino acids are apparently used for protective functions (spore-bearing bacteria capsules, antibiotics). Physico-chemical properties of amino acids Depending on the pH of the aqueous solution, a-carboxyl and a-amino groups of amino acids exist in one of the following interconvertible forms: R - CH- COOH <-> R-CH- COO - R- CH -COO" NH 3 NH 3 NH 2 Undissociated Bipolar ion Dissociated carboxyl group (zwitterion) carboxyl group Protonated Loose amino group In the pH range, at the total charge amino acids is zero amino group Cation in strongly acidic solutions Anion in strongly alkaline solutions By chemical properties, amino acids containing amine and carboxyl groups are amphoteric electrolytes. 43