Amino acids – characteristics, classification. Standard –amino acids. Isomerism, physical  properties. Amphoteric and chemical properties of amino acids. Peptides.

Amino acids – characteristics, classification. Standard Alpha – amino acids. Isomerism, physical properties. Amphoteric and chemical properties of amino acids. Peptides.

Amino acids - are molecules containing an amino group, a carboxylic acid group and a hydrocarbon (side) chain that varies between different amino acids.

The nature of the hydrocarbon chain is one criterion for amino acids classification. There are aliphatic (saturated and unsaturated) and aromatic amino acids. The number of COOH and NH2 groups can vary. According to them number distinguished are, for instance, monoamino or diamino monocarboxylic acids, mono amino or diamino dicarboxylic acids, etc.

The presence of a heterocyclic ring gives a group of heterocyclic amino acids. Amino acids are classified also as alpha, beta , grama , and so on, according to the location of the amine group on the carbon chain that contains the carboxylic acid function.

In an acid-base reaction, a proton transfer occurs internally, within the same molecule. This proton transfer is indicated with zwitterionic form of the amino acid (in German "Zwitter“ means hybrid, Hermaphrodite)

Classification of the standard alpha-amino acids.

The standard amino acids are classified according to the polarity, acidic or basic properties of the side chain into four groups:

1. (1) amino acids (AA) with nonpolar side chains;

1. (2) AA with polar, uncharged side chains;

1. (3) AA with anionic (negatively charged) side chains;

1. (4) AA with cationic (positively charged) side chains (at pH ~ 7)