<span>Amino acids are the basic structural unit of all proteins. A 'free' amino acid (a single amino acid) always has: an amino group -NH2, a carboxyl group -COOH a hydrogen -H a chemical group or side chain -"R". Since the amino acids (except glycine) have 4 different groups attached to the a-carbon, they are optically active stereoisomers (recall 23.101 Organic Chemistry) although only L-isomers are found in proteins. At a neutral pH, both the amino and the carboxyl groups are ionised, giving what is termed the zwitterionic</span>
