Peptide dehydro form

In most cases of formation of peptides containing d-amino acid, the L form of the amino acid is the substrate for the incorporating enzyme. In contrast, the free D-amino acid is ordinarily a poor substrate for the incorporation reaction. Whether the racemization occurs on the enzyme or afterwards remains to be determined in most cases. In the case of the D-valyl residue formed in penicillin, a tripeptide derivative containing L-valine is an intermediate, and the conversion is thought to occur by way of an a,/3-dehydro form of the valyl residue. 

This peptide is formed by several strains of Streptococcus lactis (Langfield-N-group). It contains a number of unusual amino acids, namely dehydroalanine, dehydro-P-methy 1-alanine,... 

Other dehydro amino acids conveniently used in the same way include A " -proline and 2,6-diaminohex-3-ynoic acid (for [ H]lysine) °. These residues usually give more nearly stoichiometric tritium incorporation upon reduction than does A" -leucine. The tritiation of A -amino acids (or peptides containing them) would require stereoselective reductions in order to obtain the desired (R)- or (5)-forms (as discussed later in this section). 

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