Further evidence for the presence of a thiazoline ring in the isoleucylcysteine dipeptide intermediate in bacitracin biosynthesis

Isoleucylcysteine dipeptide, a first intermediate peptide in bacitracin biosynthesis, was liberated from the enzyme protein and oxidized with manganese dioxide in dimethylsulfoxide. The resulting oxidation product was identified by thin-layer chromatography as 2-(2-methyl-l-oxobutyl)-thiazole-4-carboxylic acid which has been isolated from the hydrolysate of bacitracin F. This result shows that the intermediate dipeptide contains a thiazoline ring, and that the thiazoline ring is synthesized at the dipeptide stage in the process of peptide chain elongation in bacitracin biosynthesis. Improbability of non-enzymatic dehydrative cyclization of the dipeptide is discussed.