The free amino group of gramicidin S

Synge (1945) has recently shown that gramicidin S is a polypeptide composed of equimolecular proportions of the five amino-acids, I-valine, I-proline, I-leucine, 1-ornithine and d-phenylalanine, and that each stoichiometric unit contains one free amino group. Belozersky & Paschina (1944a, b, 1945), on the other hand, maintain that both amino and carboxyl groups are present and that the 8-amino group of the ornithine residue is not free. Their evidence for this latter point, nevertheless, is not very convincing and the problem was clearly one which could be readily settled by applying the new dinitrofluorobenzene (DNFB) method Accordingly at Dr Synge's suggestion, the present author has made appropriate experiments, which show that the free amino group is undoubtedly the 8-amino group of the ornithine residue. The finding provides additional support for Synge's con- tention that gramicidin S is a cyclopeptide composed of the five amino-acids linked through their cx-amino and carboxyl groups. It is interesting to note that similar results for tyrocidin have recently been obtained by Christensen (1945) using a different technique.