Role of ring size on the secondary structure and antibiotic activity of gramicidin S

In order to investigate the contribution of ring size to the secondary structure and antibiotic activity of gramicidin S, many analogs consisting of 6, 7, 8, 9, 11, 12, 13, and 14 amino acid residues were synthesized. In the analogs with smaller ring sizes than that of gramicidin S, only des-Val1-, des-Leu3- and des-Pro5-gramicidin S showed weak activity against the gram-positive micro-organisms tested. On the other hand, all analogs having larger rings, in which L- or D-Leu residues were inserted, were active. The activity of the analogs consisting of 10, 11 and 12 amino acid residues was stronger than those of the analogs with other ring sizes, and the activity of endo-Leu2a-gramicidin S against S. epidermidis SP-al-1 and B. subtilis ATCC 6633 was twice of that of gramicidin S.