Molecular Structure of t-Butyloxycarbonyl-Leu-Aib-Pro-Val-Aib-Methyl Ester, a Fragment of Alamethicin and Suzukacillin: A $3_{10}$-Helical Pentapeptide

The pentapeptide Boc-Leu-Aib-Pro-Val-Aib-OMe, a fragment of alamethicin and suzukacillin, crystallizes in the space group ${P2}_1$, with a = 11.034 (2), b = 10.894 (2) , c = 15.483 (2) A, \beta = 104.80 (2) deg and Z = 2. The crystal structure has been solved by direct methods and refined to an R value of 0.069. The peptide backbone folds into a right-handed $3_{10}$-helical conformation, stabilized by two intramolecular 4 \rightarrow 1 hydrogen bonds between the Leu(1) CO and Val(4) NH and Aib(2) CO and Aib(5) NH groups. The solid-state conformation is consistent with results of spectroscopic analysis in solution