Stereochemistry of protected ornithine side chains of gramicidin S derivatives: X-ray crystal structure of the bis-Boc-tetra-N-methyl derivative of gramicidin S.
An X-ray crystallographic analysis of the bis-Ndelta-Boc-tetra-Nalpha-methyl derivative of gramicidin S, cyclo(-Val-MeOrn(Boc)-Leu-d-MePhe-Pro-)2, was undertaken successfully (R-factor = 0.088). As expected, the main chain adopts an antiparallel pleated beta-sheet conformation, but the pleated sheet is slightly twisted, and the sense of twisting is opposite to that found in the reported crystal structures of the gramicidin S-urea complex and the bis-Ndelta-(trichloroacetyl) and bis-Ndelta-(m-bromobenzoyl) derivatives of gramicidin S. In agreement with the observed resistance toward N-methylation, the urethane NH groups of the protected Orn side chains are hydrogen bonded to the carbonyl groups of the d-Phe residues. However, the side-chain-main-chain hydrogen bonding is in the i --> i - 3 mode, although hydrogen bonding in the i --> i + 2 mode was deduced from a 1H NMR study of protected gramicidin S derivatives and was actually found in the crystal structures of the diacylated gramicidin S.
Yamada
Keiichi
K
Department of Chemistry, Gunma University, 1-5-1, Tenjin-cho, Kiryu, Japan.
Unno
Masafumi
M
Kobayashi
Kyoko
K
Oku
Hiroyuki
H
Yamamura
Hatsuo
H
Araki
Shuki
S
Matsumoto
Hideyuki
H
Katakai
Ryoichi
R
Kawai
Masao
M
D000900:Anti-Bacterial Agents / Q000737:chemistry*; D018360:Crystallography, X-Ray; D005563:Formic Acid Esters / Q000737:chemistry; D006096:Gramicidin / Q000031:analogs & derivatives* / Q000737:chemistry; D006860:Hydrogen Bonding; D009952:Ornithine / Q000737:chemistry*; D011487:Protein Conformation
D000900:Anti-Bacterial Agents; D005563:Formic Acid Esters; C000345:t-butyloxycarbonyl group; D006096:Gramicidin; D009952:Ornithine