Studies on the .BETA.-turn of peptides. VI. Nuclear magnetic resonance study on the conformations of N-(2,4-dinitrophenyl)tetrapeptide p-nitroanilides related to the .BETA.-turn part of gramicidin S.

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The conformations of N-(2,4-dinitrophenyl)-L-Leu–X–L-Pro–L-Val p-nitroanilides (I, X=D-Ala; II, X=Gly; III, X=L-Ala) were studied by nuclear magnetic resonance and circular dichroism spectroscopies. The L-Val-NH group of compound I takes part in an intramolecular hydrogen bonding, and the folded conformation is considered to be type II′ β-turn structure as in the case of [D-Ala4,4′]-gramicidin S. The populations of β-turn conformer were considered to be in the order of I>II>III≈0 in methanol and N,N-dimethylformamide solutions, and decreased in dimethyl sulfoxide solution. Compound II in dimethyl sulfoxide and III in all the three solvents do not take a β-turn conformation. The data obtained from NMR and CD experiments showed a good correlation between the magnitudes of the Cotton effects (250–400 nm) and the populations of the β-turn conformation of these peptides. The proximity of the two chromophores was ascertained directly for compound I in N,N-dimethylformamide solution by nuclear Overhauser effects experiment. In addition, the β-turn preference of these peptides were in good accordance with the antibiotic activities of the gramicidin S analogs having the same tetrapeptide sequences at their β-turn parts.
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