Nuclear magnetic resonance and conformational energy calculations of repeat peptides of tropoelastin: Correlation of 1J(15N-1H) with nonplanarity of peptide moiety

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Conformational energy calculations and 1H and 13C nuclear magnetic resonance (NMR) studies in CDCI3 of a tripeptide, N-Ac-Gly1-L-Val2-Gly3-OMe, suggested the occurrence of an II-membered hydrogen-bonded ring, or γ-turn conformation. The theoretical calculations indicated that the conformation occurs when there is a distortion of the planarity of the peptide moiety. An 15N NMR study of the tripeptide in CDCI3 was undertaken to see if these spectral parameters might correlate with deviations from planarity. 1H, 13C, and 15N NMR studies were also carried out on the dipeptide N-Ac-L-Val1-Gly2-OMe, which argue for the occurrence of an 8-membered hydrogen-bonded ring, or C8 conformation and even greater distortions from peptide planarity. Therefore a theoretical investigation of this problem, using both the empirical and molecular orbital calculations in the CNDO/2 approximation, was undertaken, the results of which indicate large distortions of the planarity of the peptide moiety. While further work is required before the detailed functional dependence of NMR parameters to planarity of the peptide moiety can be established, a systematic variation of the 1J(15N-1H) coupling constant with peptide planarity is observed for the molecules discussed.