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

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.