Conformational Dependence of One‐Bond C α ,H Spin Coupling in Cyclic Peptides

Conformational effects on the one‐bond C α , H coupling constant have been studied in cyclic oligopeptides containing glycine, alanine, sarcosine and proline. The conformational contribution to 1 J (C α , H) can be described as being composed of a positive hyperconjugative term from the neighbouring N‐ p z orbital and a negative one from the carbonyl π‐system. For glycyl units, a quantitative relation between 1 J (C α ,H) and the vicinal interorbital angles Φ′ and Ψ′ is derived with maximum values Δ J (Ψ′) = + 14.0 Hz and Δ J (Ψ′) = −4.9 Hz. Applications to conformational analysis of cyclo ( L ‐Pro‐Gly) 3 , cyclo ( D ‐Ala‐ D ‐Ala‐ L ‐Ala‐ D ‐Ala‐ L ‐Ala‐ L ‐Ala) and cyclo (Sar) 8 are presented.