Carbon-13-N.M.R.-spectra of antamanide, several analogues and their alkali ion complexes

Natural abundance carbon-13 Fourier transform n.m.r.-spectra were obtained of the cyclic decapeptides [Phe4Val6] antamanide (I); antamanide (II); [Tyr5] antamanide (III); [Ala1] antamanide (IV) and their ion complexes (I)-Na+, (II)-Na+, (II)-Li+, (III)-Na+ and (IV)-Na+. Based upon literature data, systematic comparisons and model compounds, a line assignment approach was performed for the majority of the aliphatic carbons. The spectra of the [Ala9Val6] analogues (V) (Bystrov et al., 1972) and II (measured in CD3CN Patel, 1973a, b) and their complexes were also assigned. Characteristic chemical shift variations observed upon complex formation were calculated (delta delta free peptide[Me+ complex) for a number of corresponding carbons, revealing shift changes up to 2.4 p.p.m. Preliminary calculations of the theta angle at selected prolines for some ion complexes are included.