CYCLIC PEPTIDES .15. LANTHANIDE-ASSISTED C-13 AND H-1 NMR ANALYSIS OF PREFERRED SIDE-CHAIN ROTAMERS IN PROLINE-CONTAINING CYCLIC DIPEPTIDES

For a series of cyclic dipeptides containing proline, 1H and 13C NMR spectra were obtained in the presence of varying amounts of Yb(fod)3 and Eu(fod)3 [NMR shift agents]. The resulting chemical shift and coupling constant data were used to ascertain the preferred rotamer for the nonprolyl side chain. To determine whether more than 1 rotamer is populated, a rotamer averaging method was applied. The data, taken in conjunction with minimum energy calculations, imply that aliphatic side chains adopt rotamers determined primarily by steric considerations; aromatic side chains have additional stabilizing interactions.