SIDE-CHAIN CONFORMATION AND DYNAMICS IN A SOLID PEPTIDE - CP-MAS NMR-STUDY OF VALINE ROTAMERS AND METHYL-GROUP RELAXATION IN FULLY C-13-LABELED ANTAMANIDE

The effect of the crystal lattice on the side-chain conformation and s ide-chain dynamics in peptides is investigated by solid-state NMR, usi ng the cyclic decapeptide antamanide as an example. The study takes ad vantage of the C-13 assignment of the backbone and side chains based o n the resolution-enhanced 2D spin-diffusion spectra by heteronuclear a nd homonuclear decoupling. The spectra even allow for a stereospecific assignment of the gamma-carbons of the valine residue. It is found th at the valine side chain coexists in two static rotamer conformations which have not been observed by X-ray crystallography. In addition, re markable effects of the crystal packing on the methyl-group rotation f requency are found from C-13 relaxation measurements.