STRUCTURE AND DYNAMICS OF A SYNTHETIC O-GLYCOSYLATED CYCLOPEPTIDE IN SOLUTION DETERMINED BY NMR-SPECTROSCOPY AND MD CALCULATIONS

Conformational analysis by NMR spectroscopy and restrained molecular dynamics (MD) of the O-glycosylated cyclic hexapeptide cyclo(D-Pro1-Phe2-Ala3-Ser[O-2-deoxy-D-lactopyranosyl-alpha-(1-3)]-Phe5-Phe6) (1) and the cyclic hexapeptide precursor cyclo(D-Pro1-Phe2-Ala3-Ser4-Phe5-Phe6) (II) is described. For II, an X-ray structure was obtained and compared with the structure in solution. For both compounds, the distance constraints derived from 2D NMR measurements could not be completely satisfied by a single conformation, but distance violations occurred only in the Phe5 region of the peptide. The specific pattern of NOE-derived distances in this part of the molecule suggested an equilibrium between two conformers containing beta-I- and beta-II-type turns, respectively, with Phe5 at i + 2. MD simulations with time-dependent distance constraints support the assumption of a beta-I/beta-II flip in I and II. The conformations were refined using restrained MD simulations in vacuo, in water, and in DMSO. To study the exoanomeric effect of beta(1-4)- and alpha-glycosidic linkages on conformation, new force field parameters derived from literature data were incorporated, leading to greater flexibility and significantly populated alternate conformers around the beta(1-4)-glycosidic bond, in agreement with literature data. The alpha-glycosidic linkage connecting the disaccharide moiety to the peptide prefers only one conformation. Both I and II have similar backbone conformations and hydrogen-bonding patterns. Therefore, the O-glycosylation does not affect the conformation or the overall shape of the peptide backbone or side chains.