Structure and Dynamics of 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-Ser⁴[O-2-deoxy-d-lactopyranosyl-α-(1-3)]-Phe⁵-Phe⁶) (I) 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 βI- and βII-type turns, respectively, with Phe⁵ at i + 2. MD simulations with time-dependent distance constraints support the assumption of a βI/βII 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 β(1–4)- and α-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 β(1–4)-glycosidic bond, in agreement with literature data. The α-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.