NMR and molecular modeling studies on two glycopeptides from the carbohydrate-protein linkage region of connective tissue proteoglycans

Complete 1H and 13C NMR assignments are reported for two glycopeptides representing the carbohydrate-protein linkage region of connective tissue proteoglycans. These glycopeptides are the octasaccharide hexapeptide, Ser(GlcpAbeta(1-->3) Galpbeta(1-->3)Galpbeta(1-->4)Xylpbeta)-Gly-Ser-Gly-Se r (GlcpAbeta(1-->3)Galpbeta(1-->3)Galpbeta(1-->4)Xylp beta)-Gly (1), and the tetrasaccharide dipeptide, Ser(GlcpAbeta(1-->3)Galpbeta(1-->3)Galpbeta(1-->4)X ylpbeta)-Gly (2). The vicinal coupling constant data show that the monosaccharide residues adopt4 C 1 chair conformations. Distance geometry/simulated annealing calculations using 2D NOESY derived distance constraints yielded a single family of structures for the tetrasaccharide moiety, with well defined interglycosidic linkage conformations. The straight phi torsion angles of the glycosidic C1'-O1 bonds showed a strict preference for the -sc range whereas the psi torsion angles (O1-Cn) exhibited dependence upon the interglycosidic linkage position (-ac for beta(1-->3) linkage, +ac for beta(1-->4) linkage). The predominant conformation about the glycopeptide bond is straight phi = -sc and psi = +ac. The presence of strong daN (i, i+1) NOE contacts, and the general absence of dNN (i, i+1) contacts (except for a weak Ser-5/Gly-6 dNN contact) and the dbN (i, i+1) contacts (except for Ser-1/Gly-2) in the ROESY spectrum, suggest that the backbone for 1 is predominantly in an extended conformation. A comparison of the ROESY data for 1 with those obtained from the unglycosylated hexapeptide (3) of the same sequence suggests that glycosylation has only a marginal influence on the backbone conformation of the hexapeptide.