A conformational study of the trisaccharide beta-D-Glcp-(1 -> 2)[beta-D-Glcp-(1 -> 3)]alpha-D-Glcp-OMe by NMR NOESY and TROESY experiments, computer simulations, and X-ray crystal structure analysis

Proton-proton cross-relaxation rates have been measured for the trisacchari de beta -D-Glcp-(1 --> 2)[beta -D-Glcp-(1 --> 3)]alpha -D-Glcp-OMe in D2O a s well as in D2O/[D-6]DMSO 7:3 solution at 30 degreesC by means of one-dime nsional NMR pulsed field gradient H-1,H-1 NOESY and TROESY experiments. Int eratomic distances for the trisaccharide in D2O were calculated from the cr oss-relaxation rates for two intraresidue and three interglycosidic proton pairs, using the isolated spin-pair approximation. In the solvent mixture o ne intraresidue and three interglycosidic distances were derived without th e use of a specific molecular model. In this case the distances were calcul ated from the cross-relaxation rates in combination with "model-free" motio nal parameters previously derived from C-13 relaxation measurements. The pr oton-proton distances for interglycosidic pairs were compared with those av eraged from Metropolis Monte Carlo and Langevin Dynamics simulations with t he HSEA. PARM22. and CHEAT95 force fields. The crystal structure of the tri saccharide was solved by analysis of X-ray data. Interresidue proton pairs from the crystal structure and those observed by NMR experiments were simil ar. However, the corresponding proton-proton distances generated by compute r simulations were longer. For the (1 -->2) linkage the glycosidic torsion angles of the crystal structure were found in a region of conformational sp ace populated bq all three force fields, whereas for the (1 -->3) linkage t hey occupied a region of low population density, as seen from the simulatio ns.