Motional properties of two vicinally disubstituted trisaccharides as studied by multiple-field carbon-13 NMR relaxation

Multiple-field carbon-13 NMR relaxation measurements have been performed on the two vicinally disubstituted trisaccharides beta-D-Glcp-(1 --> 2)[beta-D-Glcp-(1-->3)]-alpha-D-Glcp-OMe (1) and beta-D-Glcp-(1 --> 2)[beta-D-Glcp-(1 --> 3)]-alpha-D-Manp-OMe (2). Trisaccharide 1 has trans disubstitution of the terminal beta-glucosyl residues, while trisaccharide 2 has cis disubstitution. Carbon-13 T-1 and T-2 relaxation and NOE measurements were performed outside the extreme-narrowing regime at three temperatures 283, 303, and 323 K and four magnetic field strengths 6.3, 9.4, 11.7, and 14.1 T. The obtained relaxation rates and NOE factors, averaged for each sugar residue, were fitted to overall and internal correlation times and order parameters using the Lipari-Szabo "model-free" formalism. The 2,3-disubstituted O-methyl glycoside showed fairly high-order parameters, S-2 approximate to 0.9, for both trisaccharides at all three temperatures, while the terminal glucosyl residues of 1 and 2 showed lower and slightly temperature-dependent order parameters, S-2 approximate to 0.8. In 1, the terminal residues were found to have similar motional properties and an internal correlation time of similar to 60 ps could be obtained from a simultaneous three-parameter fit at 303 K. Similar results were obtained for trisaccharide 2. This study shows that the branch-point residue displays more restricted mobility than the terminal residues in both trisaccharides and that internal motions can be detected for small oligosaccharides using NMR relaxation measurements and the Lipari-Szabo model-free formalism.