Differential Dynamics at Glycosidic Linkages of an Oligosaccharide as Revealed by 13C NMR Spin Relaxation and Stochastic Modeling.

Among biomolecules, carbohydrates are unique in that not only can linkages be formed through different positions, but the structures may also be branched. The trisaccharide β-d-Glcp-(1→3)[β-d-Glcp-(1→2)]-α-d-Manp-OMe represents a model of a branched vicinally disubstituted structure. A 13C site-specific isotopologue, with labeling in each of the two terminal glucosyl residues, enabled the acquisition of high-quality 13C NMR relaxation parameters, T1 and T2, and heteronuclear NOE, with standard deviations of ≤0.5%. For interpretation of the experimental NMR data, a diffusive chain model was used, in which the dynamics of the glycosidic linkages is coupled to the global reorientation motion of the trisaccharide. Brownian dynamics simulations relying on the potential of mean force at the glycosidic linkages were employed to evaluate spectral densities of the spin probes. Calculated NMR relaxation parameters showed a very good agreement with experimental data, deviating <3%. The resulting dynamics are described by correlation times of 196 and 174 ps for the β-(1→2)- and β-(1→3)-linked glucosyl residues, respectively, i.e., different and linkage dependent. Notably, the devised computational protocol was performed without any fitting of parameters.