MOLECULAR RELAXATION OF SUCROSE IN AQUEOUS-SOLUTION - HOW A NANOSECOND MOLECULAR-DYNAMICS SIMULATION HELPS TO RECONCILE NMR DATA

The dynamical conformational behavior of sucrose in water was assessed through the combined use of molecular dynamics simulations and high-resolution NMR spectroscopy. Molecular dynamics simulations were performed in vacuum and in aqueous solution for 1 and 1.2 ns, respectively. Carbon relaxation data were established at 62.9 and 100.6 MHz; three-bond heteronuclear coupling constants were also determined. Two sets of phase-sensitive NOESY spectra were acquired. The presence of explicit water molecules in the simulation induces significant changes in the molecular potential. An important percentage of the glycosidic conformational space is populated, exemplifying the inherent conformational flexibility of sucrose. Hydration is inducing some conformational shifts, both in the glycosidic space acid in the conformational space of the five-membered ring. The sucrose molecule is found to be extensively hydrogen bonded to water molecules. All of the potential intramolecular hydrogen bonds are exchanged to surrounding water molecules; of particular interest is the observation of a 25% populated water bridging conformation: O2-g ... Ow ... O3-f. However, neither of the two crystallographic intramolecular hydrogen bonds (O2-g ... HO-1f and O5-g ... HO-6f) persists durably in aqueous solution. A strong damping effect on high frequency motions is observed, but root-mean-square fluctuations are larger than those of the vacuum simulations. The softening of the molecular potential allows the crystal conformation of the sucrosyl raffinose to appear in a highly populated area of the conformational space. The radius of gyration, overall molecular tumbling time, and self diffusion coefficient of the sucrose in aqueous solution were established from the molecular dynamics simulations; they compare extremely well with the corresponding experimental values. Equally satisfactory is the good agreement obtained with the glycosidic heteronuclear coupling constant. The molecular dynamics simulation shows that the high-frequency oscillations of sucrose are severely damped by the presence of explicit water and that internal motions occur on the same time scale as the overall tumbling. For such a motional regime the second term in the model-free spectral densities cannot be ignored. Theoretical carbon longitudinal relaxation were fitted to experimental ones with the molecular dynamics model by adjusting the correlation times for internal motions. This model is very different from that previously proposed for sucrose in which internal motions are considered to be extremely rapid. The motional model was shown to be very satisfactory for calculating the NOESY volumes. Thus, the MD simulations were able to distinguish between two otherwise equally good motional models based on NMR relaxation data. The selected model would appear to be a fairly universal motional model for small carbohydrate molecules consistent with both proton and carbon relaxation data.