13334 zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA J. Phys. Chem. 1995, 99, zyxwvu 13334-13351 Molecular Relaxation of Sucrose in Aqueous Solution: How a Nanosecond Molecular Dynamics Simulation Helps To Reconcile NMR Data Smen Balling Engelsen: Catherine Herd du Penhoat,$ and Serge PCrez*.$ Ingtnierie MolCculaire, Institut National de la Recherche Agronomique, BP1627, 44316 Nantes, France, and Laboratoire de Chimie de 1’Ecole Normale Suptrieure, 24 rue Lhomond, 75231 Paris, France Received: February 14, 1995; zyxwvutsr In Final Form: June 27, zyxwvu 1995@ The dynamical conformational behavior of sucrose in water was assessed through the combined use of molecular dynamics simulations and high-resolution Nh4R 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 and 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: 02-g. Ow 03-f. However, neither of the two crystallographic intramolecular hydrogen bonds (02-g. *HO- If and zyx 05-g. zyx * aHO-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 intemal 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 intemal 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. Introduction Sucrose has been a leading world commodity for centuries. Since the beginnings of organic chemistry, it has always been considered a prototype molecule for establishing and refining general methods for structural elucidation. Sucrose @-D- fructofuranosyl-a-D-glucopyranoside abbreviated as p-~-Fruf- (2- 1)-a-D-Gkp) is a nonreducing disaccharide; its molecular conformation in anhydrous single crystals has been established from neutron and X-ray diffraction studies,’-3 showing an almost spherical molecule stabilized by two intramolecular hydrogen bonds (0-lf-Ha- 0-2g and 0-6f-Ha.0-5g). Despite several investigations, the conformational behavior of sucrose in solution is still a matter of debate. In one of the first combined modeling and NMR studies: sucrose was described as being nearly spherical, similar to its shape in crystalline sucrose and quite rigid. It was postulated that this preferred conformation was largely independent of the solvent used. Its Institut National de la Recherche Agronomique. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 7 Laboratoire de Chimie de I’Ecole Normale SupCrieure. * Author for correspondence. @ Abstract published in Advance ACS Abstracts, August 15, 1995. 0022-365419512099-13334$09.00/0 inability to adjust to the exo-anomeric effect was explained by steric effects, and it was suggested that dilute aqueous sucrose has one intramolecular hydrogen bond. In an NMR investiga- tion of sucrose in DMSO, Christofides and Davie@ proposed the existence of two competing intramolecular hydrogen bonds (0-2g.*0-lf, and 0-2g.0-3f). However, it has been con- cluded that there is no evidence for persistent intramolecular hydrogen bonds based on the temperature coefficients of the NMR data for the hydroxyl protons of sucrose in a mixture of water and acetone.’ Another joint NOE and molecular modeling study indicated that sucrose in aqueous solution is flexible and argued against the importance of the intramolecular hydrogen bond between 0-2g and 0-lf, with respect to the molecular conformation.8 The results provided by other numerous ex- perimental studies9-’* do not provide a unified picture of the conformational behavior of sucrose in solution. Sucrose has been extensively studied by molecular modeling methods. Tran and Bradyi9.*0 used the CHARMm2’ program and a potential function for carbohydrates22 to investigate the sucrose energy surface through molecular statics and dynamics simulations. Subsequent conformational studies were conducted 0 1995 American Chemical Society