A structural study of 2-O-lauroylsucrose with molecular modeling and NMR methods

The preferred conformations of sucrose, 2-O-acetylsucrose and 2-O-lauroylsucrose have been explored with the MM3 force field. The main effect of 2-O-substitution has been shown to be a slight decrease in accessible Φ, Ψ-space. The CICADA program, a heuristic searching algorithm, was used to investigate the favored conformers of 2-O-lauroylsucrose. The global minimum (Φ, Ψ = 73°, −81°) has been located in the A-well which contains the crystal structure of sucrose in the corresponding energy map. As this potential energy well is 2.9 kcal/mol below the other low-energy regions it represents the only conformational family which is expected to contribute significantly to the time-averaged properties. The other major conformational change with respect to sucrose concerns the CH2-1f primary hydroxyl group which preferentially adopts the TG orientation instead of an equal distribution of rotamers in the case for sucrose. Restriction of the internal rotation of the CH2-1f exocyclic results in distinct coupling constants between the HO-1f proton and the H-1Rf and H-1Sf spins and well-separated multiplets for the latter methylene protons. From the 13C relaxation data, it appears that the reorientation of the sucrosyl moiety is slightly anisotropic. A motional model similar to the one recently proposed for sucrose (internal motion of considerable amplitude, Sang2 ∼ 0.7, occurring on the same timescale as overall tumbling) reproduced all the relaxation data. Finally, molecular modeling has shown that the favored position of the sidechain is an axial orientation with respect to the pyranose ring.