A STRUCTURAL STUDY OF 2-O-LAUROYLSUCROSE WITH MOLECULAR MODELING AND NMR METHODS

The preferred conformations of sucrose, 2-O-acetylsucrose and 2-O-laur oylsucrose have been explored with the MM3 force field. The main effec t of 2-O-substitution has been shown to be a slight decrease in access ible Phi, Psi-space. The CICADA program, a heuristic searching algorit hm, was used to investigate the favored conformers of 2-O-lauroylsucro se. The global minimum (Phi, Psi = 73 degrees, -81 degrees) has been l ocated in the A-well which contains the crystal structure of sucrose i n the corresponding energy map. As this potential energy well is 2.9 k cal/mol below the other low-energy regions it represents the only conf ormational 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 pr eferentially adopts the TG orientation instead of an equal distributio n of rotamers in the case for sucrose. Restriction of the internal rot ation of the CH2-1f exocyclic results in distinct coupling constants b etween the HO-1f proton and the H-1Rf and H-1Sf spins and well-separat ed multiplets for the latter methylene protons. From the C-13 relaxati on data, it appears that the reorientation of the sucrosyl moiety is s lightly anisotropic. A motional model similar to the one recently prop osed for sucrose (internal motion of considerable amplitude, S-ang(2) similar to 0.7, occurring on the same timescale as overall tumbling) r eproduced all the relaxation data. Finally, molecular modeling has sho wn that the favored position of the sidechain is an axial orientation with respect to the pyranose ring. (C) 1998 Elsevier Science Ltd.