COMBINED NMR, GRID SEARCH MM3 AND METROPOLIS MONTE-CARLO GEGOP STUDIES OF 2 L-FUCOSE CONTAINING DISACCHARIDES - ALPHA-L-FUC-(1,4)-BETA-D-GLCNAC-OME AND ALPHA-L-FUC-(1,6)-BETA-D-GLCNAC-OME

Complete proton NMR data provided a firm experimental basis to infer t he conformational properties of the Lewis type disaccharide alpha-L-Fu c-(1,4)-beta-D-GlcNAc-OMe 1 and the N-glycoprotein type disaccharide a lpha-L-Fuc-(1,6)-beta-D-GlcNAc-OMe 2 in aqueous solution. Relaxed pote ntial energy maps from systematic grid searches (GS) using the MM3 for ce field and Metropolis Monte Carlo (MMC) simulations employing the GE GOP force field were used to calculate corresponding ensemble average NMR data such as 1D transient NOE curves and vicinal coupling constant s J(H,H). For the disaccharide with a flexible (1,6) linkage (2), the relaxed potential energy surface based on the MM3 force field was calc ulated with three variable dihedral angles, phi, psi and omega. R-fact ors derived from a comparison of experimental and theoretical NOE data allowed an evaluation of the quality of the conformational models der ived from the calculations. Seven inter glycosidic 1D transient NOE cu rves were measured for each of both disaccharides, 1 and 2. Overall R- factors of approximately 17% for the 1-4 linked disaccharide and 20% f or the 1-6 linked disaccharide indicate a very satisfying agreement be tween theoretical calculations, both GS/MM3 and MMC/GEGOP. and experim ental NOE data, indicating that the gross conformational picture devel oped is realistic. On the other hand, a close inspection of individual NOE curves and vicinal coupling constants (3)J(HS,H6-pro-R) and (3)J( HS,H6-pro-S) with corresponding theoretical values revealed shortcomin gs of the computational methods applied. Firstly, the conformational e quilibrium around the C5-C6 bond of the GlcNAc unit in disaccharide 1 is not correctly described by the GS/MM3 method. This can lead to a fa lse interpretation of NOE data involving protons attached to C6 of Glc NAc. Secondly, relaxation of ring geometry (GS/MM3) was found to have a measurable improvement of intra glycosidic NOEs in both disaccharide s, 1 and 2. In summary, the conformational models derived for disaccha rides 1 and 2 represent a starting point for further analysis of poten tial conformational changes that may occur upon binding of these compo unds to specific receptor proteins such as lectins or antibodies. (C) 1997 Elsevier Science B.V.