Ab initio conformational study of two Lewis X analogues

This paper presents the first ab initio conformational study for analogues of a histo-blood group carbohydrate antigen, Le(x) (Gal-beta-1,4-[Fuc-alpha-1,3]-GlcNAc). In these analogues, the GlcNAc group of Le(x) was replaced by a cyclohexanediol or an ethanediol group. The lowest energy conformers of these molecules were first found by the MM2*-SUMM conformational search technique. The molecular geometries and energies of lowest energy rotamers (within a 3 kcal/mol energy window) were further analyzed at the HF/6-31G(d) level of theory. This study provides a detailed description of the hydrogen-bonding properties of the low-energy conformers yielded by the MM2* and ab initio methods. The key torsion angles for Fuc-alpha-1,3-GlcNAc and Gal-beta-1,4-GlcNAc glycosidic bonds in Le(x) mostly keep their value in the different environments (solid, liquid, and gas phase). The ab initio torsion angles agree considerably better with the experimental results than the MM2* results. Another essential difference between the MM2* and ab initio results is that the latter provide better differentiation of the rotamers. Complexes with selectins introduced varying levels of distortion of Le(x), with the most tightly bound structure being most distorted. Nonstacked rotamers occur only once among the rotamers of 1,2-cyclohexanediol analogue, and that rotamer is not particularly stable (Delta E = 2.3 kcal/mol). However, such kind of rotamers are more frequent among the rotamers of 1,2-ethanediol analogue. This clearly shows that while the conformational space of 1,2-cyclohexanediol analogue is rather similar to that of Le(x), the conformational space of 1,2-ethanediol analogue is considerably less similar.