BINDING-PROPERTIES OF CARBOHYDRATE O-SULFATE ESTERS BASED ON AB-INITIO 6-31-ASTERISK-ASTERISK CALCULATIONS ON METHYL AND ETHYL SULFATE ANIONS(G)

Ab initio 6-31+G* calculations on methyl and ethyl sulfate anions and four different water complexes with methyl sulfate are presented. Top ological analysis of these structures demonstrates that all the termin al S=O bonds are highly polarized covalent double bonds, while the CO- S bond is a single covalent bond. As well, the cyclic nature of three bifurcated structures suggest hydrogen bonding motifs for protein-carb ohydrate complexes. Systematic variations of bond angles, bond lengths , and torsion angles allowed for the parameterization of the MM2 force field. It was necessary to introduce the four atom types: sulfate (S) , two terminal sulfate oxygens (O(a) and O(g)), and the oxygen attache d to both carbon and sulfur (O(s)). With these parameters satisfactory agreement between calculated and experimental values for a series of solid-state structures was found. Analysis of these calculations sugge sts two distinct modes of binding for O-sulfate groups namely, a nonsp ecific type that does not involve ''freezing'' out of the O-S bond rot ations and specific hydrogen bonded structures that do ''freeze'' out the motions. Finally some probable conformations of a O-sulfated tetra saccharide NOD factor for Rhizobium meliloti are presented.