Complexation of carboxyl groups in bacterial lipopolysaccharides: Interactions of H+, Mg2+, Ca2+, Cd2+, and UO22+ with Kdo and galacturonate molecules via quantum mechanical calculations and NMR spectroscopy

Quantum chemical calculations were combined with acid/base titrations and 13C NMR experiments to determine the structures, Gibbs free energy changes, vibrational frequencies and NMR chemical shifts characterizing the interactions of H+, Mg2+, Ca2+, Cd2+ and UO22+ with the carboxyl group of the 2-Keto-3-deoxyoctanoate (Kdo) and galacturonate molecules as models for reactive sites on bacterial surfaces. Monodentate, bidentate and outer-sphere complexation modes were considered. Results are also consistent with previous experimental and theoretical work on metal–carboxyl complexation, in particular with EXAFS studies. Among the four metals considered here, the theoretical results suggest that Mg2+ complexation is most thermodynamically favorable and UO22+ complexation is least favorable. On the other hand, our NMR experimental results suggested that Cd2+ complexation is the most favorable, whereas Mg2+ and Ca2+ complexation are weak processes, hindered by competition with H+. Apart from this discrepancy, there was a reasonable agreement between the theoretical and experimental results, which, taken together, also indicated that outer-sphere complexation was the dominant mode of complexation in our systems.