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.