Comparison of the proton (H+) and alkali metal ion (Li+, Na+ and K+) binding affinities of pyruvate and oxamate anions in the gas phase.: Quantum-chemical studies

Proton affinity (PA), lithium, sodium and potassium cation affinities (CA) in the gas phase were estimated by Gaussian-2 (G2), Moller-Plesset (MP2) methods and/or hybrid density functional theory calculations (B3LYP) for pyruvate and oxamate anions. Comparison of these affinities shows that the COO- groups of both anions have similar basicities. Differences between their calculated PAs as well as between their calculated CAs are not larger than 3 kcal mol(-1). Somewhat larger differences are evidenced for the formations of the alkali metal complexes with the cation located between the oxygen atoms of the C-alpha=O and COO- groups for which the lithium, sodium and potassium cation affinities are larger for oxamate than for pyruvate (by 4 +/- 0.5 kcal mol(-1) at the G2 level and by 6 +/- 0.5 kcal mol(-1) at the DFT(B3LYP)/6-311++G** level). This is a consequence of the chelate effect on the cation binding seen on the calculated most stable structures of the adducts.