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), Möller-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 alfaC=O and COO– groups for which the lithium, sodium and potassium cation affinities are larger for oxamate than for pyruvate (by 40.5 kcal mol–1 at theG2 level and by 60.5 kcal mol–1 at the DFT(B3LYP)/6-31++G** level). This is a consequence of the chelate effect on the cation binding seen on the calculatedmost stable structures of the adducts.