Binding of Mg(2+) and Ca(2+) to palmitic acid and deprotonation of the COOH headgroup studied by vibrational sum frequency generation spectroscopy.

At the air/liquid interface, cation binding specificity of alkaline earth cations, Mg(2+) and Ca(2+), with the biologically relevant ligand carboxylate (COO(-)) using vibrational sum frequency generation spectroscopy is reported. The empirical evidence strongly supports that the ionic binding strength is much stronger for Ca(2+) to COO(-) than that for Mg(2+). We conclude that at a near-neutral pH, the mechanism that governs Ca(2+) binding to COO(-) is accompanied by commensurate deprotonation of the carboxyl headgroup. In addition, surface molecular structure and ion concentration influence the cation binding behavior at the air/liquid interface. In a 0.1 M Ca(2+)(aq) solution, Ca(2+) initially favors forming ionic complexes in a 2:1 bridging configuration (2Ca(2+):1COO(-)) but 1:1 chelating bidentate complexes (1Ca(2+):1COO(-)) gradually emerge as secondary species as the system reaches equilibrium. As the Ca(2+) concentration rises to 0.3 M, the primary complexed species exists in the 2:1 bridging configuration. Unlike Ca(2+), Mg(2+) at 0.1 and 0.3 M favors a solvent-separated ionic complex with COO(-).