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(-).
Department of Chemistry, The Ohio State University, 100 West 18th Avenue, Columbus, Ohio 43210, United States.
Cheng Y
CY
Tang
Zishuai
Z
Huang
Heather C
HC
Allen
D002118:Calcium / Q000737:chemistry*; D002245:Carbon Dioxide / Q000737:chemistry*; D056831:Coordination Complexes / Q000737:chemistry; D006863:Hydrogen-Ion Concentration; D007477:Ions / Q000737:chemistry; D008274:Magnesium / Q000737:chemistry*; D019308:Palmitic Acid / Q000737:chemistry*; D011522:Protons; D012997:Solvents / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D014732:Vibration
D056831:Coordination Complexes; D007477:Ions; D011522:Protons; D012997:Solvents; D002245:Carbon Dioxide; C041069:carboxyl radical; D019308:Palmitic Acid; D008274:Magnesium; D002118:Calcium