title: Ionic binding of Na+ versus K+ to the carboxylic acid headgroup of palmitic acid monolayers studied by vibrational sum frequency generation spectroscopy.
abstract: Ionic binding of alkali ions Na(+) and K(+) to the carboxylic acid headgroups of fatty acid monolayers is studied as a proxy toward understanding the fundamental chemistry in cell biology. In this study, we used broad-bandwidth sum frequency generation (BBSFG) vibrational spectroscopy to investigate the ionic binding event that leads to deprotonation and complex formation of fatty acid headgroups. Palmitic acid (C(15)H(31)COOH) exists as a monolayer on aqueous surfaces. Surface vibrational stretch modes of palmitic acid from 1400 cm(-1) to 3700 cm(-1) were observed (nu(s)-COO(-), nu-C horizontal lineO, nu-C-H, nu-O-H of -COOH, free OH). Palmitic acid is mostly protonated at the aqueous surface at neutral pH (approximately 6). However, various degrees of deprotonation are initiated by the introduction of Na(+) and K(+) that results in the complexation of K(+):COO(-) and solvent separated Na(+):COO(-). Evidence in several spectral regions indicates that K(+) exhibits stronger ionic binding affinity to the carboxylate anion relative to Na(+).
authors.affiliation: The Ohio State University, Department of Chemistry, 100 West 18th Avenue, Columbus, Ohio 43210, USA.
authors.forename: Cheng Y
authors.initials: CY
authors.lastname: Tang
authors.forename: Heather C
authors.initials: HC
authors.lastname: Allen
mesh_terms: D001665:Binding Sites; D002264:Carboxylic Acids / Q000737:chemistry*; D002414:Cations, Monovalent / Q000737:chemistry; D008567:Membranes, Artificial; D008954:Models, Biological; D019308:Palmitic Acid / Q000737:chemistry*; D011188:Potassium / Q000737:chemistry*; D011522:Protons; D012964:Sodium / Q000737:chemistry*; D013059:Spectrum Analysis, Raman; D013499:Surface Properties; D013816:Thermodynamics; D014732:Vibration
chemical_list: D002264:Carboxylic Acids; D002414:Cations, Monovalent; D008567:Membranes, Artificial; D011522:Protons; D019308:Palmitic Acid; D012964:Sodium; D011188:Potassium