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(+).

Assembly and molecular order of two-dimensional peptoid nanosheets through the oil–water interface

“…A third peak appears in the spectrum of 1 near 1,440 cm −1 . Similar carboxylate peak frequencies have been observed in VSF spectroscopic studies of carboxylate-metal ion complexes, which were attributed to the disruption of carboxylate solvation shells due to the closeness of the cations (32,33). FTIR studies of simple carboxylic acids have additionally shown that the frequency of the peak attributed to the COO − antisymmetric stretch of a carboxylate-amine complex (34) is blue-shifted by ∼40 cm −1 relative to that of a fully water-solvated carboxylate group (35).…”

“…Interfacial tension measurements were obtained at the oil-water interface using the Wilhelmy plate method according to a previously described procedure (32). Briefly, a platinum plate attached to a balance (KSV) was lowered to the neat CCl 4 -H 2 O interface and the interfacial tension was measured to be 44-46 mN/m.…”

Revealing structural properties of the marine nanolayer from vibrational sum frequency generation spectra

“…For a typical fatty acid monolayer, however, as a result of the different protonation state of the carboxy group, more pronounced variations of the OH network feature have been reported. Qualitatively, a broad peak centered at 3560-3620 cm −1 becomes prominent, which has been attributed to weakly interacting OH groups between fatty acid and water molecules [Miranda et al, 1998;Johnson et al, 2005;Tang and Allen, 2009]. For seawater with pH values around 8, the carboxy group of short-chain fatty acids with pK a values around 4.8 is largely deprotonated, whereas long-chain fatty acids with pK a > 8.5 can be assumed to be largely protonated [Kanicky and Shah, 2002].…”

Palmitic Acid on Salt Subphases and in Mixed Monolayers of Cerebrosides: Application to Atmospheric Aerosol Chemistry

“…Several studies have shown that the replacement of a pure water subphase with inorganic salt solutions causes alterations in the phase behavior of the monolayer, especially for FAs and phospholipids [31][32][33][34][35][36][37]. The effect of Ca 2+ has been widely investigated due to its relevance in biological membranes [38][39][40][41].…”