Binding of Mg²⁺ and Ca²⁺ to Palmitic Acid and Deprotonation of the COOH Headgroup Studied by Vibrational Sum Frequency Generation Spectroscopy

Abstract

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(-).

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).…”

Surface charge of acidic sophorolipid micelles: effect of base and time

“…28 In our specific case, in comparison to these studies, we make the hypothesis that the deprotonated SL-COOH micellar system experiences a strong Ca 2+ binding. In this case, the water content is expected to be reduced at the micellar outer interface, as actually found in all our fits and as represented in Figure 6d for Fit 2.…”

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

“…A measured volume of PA, cerebrosides or mixtures was spread dropwise on the subphase using a microsyringe (Hamilton) and 10 min was allowed for solvent evaporation. The barriers were then BAM images of the monolayers were collected simultaneously with π-A isotherms using a custom-built BAM similar to one previously described [40,56]. The laser source (Research Electro-Optics) emits 5 mW p-polarized light at 543 nm.…”

“…The effect of Ca 2+ has been widely investigated due to its relevance in biological membranes [38][39][40][41]. Imaging techniques such as Brewster angle microscopy (BAM) and fluorescence microscopy have shown that introduction of salts into the aqueous subphase results in different surface morphologies compared to pure water subphases [42,43].…”

“…Imaging techniques such as Brewster angle microscopy (BAM) and fluorescence microscopy have shown that introduction of salts into the aqueous subphase results in different surface morphologies compared to pure water subphases [42,43]. Furthermore, spectroscopic studies indicate that ions slightly influence the orientation of alkyl chains, but more so the hydration and dehydration of polar headgroups [40,41]. These properties of marine aerosols may be affected by the chemical composition pertaining to ions in the aqueous core.…”