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  • Title: Solvation of Calcium-Phosphate Headgroup Complexes at the DPPC/Aqueous Interface.
    Author: Hua W, Verreault D, Allen HC.
    Journal: Chemphyschem; 2015 Dec 21; 16(18):3910-5. PubMed ID: 26486616.
    Abstract:
    The effects of sodium (Na(+) ) and calcium (Ca(2+) ) cations on model zwitterionic dipalmitoylphosphatidylcholine (DPPC) monolayers spread on metal chloride salt solutions are investigated by means of vibrational sum frequency generation (VSFG) and heterodyne-detected (HD)-VSFG spectroscopy. VSFG and HD-VSFG spectra in the OH stretching region reveal cation-specific effects on the interfacial water's H-bonding network, knowledge of which has been limited to date. It is found that low-concentrated Ca(2+) more strongly perturbs interfacial water organization relative to highly concentrated Na(+) . At higher Ca(2+) concentrations, the water H-bonding network at the DPPC/CaCl2 interface reorganizes and the resulting spectrum closely follows that of the bare air/CaCl2 interface up to ∼3400 cm(-1) . Most interesting is the appearance of a negative band at ∼3450 cm(-1) in the DPPC/CaCl2 Im χs ((2)) spectra, likely arising from an asymmetric solvation of Ca(2+) -phosphate headgroup complexes. This gives rise to an electric field that orients the net OH transition moments of a subset of OH dipoles toward the bulk solution.
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