Solvation of Calcium-Phosphate Headgroup Complexes at the DPPC/Aqueous Interface

Area of Science:

• Physical Chemistry
• Surface Science
• Biophysical Chemistry

Background:

• Understanding ion-lipid interactions is crucial for cell membrane function.
• The influence of specific cations on interfacial water structure at lipid monolayers remains incompletely understood.

Purpose of the Study:

• To investigate the distinct effects of sodium (Na+) and calcium (Ca2+) cations on the hydrogen-bonding network of water at dipalmitoylphosphatidylcholine (DPPC) monolayer interfaces.
• To elucidate cation-specific perturbations of interfacial water organization using advanced spectroscopic techniques.

Main Methods:

• Utilized vibrational sum frequency generation (VSFG) and heterodyne-detected (HD)-VSFG spectroscopy.
• Analyzed spectra in the OH stretching region to probe interfacial water structure.
• Studied model zwitterionic DPPC monolayers spread on metal chloride salt solutions.

Main Results:

• Observed cation-specific alterations in the interfacial water H-bonding network.
• Found that low concentrations of Ca(2+) more strongly perturb water organization compared to high concentrations of Na(+).
• Identified a reorganization of the water H-bonding network at higher Ca(2+) concentrations, with a unique negative band in DPPC/CaCl2 spectra indicating asymmetric solvation and electric field generation.

Conclusions:

• Cations exert distinct influences on interfacial water structure at DPPC monolayers.
• Calcium ions induce significant structural changes in interfacial water, including the formation of Ca(2+)-phosphate complexes and orientation of water dipoles.
• The findings provide new insights into ion-surface interactions at biological interfaces.