Lipid membranes interact with the surrounding water through their hydrophilic headgroups. This thin layer of water molecules at the membrane/water interface plays a crucial role in maintaining the structure and fluidity of the membrane. Yet, molecular level understanding of phospholipid membrane/water interfaces remain incomplete. One of the major challenges in probing the interfacial hydrogen bonding network of water using linear spectroscopic techniques such as infrared and Raman is the overwhelming contribution of bulk water to the signal. Vibrational sum frequency generation spectroscopy has proven to be an important technique to probe the hydrogen bonding network of interfacial water along with other molecular vibrations due to its inherent interface specificity. A Langmuir monolayer of zwitterionic phospholipids at the air/water interface was used as a model system to investigate factors regulating the sum frequency response of interfacial water molecules. Systematic variation of the types and densities of surface charges revealed that interfacial water structure follows the structural changes of the phospholipid headgroups in membranes. Measurement of surface pressure-area isotherms using monolayer compression experiments revealed a direct correlation between the interfacial hydrogen bonding network and the area per lipid headgroup in lipid monolayers. Our findings suggest that understanding the interfacial hydrogen bonding network of lipid membranes can be useful for applications involving model lipid membranes.

Probing water structure next to lipid monolayers using vibrational sum frequency spectroscopy

254th National Meeting (2017)

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