Vibrational Sum-Frequency Studies of a Series of Phospholipid Monolayers and the Associated Water Structure at the Vapor/Water Interface

The orientation and conformation of phospholipid monolayers and the structure of water associated with these monolayers adsorbed at the vapor/water interface have been investigated using vibrational sum-frequency spectroscopy. The phospholipids studied are saturated diacyl phosphatidylcholines, phosphatidylethanolamines, phosphatidylglycerols, and phosphatidylserines with chain lengths of 14, 16, and 18 carbons. Spectra have been acquired under different polarization schemes for monolayers at the vapor/D 2 O interface to examine chain ordering and at the vapor/H 2 O interface to examine water structure. Comparisons are made between the monolayers based on chain length and headgroup differences. These studies show significant differences between the aqueous environments associated with the zwitterionic lipids and the charged lipids.