Hydration of phosphatidyl serine multilayers and its modulation by conformational change induced by correlated...

Hydration of the various residues of phospholipids was inferred from the shift in the wave number of their vibration bands, obtained from the amplitudes of their positive and negative peaks in the difference spectra between those of the hydrated and the dry phospholipid multibilayers. The effect of aligned phospholipid layers on the orientation of their hydrating water molecules was inferred from the dichroic ratio of the OH stretching band, measured by polarized attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR) with a germanium prism, as a function of the water-to-lipid ratio in the surface film. The results indicate that about seven water molecules are oriented by one phosphatidyl serine molecule in the surface film. About 8 to 11 additional water molecules contribute to the hydration of the polar residues as revealed by the effect on the difference spectra. The hydration appears to be cooperative. A water molecule that initiates hydration of a site facilitates access of additional water molecules, until the hydration of the whole site composed of many different interacting polar residues is completed.