Water State and Carbonyl Distribution Populations in Confined Regions of Lipid Bilayers Observed by FTIR Spectroscopy

It has been suggested that water in confined regions presents different properties than bulk water, mainly because of the changes in water population species that may be induced by the adjacent walls of different polarities in terms of hydrogen bond formation. In this context, it would be expected that lipids in the gel and the fluid states should offer different templates for water organization. The presence of water pockets or defects in lipid bilayers has been proposed to explain the insertion of charged and polar peptides and amino acids in membranes. In this work, we provide direct evidence by means of FTIR spectroscopy that water band profiles are changed whether lipids are in the solid state, in the gel state after heating and cooling across the phase transition, or in the fluid state. The different bands found in each case were assigned to different H-bonded water populations in agreement with the exposure of carbonyl groups.