Estimations of lipid bilayer geometry in fluid lamellar phases

The excess water bilayer thickness, d(l,0), and molecular area, A(0), of lipid amphiphiles in the fluid lamellar phases of dioleoylphosphatidylcholine (DOPC) and dipalmitoleoylphosphatidylcholine (DPolPC) have been estimated between 15 and 50 degrees C and for dimyristoylphosphatidylcholine (DMPC) between 25 and 50 degrees C. These determinations have been made from X-ray measurements on samples of known water composition. With respect to temperature, T, d(l,0) and A(0) are well fitted to a linear equation. We find d(l,0) (A)=(35.68+/-0.02)-(0.0333+/-0.0006)T (degrees C) and A(0) (A(2))=(70.97+/-0.05)+(0.136+/-0.001)T (degrees C) for DOPC, d(l,0) (A)=(35.2+/-0.1)-(0.068+/-0.003)T (degrees C) and A(0) (A(2))=(59.7+/-0.2)+(0.210+/-0.006)T (degrees C) for DMPC, and d(l,0) (A)=(34.54+/-0.03)-(0.0531+/-0.0009)T (degrees C) and A(0) (A(2))=(67.12+/-0.09)+(0.173+/-0.003)T (degrees C) for DPolPC. The accuracy of these estimates depends largely on how accurately the excess water point is determined. Ideally, reliable X-ray and compositional data will be available around the excess water and it may be found by simple inspection, but this is the exception rather than the rule, since samples close to water excess normally sequester sizeable amounts of water in defects, which lead to an underestimate of d(l,0). and overestimate of A(0). In this paper, we report a methodology for identifying and removing such data points and fitting the remaining data in order to determine the excess water point.