Estimations of lipid bilayer geometry in fluid lamellar phases

The excess water bilayer thickness, d(1.0), and molecular area, A(0), of li pid amphiphiles in the fluid lamellar phases of dioleoylphosphatidylcholine (DOPC) and dipaltnitoleoylphosphatidylcholine (DPolPC) have been estimated between 15 and 50 degrees C and for dimyristoylphosphatidylcholine (DMPC) between 35 and 50 degrees C. These determinations have been made from X-ray measurements on samples of known water composition. With respect to temper ature, T, d(1,0) and A(0) are well fitted to a linear equation. We find d(1 ,0) (Angstrom) = (35.68 +/- 0.02)-(0.0333 +/- 0.0006)T(degrees C) and A(0) (Angstrom(2)) (70.97 +/- 0.05)+(0.135 +/- 0.001)T (degrees C) for DOPC, d(1 ,0) (Angstrom)= (35.2 +/- 0.1)-(0.068 +/- 0.003)T (degrees C) and A(0) (Ang strom(2)) (59.7 +/- 0.2)+(0.210 +/- 0.006)T (degrees C) for DMPC, and d(1,0 ) (Angstrom) = (34.54 +/- 0.03)-(0.0531 +/- 0.0009)T (degrees C) and A(0) ( Angstrom(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 exces s 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 in spection, but this is the exception rather than the rule, since samples clo se to water excess normally sequester sizeable amounts of water in defects, which lead to an underestimate of d(1,0) and overestimate of A(0). In this paper, we report a methodology for identifying and removing such data poin ts and fitting the remaining data in order to determine the excess water po int. (C) 2000 Elsevier Science B.V. All rights reserved.