Organization of water molecules by adhering to oriented layers of dipalmitoylphosphatidyl serine in the presence of varying concentrations of cholesterol

About seven water molecules adhere to one molecule of dipalmitoylphosphatidyl serine (DPPS) in an oriented surface layer as inferred from the increase of the dichroic ratio R of their OH stretching vibration band (3400 cm(-1)) from 2 in the random bulk state to about 2.8 when adhering to DPPS. In DPPS-cholesterol mixtures the number of water molecules adhering to the phospholipid molecules and oriented by them increases as cholesterol content increases. This increase is very steep between molar fractions of cholesterol X(chol) = 0.2-0.4 and at X(chol) = 0.6 about 13 water molecules adhere and are oriented by one DPPS molecule. (C) 2000 Elsevier Science B.V. All rights reserved.