EFFECTS OF CHOLESTEROL ON MEMBRANE SURFACES AS STUDIED BY HIGH-PRESSURE FLUORESCENCE SPECTROSCOPY

We have studied the effects of cholesterol on membrane surfaces using fluorescence spectroscopy at high pressure. At atmospheric pressure, t he dissociation state of a pH-sensitive fluorophore (6-decanylnaphthol or DECNA) incorporated into several types of membranes showed an appa rent increase in dissociation with cholesterol content coming somewhat closer to its dissociation state in solution. Previous studies have s hown that when DECNA is free in solution, pressure induces proton diss ociation due to the volume decrease that occurs when water electrostri cts around the ions. But in phosphatidylcholine (PC) bilayers, proton dissociation is inhibited, either due to the inability of the surface to expand and allow for increased hydration, or other changes in lipid structure. The pressure behavior of DECNA in dioleoyl-PC, dioleoylpho sphatidic acid and dioleoylphosphatidylglycerol bilayers shows that in corporation of 5-10% cholesterol causes DECNA to behave like it is in a more unrestricted environment. This trend is reversed at higher chol esterol concentrations. These data, together with compressibility meas urements, support the model of Sankaram and Thompson [M. Sankaram, T.E . Thompson, Biochemistry 29 (1990) 10676.] whereby in the disordered p hase, cholesterol can span the two leaflets causing an increase in the area between the head groups; whereas in the ordered phase, no expans ion occurs. Thus, the effect of cholesterol on membrane surfaces depen ds on its phase diagram. (C) 1997 Elsevier Science B.V.