Effects of cholesterol on acyl chain dynamics in multilamellar vesicles of various phosphatidylcholines

Abstract

Phase modulation fluorescence spectroscopy was used to investigate the influence of cholesterol (0 to 50 mol%) on acyl chain dynamics in multilamellar vesicles of phosphatidylcholine. Four different phosphatidylcholines (DPPC, DOPC, POPC, and egg PC) and six different fluorescent probes (diphenylhexatriene and five anthroyloxy fatty acids) were employed. We found that: (1) Increased cholesterol content had only slight effects on fluorescence lifetimes of the six probes. (2) Increased cholesterol content increased the steady-state fluorescence anisotropy (r) of all the probes except 16-anthroyloxy palmitate (16-AP) in each of the four phosphatidylcholines. (3) Added cholesterol tended to limit the extent of probe rotation (as reflected by r ∞ , the infinite-time anisotropy) to a much greater extent than it altered the rate of probe rotation. (4) The tendency for cholesterol to order the structure of the bilayer was greatest in the proximal half of the acyl chains and diminished toward the center of the bilayer. (5) In some phosphatidylcholines the rotation rates of probes located near the bilayer center (diphenylhexatriene and 16-AP) were apparently increased by increasing levels of cholesterol. (6) In several respects dipalmitoylphosphatidylcholine (DPPC) vesicles responded differently to increased cholesterol than vesicles of the other three phosphatidylcholines. (7) A single second-order equation described the relationship between r ∞ and r for the five anthroyloxy fatty acid probes in the four different phosphatidylcholines over a wide range of cholesterol content. The data for diphenylhexatriene in the different phosphatidylcholines could not be fit by a single equation.