Cholesterol in condensed and fluid phosphatidylcholine monolayers studied by epifluorescence microscopy.
Epifluorescence microscopy was used to investigate the effect of cholesterol on monolayers of dipalmitoylphosphatidylcholine (DPPC) and 1 -palmitoyl-2-oleoyl phosphatidylcholine (POPC) at 21 +/- 2 degrees C using 1 mol% 1-palmitoyl-2-[12-[(7-nitro-2-1, 3-benzoxadizole-4-yl)amino]dodecanoyl]phosphatidylcholine (NBD-PC) as a fluorophore. Up to 30 mol% cholesterol in DPPC monolayers decreased the amounts of probe-excluded liquid-condensed (LC) phase at all surface pressures (pi), but did not effect the monolayers of POPC, which remained in the liquid-expanded (LE) phase at all pi. At low pi (2-5 mN/m), 10 mol% or more cholesterol in DPPC induced a lateral phase separation into dark probe-excluded and light probe-rich regions. In POPC monolayers, phase separation was observed at low pi when > or =40 mol% or more cholesterol was present. The lateral phase separation observed with increased cholesterol concentrations in these lipid monolayers may be a result of the segregation of cholesterol-rich domains in ordered fluid phases that preferentially exclude the fluorescent probe. With increasing pi, monolayers could be transformed from a heterogeneous dark and light appearance into a homogeneous fluorescent phase, in a manner that was dependent on pi and cholesterol content. The packing density of the acyl chains may be a determinant in the interaction of cholesterol with phosphatidylcholine (PC), because the transformations in monolayer surface texture were observed in phospholipid (PL)/sterol mixtures having similar molecular areas. At high pi (41 mN/m), elongated crystal-like structures were observed in monolayers containing 80-100 mol% cholesterol, and these structures grew in size when the monolayers were compressed after collapse. This observation could be associated with the segregation and crystallization of cholesterol after monolayer collapse.
Worthman
L A
LA
Department of Biochemistry, Memorial University of Newfoundland, St. John's, Canada.
Nag
K
K
Davis
P J
PJ
Keough
K M
KM
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D009327:4-Chloro-7-nitrobenzofurazan / Q000031:analogs & derivatives; D000818:Animals; D055592:Biophysical Phenomena; D001703:Biophysics; D002784:Cholesterol / Q000737:chemistry*; D005456:Fluorescent Dyes; D066298:In Vitro Techniques; D008563:Membrane Lipids / Q000737:chemistry*; D008856:Microscopy, Fluorescence / Q000379:methods*; D010713:Phosphatidylcholines / Q000737:chemistry*; D011312:Pressure; D013499:Surface Properties
C078632:1-acyl-2-(12-((7-nitrobenz-2-oxa-1,3-diazol-4-yl)amino)dodecanoyl)phosphatidylcholine; D005456:Fluorescent Dyes; D008563:Membrane Lipids; D010713:Phosphatidylcholines; D015060:1,2-Dipalmitoylphosphatidylcholine; D002784:Cholesterol; D009327:4-Chloro-7-nitrobenzofurazan; C028694:1-palmitoyl-2-oleoylphosphatidylcholine