Structural, dynamic and mechanical properties of POPC at low cholesterol concentration studied in pressure/temperature space.
We have studied the structural, dynamic and mechanical properties of 1-palmitoyl-2-oleoyl- sn-glycero-3-phosphatidylcholine (POPC)/cholesterol binary mixtures by small-angle X-ray scattering. Our investigations were concentrated on the biologically most relevant pressure-temperature-cholesterol regime, i.e. the liquid crystalline phase and its phase boundary to the lamellar gel phase within a cholesterol concentration up to 25 mol%. From the dependence of the transition pressure we derived a value of 19 kJ/mol for the transition enthalpy Delta H(m) of POPC in excess water. With increasing cholesterol concentration, Delta H(m) drops to about 7 kJ/mol at 20 mol% cholesterol. Time-resolved pressure-scan (p-scan) and temperature-jump (T-jump) experiments reveal that at low cholesterol content (<5-8 mol%) the fluidity and also the bilayer compressibility increase remarkably. In contrast, at concentrations between 5 and 25 mol% cholesterol the bilayer becomes again more rigid and the lipid bilayer spacing increases about 2 A. Theses changes are attributed to the onset of phase separation between liquid disordered and liquid ordered phases. The fluid-fluid miscibility gap for this mono-unsaturated lecithin species is strongly enlarged compared with saturated lecithins.
Rappolt
Michael
M
Institute of Biophysics and X-Ray Structure Research, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria.
Vidal
Mónica Fernández
MF
Kriechbaum
Manfred
M
Steinhart
Milos
M
Amenitsch
Heinz
H
Bernstorff
Sigrid
S
Laggner
Peter
P
D002784:Cholesterol / Q000737:chemistry*; D005782:Gels / Q000737:chemistry; D008051:Lipid Bilayers / Q000737:chemistry*; D008422:Materials Testing / Q000379:methods; D008968:Molecular Conformation; D010713:Phosphatidylcholines / Q000737:chemistry*; D011312:Pressure; D012996:Solutions / Q000737:chemistry; D013696:Temperature; D014961:X-Ray Diffraction / Q000379:methods*
D005782:Gels; D008051:Lipid Bilayers; D010713:Phosphatidylcholines; D012996:Solutions; D002784:Cholesterol; C028694:1-palmitoyl-2-oleoylphosphatidylcholine