Effect of ionic strength on the transfer of 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate between bilayer vesicles containing phosphatidylserine.
The influence of ionic strength or the concentration of K+ ([K+]) of the aqueous phase on the spontaneous transfer of cholesterol between negatively charged bilayer vesicles composed of dipalmitoylphosphatidylcholine (DPPC) and dipalmitoylphosphatidylserine (DPPS) (1:1, mole:mole) was studied using a pyrene-labelled cholesterol analogue, 1-pyrenemethyl-3 beta-hydroxy-22,23-bisnor-5-cholenate (PMC), as the probe. The decrease in PMC excimer fluorescence was best fitted to a bi-exponential function. Increasing [K+] from 0.1 M to 0.3 M had little effect on the shorter half-time (1.4 +/- 0.2 min) but increased the longer half-time from 16.3 +/- 1.9 min to 26.7 +/- 2.1 min. Fluorescence quenching and titration of an interface-located fluorophore, 1-anilinonaphthalene-8-sulfonic acid (ANS) revealed an increase in interfacial hydrophobicity upon increasing in ionic strength. The physical state of the acyl chains was not affected by ionic strength as indicated by a constant PMC excimer:monomer fluorescence intensity ratio. However, an increase in enthalpy change of the lipid phase transition from 15.7 kJ/mol ([K+] = 0.1 M) to 21.3 kJ/mol ([K+] = 0.3 M), together with a slight increase in the transition temperature, implies that interactions between adjacent molecules in the charged lipid bilayer vesicles became stronger at higher ionic strength. Our results suggest that the van der Waals attraction between PMC and phospholipid molecules could be affected by conformation changes in the charged head group region brought about by changes of ionic strength in the aqueous phase, with consequent effects on the desorption of cholesterol from the bilayer surface.
Li
Q T
QT
Department of Biochemistry, Faculty of Medicine, National University of Singapore.
Sawyer
W H
WH
D015060:1,2-Dipalmitoylphosphatidylcholine / Q000737:chemistry; D000817:Anilino Naphthalenesulfonates; D002152:Calorimetry, Differential Scanning; D002784:Cholesterol / Q000031:analogs & derivatives* / Q000737:chemistry; D005456:Fluorescent Dyes; D008051:Lipid Bilayers / Q000737:chemistry*; D008560:Membrane Fluidity; D015335:Molecular Probes; D009994:Osmolar Concentration; D010718:Phosphatidylserines / Q000737:chemistry*; D011721:Pyrenes / Q000737:chemistry*; D013050:Spectrometry, Fluorescence
D000817:Anilino Naphthalenesulfonates; D005456:Fluorescent Dyes; D008051:Lipid Bilayers; D015335:Molecular Probes; D010718:Phosphatidylserines; D011721:Pyrenes; C079328:1-pyrenemethyl-3-hydroxy-22,23-bisnor-5-cholenate; D015060:1,2-Dipalmitoylphosphatidylcholine; C038694:dipalmitoylphosphatidylserine; D002784:Cholesterol