Modulation of phospholipid acyl chain order by cholesterol. A solid-state 2H nuclear magnetic resonance study.
The effect of cholesterol on the acyl chain order of three glycerophosphocholines with 14, 16, and 18 carbons per acyl chain, namely, di(14:0)PC, di(16:0)PC, and di(18:0)PC, above the gel to liquid-crystalline phase transition temperature was investigated by using 2H nuclear magnetic resonance spectroscopy. Average acyl chain lengths were calculated from the segmental order parameters (Smol) for the sn-1 and the sn-2 chains in the absence of cholesterol and at 3:1, 2:1, and 1:1 mole ratios of phospholipid-cholesterol. The three binary mixtures of cholesterol with phosphatidylcholines are in the liquid-ordered (lo) phase. For all the three phosphatidylcholine-cholesterol systems, the distance from the carbonyl groups to the terminal methyl groups is shorter than the length of the cholesterol molecule. A molecular model for the lo phase consistent with these observations has in a statistical sense a part of each cholesterol molecule in one monolayer extending into the other monolayer. This results in a packing arrangement akin to that in interdigitated systems. On the basis of the effect of cholesterol on phospholipid acyl chain orientational order, it is suggested that the liquid-disordered (ld) phase at low cholesterol concentrations corresponds to a packing mode in which the cholesterol molecule spans the entire transbilayer hydrophobic region. A molecular mechanism is proposed in which increasing the concentration of cholesterol has the effect of stretching the acyl chains of phospholipids by increasing the population of trans conformers up to a stage where the hydrophobic length is considerably longer than the cholesterol molecule. Beyond this concentration, the partially interdigitated phase forms.(ABSTRACT TRUNCATED AT 250 WORDS)
Sankaram
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MB
Department of Biochemistry, University of Virginia Health Sciences Center, Charlottesville 22908.
Thompson
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TE
D000085:Acetates; D019342:Acetic Acid; D002784:Cholesterol / Q000502:physiology*; D003903:Deuterium; D004134:Dimyristoylphosphatidylcholine / Q000378:metabolism*; D008094:Lithium; D009682:Magnetic Resonance Spectroscopy; D008560:Membrane Fluidity; D008567:Membranes, Artificial; D008956:Models, Chemical; D010713:Phosphatidylcholines / Q000378:metabolism*; D013816:Thermodynamics
D000085:Acetates; D008567:Membranes, Artificial; D010713:Phosphatidylcholines; D002784:Cholesterol; D008094:Lithium; D003903:Deuterium; C010942:1,2-distearoyllecithin; D019342:Acetic Acid; D004134:Dimyristoylphosphatidylcholine