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)