Differential effects of sterols on phospholipid acyl chain packing in binary and ternary mixes

We examined the effects of the cholesterol, ergosterol and cholestane on phospholipid acyl chain packing in 16:0,18:1 PC and ternary mixtures where di-18:0 PC/di-18:1 PC/cholesterol forms coexisting liquid ordered (Lo) and liquid disordered (Ld) phases. Ensemble acyl chain order was assessed via time-resolved fluorescence measurements of DPH. Analysis in terms of the P2-P4 Model enabled separation of probe motion and orientational order. In 16:0,18:1 PC ergosterol and cholesterol increased probe order to a similar extent. Cholestane had essentially no effect on probe order and a small effect on probe motion. Effects of the 3 sterols were also examined in a ternary mix of di-18:1 PC/di-18:0 PC/sterol selected from the middle of the Lo/Ld coexistence region of the di:18:1/di-18:0/chol phase diagram at 23C. In the ternary mix the effects of cholesterol and ergosterol on ensemble acyl chain order were distinguishable, with ergosterol having a smaller effect. In this mix cholestane had a pronounced ordering effect and reduced the rate of probe motion. The results suggest that sterol structure plays a more significant role in altering acyl chain interactions in mixtures with the potential to form coexisting Lo/Ld domains than in single component bilayers.