Conformation of phosphatidylcholine in neat and cholesterol-containing liquid-crystalline bilayers. Application of a novel method.

The conformation of phosphatidylcholine in liquid-crystalline bilayers was studied with a novel, high-resolution method employing phosphatidylcholine species containing pyrenyl moieties in both acyl chains of variable length. Analysis of the intramolecular pyrene-pyrene collision data obtained for 30 such species in terms of a simple geometrical model showed that the sn-1 acyl chain penetrates, on the average, 0.84 +/- 0.11 methylene units (0.8 A) deeper into the bilayer than the sn-2 chain at 22 degrees C. A similar value was obtained at 37 degrees C. Since the penetration difference of the sn-1 and sn-2 acyl chains is inherently coupled to the conformation of the glycerol moiety, these data mean that the glycerol moiety of phosphatidylcholine is, on the average, only moderately tilted with respect to the bilayer plane in the liquid-crystalline state. This contrasts the perpendicular orientation observed previously for phosphatidylcholine crystals [Pearson, R. H., & Pascher, I. (1979) Nature 281, 499-501]. Importantly, addition of 50 mol % cholesterol, which is known to reduce dramatically the interactions between phosphatidylcholine molecules in bilayers, had only a small effect on the penetration difference of the acyl chains, strongly suggesting that the conformation of phosphatidylcholine in the liquid-crystalline state is determined largely by intramolecular, rather than intermolecular, interactions.