INTERACTIONS OF CHOLESTEROL AND SYNTHETIC STEROLS WITH PHOSPHATIDYLCHOLINES AS DEDUCED FROM INFRARED CH2 WAGGING PROGRESSION INTENSITIES

The intensities of the coupled CH2 wagging vibrations in the IR spectr a of relatively ordered phases of phospholipids have been used to char acterize acyl chain conformations in a set of sterol/phosphatidylcholi ne (PC) mixtures. A series of sterols, with side chain lengths at C-17 varying from 0 to 10 carbon atoms, has been synthesized. The thermotr opic behavior of 1,2-dipalmitoylphosphatidylcholine (DPPC) with these sterols has been monitored with Fourier transform IR (FT-IR) spectrosc opy. As the sterol side chain lengthens, the all-trans conformation of the acyl chains in DPPC persists to progressively higher temperatures . In complementary experiments, cholesterol was mixed with a series of disaturated PCs varying in chain length from C-12 to C22. Remarkable variations in the thermotropic response of the CH2 wagging progression intensity of the PCs were noted at molar ratios of 2:1 (PC/cholestero l). For short-chain PCs (C-12 and C-14) the wagging progressions persi sted to 35-40-degrees-C above the main phase transition temperature (T (m)) for the pure phospholipid. In contrast, mixtures of longer chain PCs with cholesterol displayed progressions which vanished close to th eir respective T(m) values. These trends were unchanged by the presenc e of a single C=C bond in the acyl chains. A semiquantitative model is used to convert the measured intensity changes into the extent of gau che bond formation in the acyl chains. These results are discussed in terms of structural models for cholesterol/PC interaction. The current investigation represents the first systematic evaluation of conformat ional changes in model membranes in which both the sterol side chain l ength and the phospholipid acyl chain length have been varied.