Phase transitions in phosphatidylcholine multibilayers

Abstract

The 2H NMR spectrum of a multilamellar dispersion of 1-myristoyl-2-[14,14,14-2H3]myristoyl-sn-glycero-3-phosphocholine with 1 mol % cholesterol in excess water was recorded at temperatures between -15.degree. C and 36.degree. C. Motionally averaged quadrupole coupling constants .ltbbrac..nu.Q.rtbbrac. and motionally induced asymmetry parameters .eta. are obtained by spectral analysis. Values of these quantities indicate that, at temperatures below -4.degree. C, any rotational motion of the molecules about their molecular long axis is slow on the NMR time scale. At temperatures immediately above the pretransition these same parameters show that a fast-rotational motion is occurring about the molecular long axis. This rotational motion is hindered in that the molecules flip about a 2-fold symmetry axis. Between -4.degree. C and the pretransition, spectral appear as the superposition of 2 powder patterns, one corresponding to the pattern observed below -4.degree. C and the other to the pattern above the pretransition. The relative contribution of the latter increases with temperature until the pretransition is reached. Either the sample between -4.degree. C and the pretransition contains 2 populations of rapidly and slowly rotating molecules, or there is only a single population of molecules undergoing a 180.degree. flipping motion on the time scale of the NMR measurement. The latter interpretation is more consistent with other experimental findings. At the temperature of the main transition the hydrocarbon chains melt. In the absence of cholesterol, spectra are more complex in that the line shape is reproduced by the superposition of 3 spectral powder patterns between -4.degree. C and the pretransition, and by the superposition of 2 spectral patterns above the pretransition. These 2 patterns observed above the pretransition may be in direct correspondence to the 2 ripple structures observed by freeze-fracture EM in the absence of cholesterol.