Neutron diffraction studies on phosphatidylcholine model membranes. II. Chain conformation and segmental disorder.

Abstract In this paper neutron diffraction experiments on 1,2-dipalmitoyl- sn -glycero-3-phosphocholine selectively deuterated in the hydrocarbon chains are reported. The experiments were carried out in the gel phase L β′ and in the liquid crystalline phase L α . The labelled segments were assumed to have a Gaussian distribution in the projection on the bilayer normal and their mean positions were derived with an accuracy of ±1 angstrom unit from a fit to the observed structure factors. The values obtained in the L β′ phase confirm the model with chains in all trans configuration tilted with respect to the bilayer normal by an angle that increases with water content. From samples that were deuterated in both chains separately and studied at low water content it was seen that the chains of the molecule are out of step by as much as 1.5 carbon-carbon bond lengths. A constant width of the label distribution in the projection on the bilayer normal was observed for segments at the beginning and end of the chains. This is an additional indication for the chains being in the all trans state in L β′ phase. In the L α phase, the present experiments show that consecutive segments are well-separated in the profile. The whole chain region is shortened by a factor of ~0.75 compared to the L β′ phase. In contrast to the gel phase, the width of the label distribution is not constant over the entire region, but is found to be increased by more than a factor of two at the end of the chains. This complements the picture derived by deuterium magnetic resonance experiments, where order parameters and correlation times of segmental motions along the chains, which essentially determine the orientational disorder and angular fluctuations of the segments, were obtained.