A 2H-NMR analysis of dihydrosterculoyl-containing lipids in model membranes: structural effects of a cyclopropane ring

The molecular properties of lipid multilayers of 1-palmitoyl-2-[dideutero]dihydrosterculoyl-sn-glycero-3-phosphocholine (PDSPC) were investigated by means of 2H-NMR. The transition from the liquid-crystalline phase to a more highly ordered phase was found to take place between -10 degrees C and -15 degrees C. The temperature variation of the quadrupolar splittings of specifically dideuterated PDSPC molecules was analyzed in terms of 'segmental' and 'geometrical' order parameters: the lower half of the sn-2 chain (from the site of the cyclopropane ring to the terminal methyl group) was more conformationally disordered than the upper half. The apparently abnormal increase of the quadrupolar splitting of the pro-S deuteron at the C-2' position, with increasing temperature, was attributed to a change in the average orientation of that C-2H bond with respect to the axis of motion, resulting in an increase of the 'geometrical' order parameter, S gamma. The molecular order parameter matrix elements, Sij, of the cyclopropane ring were derived from the experimental SC-2H order parameters using similarity transformations. The matrix S was diagonalized and the molecular order parameter of the cyclopropane ring, Smol (or S*33), was determined by assuming axial symmetry for such matrices associated with molecules in a liquid-crystal medium. A value of Smol = 0.59 +/- 0.04 at 25 degrees C was thus calculated. This value represents a discontinuity in the positional dependence of the molecular order parameter for the sn-2 chain of PDSPC, indicating that the cyclopropane ring provides a rigid barrier separating the lipid bilayer into two regions: an ordered region from the bilayer surface to the site of the cyclopropane ring and a much more disordered region thereafter to the center of the bilayer.