Studies in membrane processes X: A deuterium magnetic resonance study of dipalmitoyl lecithin and palmitic acid guests in magnetically-oriented hexadecyltrimethyl-ammonium bromide liquid crystalline system

Abstract

The membrane phospholipid, dipalmitoyl lecithin, deuterium labelled in its fatty acyl chains, and palmitic acid-d 31 have been incorporated into the same bilayer model membrane, a lyotropic liquid crystalline hexadecyltrimethylammonium bromide mesophase which spontaneously orients in an applied magnetic field. The order parameter profiles for the lecithin and palmitic acid, and that of the host detergent are quite different indicating that the ordering of the incorporated lipids is not dictated by surrounding detergent molecules, but rather the order imposed is a function of the nature of chemical anchoring of the hydrophilic headgroups of the individual molecules at the bilayer interface. Dissimilarities in the order profiles are interpreted in terms of variations in the case of formation of random gauche conformers along the length of the acyl chains. In addition, the validity of the use of perdeuterated fatty acids as probes of the order of other membrane components is questioned. For the host detergent the relaxation rate ‘ 1 T′ 2 obtained from the line width is directly proportional to the deuterium quadrupole splitting of the —CD 2 — segments for that flexible part of the chain beyond the plateau region of constant degree of order. This indicates that for an aligned chain, the increasing motional freedom for each successive segment toward the chain end, which is linked with the increasing probability of single gauche rotations, is the motion responsible for both the decrease in degree of order and thus the increase in line width.