Bruce J. Forrest

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-d31 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 —CD2 — 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.

The Aqueous Lyotropic Analogues of Thermotropic Nematics and Cholesterics

Abstract The motional non-equivalence in a deuteriated -CD2-segment of a hydrocarbon chain near a chiral carbon atom has been investigated in a series of nematic and cholesteric phases. The deuterium nuclear magnetic resonance signals show the number of non-equivalent motions as equal to the number of different quadrupole doublet splittings. It is shown that a necessary but not sufficient condition for motional non-equivalence is the presence of a chiral carbon. The chiral carbon is not sufficient because in some cases the two quadrupole doublets may accidentally overlap. The composition of a number of nematic and cholesteric meso-phases has been located and recorded.The non-equivalence is affected to a large extent by the protonation of the chiral head group, showing that head group anchoring is changed in this chemical modification.

Conformational Studies of a Carboxylic Acid Incorporated into Lyotropic Liquid Crystalline Systems. A Deuterium Nuclear Magnetic Resonance Investigation

Abstract Butanoic acid-d7 and its corresponding anion have been incorporated into two Type II disc micelle lyotropic liquid crystalline systems based on hexadecyltrimethylammonium bromide, and on sodium decylsulfate. The order parameter profiles for the perdeuteriated guests have been determined. Calculations based on these order profiles have been performed in order to describe the conformation of the guest carboxylic acid present in the bilayer micelle. These calculations indicate that two conformations predominate, one an all trans arrangement (63%), and the second possessing a single gauche rotation about the α-β C—C bond (37%). The model used was based on two possible methods of anchoring the polar group at the aqueous interface.

Interface anchoring of p-alkyl phenols in disc-micelles of mesophases

Abstract Five para-alkyl substituted phenols have been studied as solutes in two chemically different type II DM mesophases. The degrees of order for the aromatic ring, are not significantly affected by the different short alkyl chains, showing that the anchoring of the -OH group at the aqueous interface is the dominant determinant of local motion. The principal change in the diagonal degrees of order between the chemically different mesophases resides in the change in size of disc micelles with consequent changes in their motional freedom. The anchoring of the phenol -OH at the aqueous interface shows little or no perturbation from the chemical variation of neighboring amphiphiles in the micelle bilayer.