A P-31 NMR-STUDY OF IMMOBILIZED ARTIFICIAL MEMBRANE SURFACES - STRUCTURE AND DYNAMICS OF IMMOBILIZED PHOSPHOLIPIDS

Chromatography surfaces were prepared by immobilizing a single-chain ether phospholipid at approximately a monolayer density on silica particles. The chromatography particles are denoted as (ether)IAM.PC(C10/C3), and they are stable to all solvents. The structure and dynamics of the interphase created by immobilizing phospholipids on silica particles were studied by P-31 NMR methods. (ether)IAM.pC(C10/C3) spontaneously wets when suspended in both aqueous and organic solvents, and P-31 NMR spectra were obtained in water, methanol, chloroform, acetonitrile, and acetone. P-31 NMR spectra were subjected to line-shape analysis. From line-shape analysis, the correlation times for rapid internal motion (tau(parallel-to)) and wobbling (tau(perpendicular-to)) of the phospholipid headgroup were calculated for each solvent. Immobilized phospholipid headgroups comprising the IAM interfacial region undergo rapid reorientation similar to the case of the phospholipids forming liposome membranes with tau(parallel-to) approximately 1 ns. Phospholipids in liposome membranes exhibit slower wobbling motion (tau(perpendicular-to) approximately 1 ms) in the plane of the membrane. However, the immobilized phospholipids on (ether)IAM.PC(C10/C3) surfaces wobble with correlation times tau(perpendicular-to) that depend on the solvent bathing the (ether)IAM.PC(C10/C3) surface. In methanol suspensions, tau(perpendicular-to) is approximately 1 ns. In water suspensions of (ether)IAM.PC(C10/C3), tau(perpendicular-to) is approximately 1 mus. In less polar solvents like chloroform, acetonitrile, and acetone, the tau(perpendicular-to) values are approximately 0.1 ms. Temperature-dependent T1 measurements of (ether)IAM.PC(C10/C3) suspensions in methanol and water were performed, and a T1 minimum for each suspension was observed. The experimental T1 data indicated that relaxation of the P-31 nucleus is completely dominated by chemical shift anisotropy (CSA). On the basis of the pure CSA relaxation theory, the temperature-dependent T1 data were thus converted into the motional correlation times tau(c) of the phosphate group on the (ether)IAM.PC(C10/C3) surface. For both methanol and water suspension of (ether)IAM.PC(C10/C3), tau(c) is approximately 0.5 ns at 25-degrees-C, which is consistent with the tau(parallel-to) calculated from spectral line-shape analysis. The motional activation energies obtained from Arrhenius plots of tau(c) vs temperature were used to monitor rotational energy barriers of the immobilized phosphate group regarding solvent effects and phospholipid density. The phosphate motional activation energies of nonfrozen (3.45 kcal/mol) and frozen (7.59 kcal/mol) (ether)IAM.PC(C10/C3) aqueous suspensions are similar to the values of nonfrozen and frozen liposomes.