19 F NMR of monofluorostearic acids in lecithin vesicles

In vivo incorporation of a cholesterol-like fluorescent probe into rat brain membranes

The incorporation of the fluorescent sterol analog, cholesta-5,7,9-trien-3 beta-ol, into rat brain P2 fractions and its feasibility to detect membrane perturbations by polarization analysis were evaluated. This investigation involved the administration (i.c.v.) of the fluorescent compound in sesame oil with subsequent determination of optimal dose and the half-life of this compound within the membrane fraction. The dose giving maximum polarization was found to be 0.65 mumole, and pharmacokinetic analysis revealed that the disappearance of this analog followed a two-compartment open model with an elimination half-life from the membrane fraction of 655 hr. Increasing temperature of the membrane preparation showed an increase in polarization, indicating a consequent decrease in mobility of the molecule, while addition of a fluidizing agent like ethanol caused a decrease in polarization, and thus the expected increase in fluidity.

Differential thermal analysis of dipalmitoylphosphatidylcholine--fatty acid mixtures

Mixtures of dipalmitoylphosphatidylcholine (DPPC) with palmitic, stearic, and myristic acids and the sodium salts of these acids were analyzed by differential thermal analysis (DTA) over a wide range of lipid compositions, all in excess water. All three fatty acids raise the liquid-crystal phase transition temperature and form sharp-melting complexes, with 1:2 DPPC--fatty acid stoichiometry observed for palmitic and stearic acids and suggested for myristic acid. Phase diagrams of the peritectic type, indicating nonideal mixing, was fitted to the DPPC--palmitic acid and DPPC--stearic acid data. In contrast, DPPC forms nearly ideal mixtures with the putative DPPC--myristic acid complex. At levels of only a few mole percent, both sodium stearate and myristate remove the pretransition and main transition and produce new peaks at approximately 30 and approximately 48 degrees C; the relative areas of the new peaks were unreproducible for the DPPC--myristate system. Sodium palmitate is the least disruptive of any of the sodium soaps or fatty acids; up to 80 mol % palmitate, the transition is lowered 3 degrees C and approximately doubled in width. The pretransition is detectable up to 36 mol %, and the main transition persists up to 88 mol % palmitate. The apparent pK of palmitic acid (12 mol %) in DPPC bilayers was determined to be 10.2 by direct pH measurement of ternary DPPC mixtures with known palmitic acid/sodium palmitate ratios; the intrinsic pK is estimated to be less than or approximately 8.5.

Phase transition in a lipid bilayer. II. Influence of adamantane derivatives

The influence of thirty-four adamantane, protoadamantane, and homoadamantane derivatives on the phase transition characteristics of the bilayer in dipalmitoyl lecithin liposomes has been determined by differential scanning calorimetry. Each of these compounds induces a broadening of the phase transition profile of the lipid bilayer that is dependent upon the concentration of the solute and its molecular structure. The concentration--response curves obtained for these solutes suggest that the cage compound derivatives modify the phase properties and under some conditions may induce a phase separation in the doped bilayer. The relative activity sequences obtained for the compounds examined cannot be accounted for by simple considerations of lipid/water partition coefficients, substitution constants based on free energy relationships, or the relative polarities or sizes of substituent groups. The observations are consistent with the hypothesis that the position and orientation of a solute within the bilayer are critical factors in determining its relative potency. The position of a solute within the bilayer is significantly controlled by the presence of polar substituents and by the relative geometric relationships of these groups. For a given substituent group, the shape and size of the hydrocarbon cage becomes increasingly important. It is apparent that seemingly minor modifications in the structure of a solute can significantly alter its influence on the phase transition behavior of a bilayer.

Physical studies on phosphonium phosphatidylcholine. A unique [31P]phosphorus nuclear-magnetic-resonance probe for model and biological membranes

1. Distearoyl phosphatidylcholine and the phosphonium analogue, in which the nitrogen atom is replaced by phosphorus, show similar gel-liquid crystalline transition temperatures as detected by differential scanning calorimetry. 2. The temperature-dependence of the 31P n.m.r. (nuclear-magnetic-resonance) linewidths of the phosphate resonances of sonicated vesicles of distearoyl phosphatidylcholine and the phosphonium analogue are similar. Below the phase-transition temperature the linewidths decrease as the temperature is raised. Above the phase-transition temperature the phosphate resonances are relatively temperature-independent. The phosphonium 31P n.m.r. signal exhibits the same pattern of temperature-dependence. 3. The 31P n.m.r. phosphonium resonance is sensitive to the paramagnetic shift reagent, K3Fe(CN)6. Use of K3Fe(CN)6, together with Nd(NO3)3, enabled the determination of the trans-bilayer distribution of egg-yolk phosphatidylcholine and its phosphonium analogue in co-sonicated vesicles. Both are distributed comparably across the bilayer of the vesicles. 4. The phosphonium 31P n.m.r. signal is much sharper than the corresponding phosphate resonance in both sonicated and unsonicated dispersions of the phosphatidylcholine analogue. 5. The properties of the phosphonium analogue of phosphatidylcholine are discussed in terms of its suitability as a probe of membrane structure.

13C nuclear magnetic resonance spectroscopy of native and recombined lipoproteins

(13)C nuclear magnetic resonance data on native and recombined lipoproteins are reported. [Methyl-(13)C]phosphatidylcholine, [methyl-(13)C]sphingomyelin, 1,2-[dioleoyl-1-(13)C]-sn-phosphatidylcholine and cholesteryl-[1-(13)C]oleate were enriched with 90% carbon-13 in respective molecules by chemical synthesis and used for recombination experiments with high density lipoprotein apoproteins. Relaxation times for these specifically enriched lipids in organic solvents, (2)H(2)O, and lipid-protein complexes isolated by ultracentrifugal flotation, were measured. The results show that both the phosphatidylcholine and sphingomyelin polar headgroups have the same hydrophilic environment in either sonicated lipid particles or reassembled lipoproteins, and suggest that ionic interaction of lipid and apolipoproteins is of minor importance in the formation of plasma lipoprotein complexes. Our experiments indicate that (13)C nuclear magnetic resonance spectroscopy will contribute to the understanding of lipidprotein interaction in lipoproteins and membranes.