Phospholipid Headgroup Conformation Research Articles

Effects of antidepressants on the conformation of phospholipid headgroups studied by solid-state NMR.

The effect of tricyclic antidepressants (TCA) on phospholipid bilayer structure and dynamics was studied to provide insight into the mechanism of TCA-induced intracellular accumulation of lipids (known as lipidosis). Specifically we asked if the lipid-TCA interaction was TCA or lipid specific and if such physical interactions could contribute to lipidosis. These interactions were probed in multilamellar vesicles and mechanically oriented bilayers of mixed phosphatidylcholine-phosphatidylglycerol (PC-PG) phospholipids using (31)P and (14)N solid-state NMR techniques. Changes in bilayer architecture in the presence of TCAs were observed to be dependent on the TCA's effective charge and steric constraints. The results further show that desipramine and imipramine evoke distinguishable changes on the membrane surface, particularly on the headgroup order, conformation and dynamics of phospholipids. Desipramine increases the disorder of the choline site at the phosphatidylcholine headgroup while leaving the conformation and dynamics of the phosphate region largely unchanged. Incorporation of imipramine changes both lipid headgroup conformation and dynamics. Our results suggest that a correlation between TCA-induced changes in bilayer architecture and the ability of these compounds to induce lipidosis is, however, not straightforward as imipramine was shown to induce more dramatic changes in bilayer conformation and dynamics than desipramine. The use of (14)N as a probe was instrumental in arriving at the presented conclusions.

Effect of a Polar Environment on the Conformation of Phospholipid Head Groups Analyzed with the Onsager Continuum Solvation Model

The effect of the polarity of the environment on the conformation of the zwitterionic membrane lipid head groups phosphoethanolamine (PE) and phosphocholine (PC) has been investigated with calculations at the Hartree−Fock level using the 3-21G(*), 6-31G*, and 6-31+G* basis sets together with the Onsager continuum solvation model. Results suggest that in the gas phase both PE and PC adopt cyclic minimum energy conformations, in which an ammonium or N-methyl hydrogen closely approaches one of the nonesterified phosphate oxygens. In the case of PE, intramolecular interactions result in a proton transfer from the ammonium group to the phosphate oxygen, which however is suppressed by a moderate increase in the polarity of the surrounding medium. With increasing polarity of the environment, the cyclic structures of PE and PC still remain low-energy conformers but simultaneously for both head groups an almost identical extended conformer, typical of crystal structures, becomes increasingly favored. Already at ε = 10, the extended conformer of PC is favored (−2.4 kcal/mol) relative to the cyclic one, while for PE the relative energy of the extended conformer approaches that of the cyclic one at ε = 80. The similarity and increasing stability of the extended PE/PC conformers in the monomeric state and the fact that this conformer is also adopted in all crystal structures of PE/PC lipids, regardless of hydration and interaction pattern, indicate that the geometry of this conformer is determined by energetics intrinsic to the phosphoethanolammonium backbone. In lipid aggregates or a membrane environment the extended conformer becomes additionally stabilized by intermolecular ionic and hydrogen bond interactions with neighboring molecules substituting for the internal interaction that in the monomeric state constrains the zwitterionic dipole into a cyclic structure.

Characterization of pH-Dependent Poly(acrylic Acid) Complexation with Phospholipid Vesicles

The pH-dependent complexation of poly(acrylic acid) (PAA) with phospholipid (phosphatidylcholine) vesicles was characterized by fluorescence polarization, differential scanning calorimetry (DSC), and surface pressure measurements of phospholipid monolayers. The complexation was pronounced below pH 4, when the polymer carboxyl groups are protonated, as shown by the binding of PAA to vesicles and the decrease in polymer mobility. The complexation was strong at low polymer concentrations and was weaker at higher polymer concentrations. PAA complexation increased the gel to liquid crystalline (LC) phase transition temperature (Tm) and enthalpy (ΔH) of the vesicles accompanied by a decrease in the transition cooperativity. This is most likely due to perturbations in the phospholipid headgroup upon PAA interaction. The effect of polymer adsorption on the phospholipid surface was investigated at the phospholipid/water interface. The ability of PAA to penetrate between the phospholipid molecules as a function of pH was determined by the lateral expansion of the monolayer at a constant surface pressure. The ability of the polymer to penetrate into the monolayer increased with decreasing pH. These results suggest that PAA complexation leads to expansion of the phospholipid packing of the vesicles by altering the phospholipid headgroup conformation.

The membrane potential has no detectable effect on the phosphocholine headgroup conformation in large unilamellar phosphatidylcholine vesicles as determined by 2H-NMR

In this study the effect of a transmembrane electrical potential on the phospholipid headgroup conformation was investigated using the 2H-NMR technique. Large unilamellar vesicles were prepared of dioleoylphosphatidylcholine, specifically 2H-labeled at the α- or β-position of the choline group. No conformational change of the phosphocholine headgroup could be detected after induction of a valinomycin-induced K +-diffusion potential across the bilayer. However, this method could be used to measure the redistribution of tetraphenylphosphonium across the bilayer in response to Δψ, which reorients the phosphocholine headgroups in the opposite bilayer—water interfaces.

Magic angle spinning 13C-NMR spin-lattice relaxation study of the location and effects of ?-tocopherol, ubiquinone-10 and ubiquinol-10 in unsonicated model membranes

α-Tocopherol, ubiquinone-10 and ubiquinol-10 have been studied by high resolution magic angle samples spinning 13C-nuclear magnetic resonance in egg yolk phosphatidylcholine multilamellar vesicles model membranes in order to assess their location and the induced perturbations on this model system. α-Tocopherol is placed in such a position that it is in close contact with the head group of the phospholipid and exposed to the solvent. In this position it significantly perturbs the phospholipid head group, the 5a-CH3 and the 7a-CH3 groups being the closest parts to the membrane surface. On the other hand, ubiquinol-10 perturbs the membrane surface more than ubiquinone-10, but neither compound significantly changed the phospholipid head group conformation.

A study of phospholipid phosphate groups in model membranes by Fourier transform infrared spectroscopy.

The phosphate region (1000–1300 cm–1) of the infrared spectrum of aqueous phospholipid dispersions has been studied by the infrared Fourier transform technique. The main features of this spectral region have been described for various phospholipids, including phosphatidylcholine, phosphatidylethanolamine, cardiolipin and phosphatidylglycerophosphate (alkyl ether), the major phospholipid of Halobacterium purple membranes. No changes in the phosphate region are observed due to lipid polymorphism or as a consequence of changes in fatty acyl chain structure. Shifts in the phosphoryl stretching bands are interpreted in terms of changes in hydrogen bonding, while a contribution from R—O—P vibration is considered to reflect changes in phospholipid headgroup conformation. When added in equimolar amounts to phosphatidylcholine or phosphatidylethanolamine bilayers, surfactants (Triton X-100, sodium cholate) modify the degree of hydration and/or the orientation of the headgroup with respect to the bilayer plane. Phospholipids containing two phosphate groups give rise to more complex spectral features. Infrared data suggest that the two phosphate groups of cardiolipin are conformationally non-identical when incorporated into a lipid bilayer. Controlled enzyme hydrolysis (e.g. with phospholipase D) may help in the study of these complex phospholipid headgroups.

Perturbations of phospholipid head groups by membrane proteins in biological membranes and recombinants

P-31 nuclear magnetic resonance (NMR) spin-lattice relaxation times (T1) have been used to probe the behavior of phospholipid head groups in the presence of membrane proteins. Measurements have been made on rabbit muscle sarcoplasmic reticulum and recombinants of the Ca2+ Mg2+ ATPase, rod outer segment disk membranes and recombinants of rhodopsin, and human erythrocyte ghosts and recombinants of human erythrocyte glycophorin. Recombined membranes with lipid/protein ratios greater than or equal to that found in biological membranes showed T1 behavior similar to the biological membranes and pure phosphatidylcholine. However, recombined membranes with a low lipid/protein ratio exhibited a T1 that was dramatically shorter than any of the other systems. Analysis of the relaxation mechanism and the factors contributing to it implicate a phospholipid head group conformation change at high protein content. It is suggested that this is due to trapping of phospholipid between proteins and is not the same phenomenon as motional restriction at the lipid-protein interface at higher lipid contents.

Bilayers of phosphatidyldiacylglycerol and phosphatidylcholesterol give 31P-NMR spectra characteristic for hexagonal and isotropic phases

Aqueous dispersions of phosphatidyldiacylglycerol and phosphatidylcholesterol are shown to form bilayers by differential scanning calorimetry, diphenylhexatriene fluorescence polarization, and electron microscopy; however, 31P-nuclear magnetic resonance spectra of these dispersions are characteristic of the ‘hexagonal’ and ‘isotropic’ phases, respectively. The theoretical relationship between the conformation of phospholipid molecules in bilayers and the shape of the 31P-NMR line is examined. It is shown that differences in the various characteristic spectral line shapes can result from differences in the phospholipid headgroup conformation without significant changes in the organization of the acyl chains in the bilayer phase.

Phosphorus-31 nuclear magnetic resonance spectra characteristic of hexagonal and isotropic phospholipid phases generated from phosphatidylethanolamine in the bilayer phase.

31P nuclear magnetic resonance (NMR) spectroscopy is recognized as a technique which yields information concerning both the dynamics and organization of phospholipid molecules in biological membranes and phospholipid dispersions. In this theoretical paper, we examine the relationship between the conformation of the phospholipid molecule and the shape of the predicted 31P NMR spectrum. Using a simple model of rotation of the phospholipid molecule about its long axis, we show that it is possible to generate spectra previously considered typical of the bilayer (sigma parallel to less than sigma perpendicular), isotropic (sigma parallel to congruent to sigma perpendicular), and hexagonal II (sigma parallel to greater than sigma perpendicular) packing arrangements by simply changing the phospholipid head-group conformation while retaining the molecules in the bilayer phase.

Effect of transmembrane electrical potential and micelle geometry on phospholipid head group conformation

Phospholipid head group conformation has been measured in egg phosphatidyl choline vesicles with a transmembrane electrical potential. Using 31P NMR, the proton source of the 31P[ 1H] nuclear Overhauser effect has been determined and the head group conformation shown to be unperturbed by a transmembrane electrical potential near 60 mV. The same technique was used to probe polar head group conformation in lysophosphatidyl choline micelles. The geometric constraints of the micelle structure force the head groups apart, preventing the intermolecular head group interactions found in phosphatidyl choline bilayers.

Quantum‐Chemical and semiclassical calculations of intermolecular interactions of phospholipids

AbstractBy use of empirical 0–1–6–12 atom–atom potential functions and the PCILOCC method intra‐ and intermolecular interactions of glycero–phosphoryl–ethanolamine model head groups in a planar layer crystal were calculated. Starting from investigations of the two‐dimensional energy‐contour diagrams the minima of energy as a function of all head group torsion angles were calculated using a gradient procedure. Within an interval of 15 kcal/mol above the energy of the global minimum we obtained about 30 local minima. These results demonstrate a high flexibility of the investigated phosphorylethanolamine head group in agreement with experiment. The ethanolamine moiety exists in enantiomeric conformations. With the torsion angles of the 0–1–6–12 energy minimization procedure PCILOCC calculations were carried out. These calculations yield the x‐ray conformation as the most stable one (unit‐cell stabilization energy = −36.3 kcal/mol). The PCILOCC as well as the potential function calculations show that the conformation of phospholipid head groups in layer crystals is determined by intramolecular as well as by intermolecular interactions with neighboring phospholipid molecules.

Quantum-chemical and empirical calculations on phospholipids IV. Glycero-phosphorylethanolamine in a quasihexagonal planar lattice

The energetically most stable head group conformations of a racemic mixture of diacyl-glycero-phosphorylethanolamine in a planar quasihexagonal lattice were calculated using empirical 1-6-12 atom-atom potential functions for intra- and intermolecular interactions. The results demonstrate that the conformation of phospholipid head groups in bilayer systems is determined by intramolecular interactions as well as by intermolecular interactions with neighbouring phospholipid molecules and with solvent molecules. The most stable conformers are that with a φ2 = guache − conformation of the phosphodiester group. All conformers with a φ2 = gauche (+) or trans conformation have energies more than 15 kcal ☆ mol −1 above that of the global minimum. The calculated torsional angles ϕ1 and φ1 are in very good agreement with the results of the X-ray diffraction analysis of 1,2-dilauroyl-DL-phosphatidylethanolamine (DLPE) acetic acid single crystals.