Headgroup Dipole Research Articles

NMR relaxation in micelles formed by a long zwitterionic surfactant

A frequency-dependent 14N and 13C NMR relaxation study of the micellar region formed by a very long zwitterionic surfactant, viz., 6-(dimethyleicosylammonio)hexanoate (C20AH), is presented. As a starting point the binary phase diagram of C20AH/D2O is determined. The phase diagram is similar to those found for shorter chain ionic surfactants with bulky head groups. The NMR relaxation data show a dependence upon the magnetic field strength and are discussed in terms of the two-step model of relaxation. The frequency dependence found at low frequencies in the 14N data is assumed to be caused by the rotational tumbling of the micelle, while it is argued that the frequency dependence found at high frequencies in the 13C relaxation data is caused by a local motion. The analysis of the data yields a radius of the micelle which is slightly shorter than an extended C20 chain, pointing to a situation where the methylene groups in the head-group dipole are embedded in the micelle. Moreover, the local motions in the head-group region are slow as compared to the corresponding motions in single-chain ionic surfactants.

Lateral interactions among phosphatidylcholine and phosphatidylethanolamine head groups in phospholipid monolayers and bilayers.

We develop theory for the lateral interactions among the zwitterionic head groups of phospholipids in monolayers and bilayers, particularly phosphatidylcholine (PC) and phosphatidylethanolamine (PE). With the P- end of the head group anchored at the water/hydrocarbon interface, a balance of two effects dictates the angle that the P--N+ dipole makes with respect to the plane of the bilayer: N+ is driven toward water due to the (Born) electrostatic free energy, but the hydrophobic effect drives the methyl and methylene groups around the N+ charge toward the hydrocarbon. The only adjustable parameter of the model is the average fluctuation of the oil/water interface or, alternatively, the dielectric constant of the hydrocarbon phase. The model predicts that at 5 degrees C the head group dipole should lie largely in the bilayer plane, in accord with X-ray, neutron diffraction, and NMR studies. The theory makes the novel prediction that the N+ end of the dipole becomes increasingly submerged in hydrocarbon with increasing temperature, leading to strongly enhanced lateral repulsion between PC head groups. This prediction is in good agreement with second and third viral coefficients of monolayer lateral pressures, and with the temperature dependence of the former. The theoretical model is consistent with head group fluctuations measured by neutron diffraction of PC and PE bilayers. Because PE has a smaller hydrophobic cluster near N+, its lateral repulsion should be much smaller and less temperature dependent than for PC, also in agreement with equation-of-state measurements. This suggests why at high density PE monolayers have higher melting temperatures than PC monolayers and more propensity for reversed curvature.

Structure and dynamics of the phosphatidylcholine and the phosphatidylethanolamine head group in L-M fibroblasts as studied by deuterium nuclear magnetic resonance.

Mouse fibroblast L-M cells were grown in tissue culture medium containing selectively deuterated choline or ethanolamine. Both compounds were incorporated into the corresponding phospholipids at levels greater than 50% thus leading to a selective deuteration of these phospholipid head groups. Choline and ethanolamine were labeled at either the alpha- or the beta-carbon atom and well-resolved deuterium and phosphorus n.m.r. spectra were obtained from intact cells, crude plasma membranes and lipid extracts, leading to the following conclusions. (i) A large fraction, if not all, of the phospholipids in the intact L-M cell membranes were organized in a liquid crystalline bilayer. (ii) The phosphoethanolamine and the phosphocholine head group conformation were found to be remarkably similar in pure lipid bilayers and in intact L-M cell membranes with the head group dipoles being oriented parallel to the membrane surface. (iii) The deuterium T1 spin lattice relaxation times fell in the range of 7-25 ms and were similar in intact L-M cells and in pure lipid model membranes, suggesting that the two head groups are not involved in strong interactions with membrane proteins. The rotational diffusion rate of the two head groups was reduced by at least a factor of 10 compared to molecules of the same size in aqueous solution. (iv) The phosphocholine head group was sensitive to the size and sign of membrane surface charges as verified in mixing experiments with charged lipids. In L-M cell membranes the phosphocholine appeared to sense an electrically neutral environment in spite of the fact that L-M cell membranes contain 10-20% negatively charged lipids.

How membrane chain melting properties are regulated by the polar surface of the lipid bilayer.

The principle of regulation of various membrane properties by the hydrocarbon membrane interior is now well understood. The mechanism by which the interfacial membrane region including aqueous solution affects the state of the lipid bilayer matrix, however, is as yet unclear, despite its great biological and physiological significance. Data and analysis presented in this paper show that apart from the lipid chain type, length, and degree of unsaturation the main factors determining the characteristics of lipid membranes are surface polarity and interfacial hydration. These incorporate the effects of head group dipole and multipole moments as well as the head group ability for hydrogen bonding and can account for most of the changes in the physicochemical membrane state caused by the lipid head group structure, bulk pH value, salt content, solute adsorption, etc. The effects of membrane potential are much less, only 10-30% of the former. Variations in hydration thus not only govern the short- and medium-range intermolecular and intermembrane interactions but also provide a fast and energetically inexpensive regulatory mechanism for lipid membranes to adapt their characteristics, at least locally or transiently, to new requirements.

A 31P- and 2H-NMR study on the structural perturbations induced by charged detergents in the headgroup region of phosphatidylcholine bilayers

The effect of cetyltrimethylammonium bromide (CTAB) and dodecyltrimethylammonium bromide (DTAB) on the structure and mobility of phosphatidylcholine membranes was studied by 31P- and 2H-nuclear magnetic resonance. CTAB and DTAB induced a strong increase in the effective phosphorus chemical shift anisotropy of the phospholipid. In phosphatidylcholine which had been specifically labeled by deuterium in the headgroup segments, the detergents caused an increase in the quadrupole splittings from the N- methyl deuterons whereas the splittings from both methylene segments collapsed to a singlet signal at a ratio of about 2 mol of phosphatidylcholine per mol of detergent. The effect of DTAB on the headgroup spectral parameters was largely compensated by the negatively charged detergent sodium dodecylsulfate (SDS). In contrast to the phospholipid headgroup the hydrophobic region of the membrane was almost unaffected by CTAB or DTAB. The trimethylammonium headgroups of membrane-bound detergent molecules were found to be motionally much more restricted than the corresponding headgroup segments of the host phospholipid. From these observations it is concluded that the detergent headgroups are located close to the water/hydrocarbon interface of the phospholipid membrane thereby tilting the phospholipid headgroup dipole from the membrane parallel into a more upright orientation.

Membrane lipids: preferred conformational states and their interplay. The crystal structure of dilauroylphosphatidyl-N,N-dimethylethanolamine

The conformation and molecular packing of 2,3-dilauroyl-rac-glycero-1-phospho-N,N-dimethylethanolamine (DLPEM2) has been determined by single-crystal analysis (R = 0.079). The lipid crystallizes in a triclinic space group (P1) with a unit cell of a = 5.64, b = 8.20, c = 39.86 Å and α = 94.5, β = 90.1, γ = 101.9°. The two molecules of the unit cell are related by centro-symmetry and pack tail to tail in a bilayer structure. The zwitterionic head groups are extended perpendicular to the layer plane and interdigitate with the head group dipoles of the adjacent bilayer. The packing cross-section per molecule is 45.2 Å2. The hydrocarbon chains pack with an orthorhombic (O ⊥) subcell and tilt by 33° with respect to the layer normal. The diacylglycerol moiety shows an unusual conformation. The glycerol chain is inclined by 45° with respect to the layer normal and the fatty acid substituted oxygens adopt mutually a — syn-clinal instead of the more common + syn-clinal conformation. Head group interactions and molecular conformation of DLPEM2 are compared with the corresponding structural features of phosphatidic acid, phosphatidylethanolamine and phosphatidylcholine. For the diacylglycerol part three minimum energy conformations are observed, which interconvert by rotation and axial displacement of the hydrocarbon chains.

A proposed general structure for condensed monomolecular films

A very brief review is given of the theories put forward by earlier workers to explain the observed characteristics of condensed monolayers. From the detailed structures of numerous condensed films as discussed in a previous publication, certain generalizations are now suggested. For the least compressible region ( YZ ) evidence based upon surface potential measurements, upon the X-ray structure of hydrocarbons near the melting-point, and upon the compressibility, strongly support the statement that in this region the long chains are vertically orientated and close-packed, but not as tightly as is possible in the three-dimensional crystal. In the more compressible region ( XY ) the behaviour appears to be intermediate between that in the YZ region (determined only by the hydrocarbon chain packing), and in the expanded state (determined only by the head-group dipole orientation). The most general statement possible is that the configuration at the limiting area X is determined both by the orientation of the head-group dipole with respect to the surface, and by the long-chain packing. All known classes of condensed monolayers are grouped according to the factors thought to be primarily responsible for determining the limiting area X , namely, the ‘size’ of the head group, hydrogen bond formation between the head groups, and the hydrocarbon chain packing. The physical states of monolayers, and the factors by which they are influenced, are briefly discussed. Forces both between the polar head groups and between the hydrophobic tails have to be considered.