Calculation of Partition Coefficients of Chain Anchors in Liquid-Ordered and Liquid-Disordered Phases in Model Lipid Bilayers (original) (raw)
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Biophysical Journal, 2005
Quantitative structures of the fully hydrated fluid phases of dimyristoylphosphatidylcholine (DMPC) and dilauroylphosphatidylcholine (DLPC) were obtained at 30°C. Data for the relative form factors F(q z ) for DMPC were obtained using a combination of four methods. 1), Volumetric data provided F(0). 2), Diffuse x-ray scattering from oriented stacks of bilayers provided relative form factors jF(q z )j for high q z , 0.22 , q z , 0.8 Å ÿ1 . 3), X-ray scattering from extruded unilamellar vesicles with diameter 600 Å provided jF(q z )j for low q z , 0.1 , q z , 0.3 Å ÿ1 . 4), Previous measurements using a liquid crystallographic x-ray method provided jF(2ph/D)j for h ¼ 1 and 2 for a range of nearly fully hydrated D-spacings. The data from method 4 overlap and validate the new unilamellar vesicles data for DMPC, so method 4 is not required for DLPC or future studies. We used hybrid electron density models to obtain structural results from these form factors. Comparison of the model electron density profiles with that of gel phase DMPC provides areas per lipid A, 60.6 6 0.5 Å 2 for DMPC and 63.2 6 0.5 Å 2 for DLPC. Constraints on the model provided by volume measurements and component volumes obtained from simulations put the electron density profiles r(z) and the corresponding form factors F(q z ) on absolute scales. Various thicknesses, such as the hydrophobic thickness and the steric thickness, are obtained and compared to literature values.
Structure of Fully Hydrated Fluid Phase Lipid Bilayers with Monounsaturated Chains
Journal of Membrane Biology, 2006
Quantitative structures are obtained at 30°C for the fully hydrated fluid phases of palmitoyloleoylphosphatidylcholine (POPC), with a double bond on the sn-2 hydrocarbon chain, and for dierucoylphosphatidylcholine (di22:1PC), with a double bond on each hydrocarbon chain. The form factors F(q z ) for both lipids are obtained using a combination of three methods. (1) Volumetric measurements provide F(0). (2) X-ray scattering from extruded unilamellar vesicles provides jF(q z )j for low q z . (3) Diffuse X-ray scattering from oriented stacks of bilayers provides jF(q z )j for high q z . Also, data using method (2) are added to our recent data for dioleoylphosphatidylcholine (DOPC) using methods (1) and ; the new DOPC data agree very well with the recent data and with (4) our older data obtained using a liquid crystallographic X-ray method. We used hybrid electron density models to obtain structural results from these form factors. The result for area per lipid (A) for DOPC 72.4 ± 0.5 Å 2 agrees well with our earlier publications, and we find A = 69.3 ± 0.5 Å 2 for di22:1PC and A = 68.3 ± 1.5 Å 2 for POPC. We obtain the values for five different average thicknesses: hydrophobic, steric, head-head, phosphate-phosphate and Luzzati. Comparison of the results for these three lipids and for our recent dimyristoylphosphatidylcholine (DMPC) determination provides quantitative measures of the effect of unsaturation on bilayer structure. Our results suggest that lipids with one monounsaturated chain have quantitative bilayer structures closer to lipids with two monounsaturated chains than to lipids with two completely saturated chains.
Biochimica et Biophysica Acta (BBA) - Reviews on Biomembranes, 2000
The quantitative experimental uncertainty in the structure of fully hydrated, biologically relevant, fluid (L K ) phase lipid bilayers has been too large to provide a firm base for applications or for comparison with simulations. Many structural methods are reviewed including modern liquid crystallography of lipid bilayers that deals with the fully developed undulation fluctuations that occur in the L K phase. These fluctuations degrade the higher order diffraction data in a way that, if unrecognized, leads to erroneous conclusions regarding bilayer structure. Diffraction measurements at high instrumental resolution provide a measure of these fluctuations. In addition to providing better structural determination, this opens a new window on interactions between bilayers, so the experimental determination of interbilayer interaction parameters is reviewed briefly. We introduce a new structural correction based on fluctuations that has not been included in any previous studies. Updated measurements, such as for the area compressibility modulus, are used to provide adjustments to many of the literature values of structural quantities. Since the gel (L L P) phase is valuable as a stepping stone for obtaining fluid phase results, a brief review is given of the lower temperature phases. The uncertainty in structural results for lipid bilayers is being reduced and best current values are provided for bilayers of five lipids. ß
European Biophysics Journal With Biophysics Letters, 2011
Studying membrane active peptides or protein fragments within the lipid bilayer environment is particularly challenging in the case of synthetically modified, labeled, artificial, or recently discovered native structures. For such samples the localization and orientation of the molecular species or probe within the lipid bilayer environment is the focus of research prior to an evaluation of their dynamic or mechanistic behavior. X-ray scattering is a powerful method to study peptide/lipid interactions in the fluid, fully hydrated state of a lipid bilayer. For one, the lipid response can be revealed by observing membrane thickening and thinning as well as packing in the membrane plane; at the same time, the distinct positions of peptide moieties within lipid membranes can be elucidated at resolutions of up to several angstroms by applying heavy-atom labeling techniques. In this study, we describe a generally applicable X-ray scattering approach that provides robust and quantitative information about peptide insertion and localization as well as peptide/lipid interaction within highly oriented, hydrated multilamellar membrane stacks. To this end, we have studied an artificial, designed β-helical peptide motif in its homodimeric and hairpin variants adopting different states of oligomerization. These peptide lipid complexes were analyzed by grazing incidence diffraction (GID) to monitor changes in the lateral lipid packing and ordering. In addition, we have applied anomalous reflectivity using synchrotron radiation as well as in-house X-ray reflectivity in combination with iodine-labeling in order to determine the electron density distribution ρ(z) along the membrane normal (z axis), and thereby reveal the hydrophobic mismatch situation as well as the position of certain amino acid side chains within the lipid bilayer. In the case of multiple labeling, the latter technique is not only applicable to demonstrate the peptide’s reconstitution but also to generate evidence about the relative peptide orientation with respect to the lipid bilayer.
Structural Determinants of Water Permeability through the Lipid Membrane
The Journal of General Physiology, 2007
Despite intense study over many years, the mechanisms by which water and small nonelectrolytes cross lipid bilayers remain unclear. While prior studies of permeability through membranes have focused on solute characteristics, such as size, polarity, and partition coeffi cient in hydrophobic solvent, we focus here on water permeability in seven single component bilayers composed of different lipids, fi ve with phosphatidylcholine headgroups and different chain lengths and unsaturation, one with a phosphatidylserine headgroup, and one with a phosphatidylethanolamine headgroup. We fi nd that water permeability correlates most strongly with the area/lipid and is poorly correlated with bilayer thickness and other previously determined structural and mechanical properties of these single component bilayers. These results suggest a new model for permeability that is developed in the accompanying theoretical paper in which the area occupied by the lipid is the major determinant and the hydrocarbon thickness is a secondary determinant. Cholesterol was also incorporated into DOPC bilayers and X-ray diffuse scattering was used to determine quantitative structure with the result that the area occupied by DOPC in the membrane decreases while bilayer thickness increases in a correlated way because lipid volume does not change. The water permeability decreases with added cholesterol and it correlates in a different way from pure lipids with area per lipid, bilayer thickness, and also with area compressibility.
Physical Review E, 2003
A molecular-level self-consistent-field ͑SCF͒ theory is applied to model the lipid bilayer structures composed of 1-stearoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine ͑18:0/18:19cis PC͒ and 1-stearoyl-2docosahexaenoyl-sn-glycero-3-phosphatidylcholine ͑18:0/22:63cis PC͒. As compared to earlier attempts to model ͑saturated͒ PC membranes several additional features are implemented: ͑i͒ A water model is used which correctly leads to low water concentration in the bilayers. ͑ii͒ Free volume is allowed for, which is important to obtain bilayers in the fluid state. ͑iii͒ A polarization term is included in the segment potentials; this new feature corrects for a minor thermodynamic inconsistency present in ͑all͒ earlier results for charged bilayers. ͑iv͒ The CH 3 groups in the lipid molecules are assumed to have twice the volume of a CH 2 group; this leads to stable noninterdigitated bilayers. ͑v͒ A cis double bond is simulated by forcing gauche conformations along the sn-2 acyl chain. Results of an all-atom molecular dynamics ͑MD͒ simulation, using the collision dynamics method, on the same system are presented. Both SCF and MD prove, in accordance with experimental facts, that acyl unsaturation effectively reduces the length of the chain which counteracts interdigitation. It is also found that the phosphatidylcholine head group is lying almost flat on the membrane surface and the water penetrates into the bilayer upto the glycerol backbone units. From the SCF results it further followed that the free volume is not exactly evenly distributed over the bilayer. There is a small increase in free volume in the center of the bilayer as well as in the glycerol backbone region.