Pressure−Area Isotherm of a Lipid Monolayer from Molecular Dynamics Simulations (original) (raw)
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Biophysical Journal, 1997
Molecular dynamics simulations of 500 ps were performed on a system consisting of a bilayer of 64 molecules of the lipid dipalmitoylphosphatidylcholine and 23 water molecules per lipid at an isotropic pressure of 1 atm and 50 degrees C. Special attention was devoted to reproduce the correct density of the lipid, because this quantity is known experimentally with a precision better than 1%. For this purpose, the Lennard-Jones parameters of the hydrocarbon chains were adjusted by simulating a system consisting of 128 pentadecane molecules and varying the Lennard-Jones parameters until the experimental density and heat of vaporization were obtained. With these parameters the lipid density resulted in perfect agreement with the experimental density. The orientational order parameter of the hydrocarbon chains agreed perfectly well with the experimental values, which, because of its correlation with the area per lipid, makes it possible to give a proper estimate of the area per lipid of 0.61 +/- 0.01 nm2.
Molecular dynamics simulation of the fully hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer
2003
The structural properties of lipid bilayers in biological membranes are of great interest in biochemistry, biophysics, and medicine. The main goal of the present study was to use molecular dynamic (MD) techniques to investigate physical properties of the hydrated dipalmitoylphosphatidylcholine (DPPC) bilayer. -- The bilayer model consists of 25 DPPC molecules per each monolayer and 44.8% water by total weight. A modified version of AMBER MD suit of programs with CHARMM22 force field for phospholipids was used in simulation. The isothermal-isobaric or NPT ensemble with a fully flexible simulation box in ROAR program was used in this study. Simulations were performed under different pressure and temperature conditions. -- According to experimental results, a liquid crystal phase (Lα) is expected with the DPPC bilayer simulated under 1 atm pressure and 323 K temperature conditions. However, area per lipid, bilayer thickness, chain tilt, and the order parameters resulting from the prese...
Langmuir, 2006
Understanding the lipid phase transition of lipid bilayers is of great interest from biophysical, physicochemical, and technological points of view. With the aim of elucidating the structural changes that take place in a DPPC phospholipid bilayer induced by an external isotropic surface pressure, five computer simulations were carried out in a range from 0.1 to 40 mN/m. Molecular dynamics simulations provided insight into the structural changes that took place in the lipid structure. It was seen that low pressures ranging from 0.1 to 1 mN/m had hardly any effect on the structure, electrical properties, or hydration of the lipid bilayer. However, for pressures above 40 mN/m, there was a sharp change in the lipid-lipid interactions, hydrocarbon lipid fluidity, and electrostatic potential, corresponding to the mesomorphic transition from a liquid crystalline state (L R) to its gel state (P′). The head lipid orientation remained almost unaltered, parallel to the lipid layer, as the surface pressure was increased, although a noticeable change in its angular distribution function was evident with the phase transition.
Critical pressures in multicomponent lipid monolayers
Biochimica et Biophysica Acta (BBA) - Biomembranes, 1996
Epifluorescence microscopy has been used previously to study coexisting liquid phases in lipid monolayers of dihydrocholesterol and dimyristoylphosphatidylcholine at the air/water interface. This binary mixture has a critical point at room temperature (22°C), a monolayer pressure of approx. 10 mN/m, and a composition in the vicinity of 20-30 mol% dihydrocholesterol. It is reported here that this critical pressure can be lowered, raised, or maintained constant by systematically replacing molecules of this phosphatidylcholine with molecules of a phosphatidylethanolamine, or an unsaturated phosphatidylcholine, or mixtures of the two, while maintaining the dihydrocholesterol concentration at 20 mol%. Thus, even complex mixtures of lipids may be characterized by a single, well-defined second-order phase transition. In principle, such transitions might be found in biological membranes.
Biophysical Journal, 2007
Molecular dynamics simulations of dipalmitoylphosphatidylcholine (DPPC) lipid bilayers using the CHARMM27 force field in the tensionless isothermal-isobaric (NPT) ensemble give highly ordered, gel-like bilayers with an area per lipid of ;48 Å 2. To obtain fluid (L a) phase properties of DPPC bilayers represented by the CHARMM energy function in this ensemble, we reparameterized the atomic partial charges in the lipid headgroup and upper parts of the acyl chains. The new charges were determined from the electron structure using both the Mulliken method and the restricted electrostatic potential fitting method. We tested the derived charges in molecular dynamics simulations of a fully hydrated DPPC bilayer. Only the simulation with the new restricted electrostatic potential charges shows significant improvements compared with simulations using the original CHARMM27 force field resulting in an area per lipid of 60.4 6 0.1 Å 2. Compared to the 48 Å 2 , the new value of 60.4 Å 2 is in fair agreement with the experimental value of 64 Å 2. In addition, the simulated order parameter profile and electron density profile are in satisfactory agreement with experimental data. Thus, the biologically more interesting fluid phase of DPPC bilayers can now be simulated in all-atom simulations in the NPT ensemble by employing our modified CHARMM27 force field.
Journal of Chemical Theory and Computation, 2010
Molecular dynamics simulations of fully hydrated pure bilayers of four widely studied phospholipids, 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), 1,2-dimyristoyl-sn-glycero-3-phosphocholine (DMPC), 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC), and 2-oleoyl-1palmitoyl-sn-glycero-3-phosphocholine (POPC) using a recent revision of the GROMOS96 force field are reported. It is shown that the force field reproduces the structure and the hydration of bilayers formed by each of the four lipids with high accuracy. Specifically, the solvation and the orientation of the dipole of the phosphocholine headgroup and of the ester carbonyls show that the structure of the primary hydration shell in the simulations closely matches experimental findings. This work highlights the need to reproduce a broad range of properties beyond the area per lipid, which is poorly defined experimentally, and to consider the effect of system size and sampling times well beyond those commonly used.
Physical Review E, 2004
Using neutron diffraction and a specially constructed high pressure cell suitable for aligned multibilayer systems, we have studied, as a function of pressure, the much observed anomalous swelling regime in dimyristoyl-and dilauroyl-phosphatidylcholine bilayers, DMPC and DLPC, respectively. We have also reanalyzed data from a number of previously published experiments and have arrived at the following conclusions. ͑a͒ The power law behavior describing anomalous swelling is preserved in all PC bilayers up to a hydrostatic pressure of 240 MPa. ͑b͒ As a function of increasing pressure there is a concomitant decrease in the anomalous swelling of DMPC bilayers. ͑c͒ For PC lipids with hydrocarbon chains у13 carbons the theoretical unbinding transition temperature T Ã is coupled to the main gel-to-liquid crystalline transition temperature T M. ͑d͒ DLPC is intrinsically different from the other lipids studied in that its T Ã is not coupled to T M. ͑e͒ For DLPC bilayers we predict a hydrostatic pressure (Ͼ290 MPa) where unbinding may occur.
Biophysical Chemistry, 1982
A statistical thermodynamic model of phospholipid bilayers is devclopcd. In the model. ;I nexv concept of a cloxlg packed system is opplicd. i.e.. a system of hard cylinders of equal radii. the radius being a function of the avcragc number of .SWKIIP rotations in a hydrocarbon chain. Using this concept of a closely packed system. reasonable vaiucs arc ohtaincd for the chnngc in specific volume at :he order-disorder transition of lecithin bilaycrs. In addition to intcroctinns hctwccn the lipid matrix and water molecules. between the head groups themselves and bctwcen hydrocarbon chains. as wcli as the intramolecular cnerg?; nssociated with chain conformation. the Hamiltonian of the memhronc eho includes the cncrgy of the prcssurc field. Thus. the phase transition of phobphoiipid mernbrancs induced not only by tcmpcrature hut also hy hydrostatic prcssurc is dcscribcd by this model simultaneously. In accordance with the experimental results. a linear relationship is ohtaincd between the phase transition temperature and phase transition prcssurc. The other ca!culated phase trxxsition propcrtics of lecithin homologucc. cg.. changes in enthalpy. surface area. thickness and gurcc~w number per chain arc in ngrccmcnt with the availahlc experimental data. The ratio of kink to interstitial conduction of bilayers is also catimatcd.
Biophysical Journal, 1995
We report a constant pressure and temperature molecular dynamics simulation of a fully hydrated liquid crystal (L) phase bilayer of dipalmitoylphosphatidylcholine at 500C and 28 water molecules/lipid. We have shown that the bilayer is stable throughout the 1550-ps simulation and have demonstrated convergence of the system dimensions. Several important aspects of the bilayer structure have been investigated and compared favorably with experimental results. For example, the average positions of specific carbon atoms along the bilayer normal agree well with neutron diffraction data, and the electron density profile is in accord with x-ray diffraction results. The hydrocarbon chain deuterium order parameters agree reasonably well with NMR results for the middles of the chains, but the simulation predicts too much order at the chain ends. In spite of the deviations in the order parameters, the hydrocarbon chain packing density appears to be essentially correct, inasmuch as the area/lipid and bilayer thickness are in agreement with the most refined experimental estimates. The deuterium order parameters for the glycerol and choline groups, as well as the phosphorus chemical shift anisotropy, are in qualitative agreement with those extracted from NMR measurements.