Biomedical subjects

M L Berkowitz

Publications and source records attributed to M L Berkowitz.

8 recordsLinked to original sources

PubMed2006

Hydration force between model hydrophilic surfaces: computer simulations.

We performed molecular dynamics simulations to study the interactions between model hydrophilic plates made of carbon atoms distributed on a hexagonal lattice. Although neutral, the plates carry equal amounts of positive and negative charges to represent physical dipoles. Using the thermodynamic perturbation theory we calculated the potential of mean force (PMF) acting between the plates as a function of the distance between these plates. We observed that, at distances when more than one water layer can be found between the plates, the contribution of water into the PMF can be either attractive or repulsive depending on the correlation between the charges situated on the plates.

Journal Article↗

PubMed2001

Computer simulation study of the interface width of the liquid/liquid interface.

Four molecular dynamics computer simulations have been performed to study the intrinsic width and the width due to thermal fluctuations of the water/carbon tetrachloride interface. We observed that thermal fluctuations have a capillary wave character. The surface tension calculated by using capillary wave formalism shows a very good agreement with the value obtained from the components of the pressure tensor.

Journal Article↗

PubMed2001

Molecular dynamics simulation of the structure of dimyristoylphosphatidylcholine bilayers with cholesterol, ergosterol, and lanosterol.

Five molecular dynamics computer simulations were performed on different phospholipid:sterol membrane systems in order to study the influence of sterol structure on membrane properties. Three of these simulated bilayer systems were composed of a 1:8 sterol:phospholipid ratio, each of which employed one of the sterol molecules: cholesterol, ergosterol, and lanosterol. The two other simulations were of a bilayer with a 1:1 sterol:phospholipid ratio. These simulations employed cholesterol and lanosterol, respectively, as their sterol components. The observed differences in simulations with cholesterol and lanosterol may have their implication on the form of the phospholipid/sterol phase diagram.

Carbon↗

PubMed2001

Effects of oxygenated sterol on phospholipid bilayer properties: a molecular dynamics simulation.

The properties of dipalmitoylphosphatidylcholine (DPPC):6-ketocholestanol bilayer at 50 mol% sterol were studied using the molecular dynamics simulation technique. Our simulations were performed at constant pressure and temperature on a nanosecond time scale. Data from this simulation were compared to the results of our previous simulations on DPPC and DPPC-cholesterol bilayers. We conclude that the differences in the properties of membranes with cholesterol and ketocholestanol are due to the difference in 6-ketocholestanol and cholesterol location in the bilayer. The presence of the keto group in ketocholestanol moves the sterol towards the polar region closer to interface with water. We predict that similar mechanisms would govern the properties of membranes with other oxygenated sterols, such as for example 7-ketocholesterol. Results of our simulations are in a good agreement with the data available from the experiment.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed2000

Molecular dynamics simulation of dipalmitoylphosphatidylcholine membrane with cholesterol sulfate.

Using the molecular dynamics simulation technique, we studied the changes occurring in a dipalmitoylphosphatidylcholine (DPPC):cholesterol (CH) membrane at 50 mol% sterol when cholesterol is replaced with cholesterol sulfate (CS). Our simulations were performed at constant pressure and temperature on a nanosecond time scale. We found that 1) the area per DPPC:CS heterodimer is greater than the area of the DPPC:CH heterodimer; 2) CS increases ordering of DPPC acyl chains, but to a lesser extent than CH; 3) the number of hydrogen bonds between DPPC and water is decreased in a CS-containing membrane, but CS forms more water hydrogen bonds than CH; and 4) the membrane dipole potential reverses its sign for a DPPC-CS membrane compared to a DPPC-CH bilayer. We also studied the changes occurring in lipid headgroup conformations and determined the location of CS molecules in the membrane. Our results are in good agreement with the data available from experiments.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1999

Structure of dipalmitoylphosphatidylcholine/cholesterol bilayer at low and high cholesterol concentrations: molecular dynamics simulation.

By using molecular dynamics simulation technique we studied the changes occurring in membranes constructed of dipalmitoylphosphatidylcholine (DPPC) and cholesterol at 8:1 and 1:1 ratios. We tested two different initial arrangements of cholesterol molecules for a 1:1 ratio. The main difference between two initial structures is the average number of nearest-neighbor DPPC molecules around the cholesterol molecule. Our simulations were performed at constant temperature (T = 50 degrees C) and pressure (P = 0 atm). Durations of the runs were 2 ns. The structure of the DPPC/cholesterol membrane was characterized by calculating the order parameter profiles for the hydrocarbon chains, atom distributions, average number of gauche defects, and membrane dipole potentials. We found that adding cholesterol to membranes results in a condensing effect: the average area of membrane becomes smaller, hydrocarbon chains of DPPC have higher order, and the probability of gauche defects in DPPC tails is lower. Our results are in agreement with the data available from experiments.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1999

Dynamical properties of phospholipid bilayers from computer simulation.

We present the results of a 10-ns molecular dynamics simulation of a dipalmitoylphosphatidylcholine/water system. The main emphasis of the present study is on the investigation of the stability over a long time and the dynamic properties of the water/membrane system. The motion of the lipid molecules is characterized by the center of mass movement and the displacement of individual atom groups. Because of the slow movement of the headgroup atoms, their contributions to the dipole potential vary slowly and with a large amplitude. Nevertheless, the water molecules compensate the strong fluctuations and maintain an almost constant total dipole potential. From the lateral displacement of the center of masses, we calculate the lateral diffusion coefficient to be Dlat = (3 +/- 0.6) x 10(-7) cm2/s, in agreement with neutron scattering results. The rotational motion is also investigated in our simulations. The calculated value for the rotational diffusion coefficient parallel to the molecular long axis, D = (1.6 +/- 0.1) x 10(8) s-1, is in good agreement with the experiment.

1,2-Dipalmitoylphosphatidylcholine↗

PubMed1999

Molecular dynamics simulation of DPPC bilayer in DMSO.

We performed molecular dynamics simulations on dipalmitoylphosphatidylcholine (DPPC)/dimethylsulfoxide (DMSO) system that has the same lipid:solvent weight ratio as in our previous simulation done on DPPC/water. We did not observe a large change in the size of DPPC membrane when the solvent was changed from water to DMSO. Also, we did not observe that a large number of DMSO molecules is permeating into the membrane, as it was suggested to explain the observed change in the bilayer repeat period. We found that the surface potential reverses its sign when water is replaced by DMSO. Based on the results from our simulations, we propose that the repulsion force acting between membranes is reduced when DMSO is added to solvent water and therefore membrane surfaces approach closer to each other and the extra solvent is removed into excess solution.

1,2-Dipalmitoylphosphatidylcholine↗