Phase Separation of Saturated and Mono-unsaturated Lipids as determined from a Microscopic Model

A molecular model is proposed of a bilayer consisting of fully saturated DPPC and mono unsaturated DOPC. The model not only encompasses the constant density within the hydrophobic core of the bilayer, but also the tendency of chain segments to align. It is solved within self-consistent field theory. A model bilayer of DPPC undergoes a main chain transition to a gel phase, while a bilayer of DOPC does not do so above zero degrees centigrade because of the double bond which disrupts order. We examine structural and thermodynamic properties of these membranes and find our results in reasonable accord with experiment. In particular, order-parameter profiles are in good agreement with NMR experiments. A phase diagram is obtained for mixtures of these lipids in a membrane at zero tension. The system undergoes phase separation below the main-chain transition temperature of the saturated lipid. Extensions to the ternary DPPC, DOPC, and cholesterol system are outlined.Comment: 29 pages, 4 figures, 1 table. revised versio

Extended conformations of isolated molecular bottle-brushes:Influence of side-chain topology

A Monte Carlo study is presented to discuss the influence of the side-chain topology on the enhancement of the persistence length of a molecular bottle-brush in a dilute athermal solution due to the excluded volume interactions between the side chains. The structures investigated consisted of freely jointed backbones of 100 hard spheres (beads) of diameter 1 to which 50 equally flexible side chains were grafted. The diameter of the side-chain beads was varied from 1 to 3 in the same units. For every given size of the side-chain bead, the length of the side chains was varied from 4 to 20 beads. The ratio between the persistence length and the bottle-brush diameter, which is the determining factor for lyotropic behavior of conventional semi-flexible chains, was found to be almost independent of the side-chain length. At the same time, it was found to increase considerably with increasing size of the side-chain beads, suggesting that by a proper choice of the chemistry lyotropic behavior of molecular bottle-brushes due to excluded-volume interactions between the side chains might be achieved. Moreover, relatively short side chains can be used since the side-chain length has only a minor influence on the ratio between the persistence length and the diameter. These findings are in a good agreement with recent experimental observations

Anomalies in supercooled NaCl aqueous solutions: A microscopic perspective

In this work we studied the effect of NaCl on the thermodynamic and dynamic properties of supercooled water, for salt concentrations between 0.19 and 1.33mol kg-1, using molecular dynamic simulations for TIP5PE water model and ion parameters specially designed to be used in combination with this potential. We studied the isobaric heat capacity (Cp) temperature dependence and observed a maximum in Cp, occurring at Tm, that moves to lower temperature values with increasing salt concentration. Many characteristic changes were observed at scaled temperature TTm ∼ 0.96, namely a minimum in the density of the system, a reduction of the slope of the number of hydrogen bonds vs. temperature, and a crossover from Vogel-Tamman-Fulcher to Arrhenius dynamics. Finally, at low temperatures we observed that water dynamics become heterogeneous with an apparently common relationship between the fraction of immobile molecules and T/Tm for all studied systems. © 2011 American Institute of Physics.Fil:Longinotti, M.P. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina.Fil:Corti, H.R. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Calculation of the Phase Behavior of Lipids

The self-assembly of monoacyl lipids in solution is studied employing a model in which the lipid's hydrocarbon tail is described within the Rotational Isomeric State framework and is attached to a simple hydrophilic head. Mean-field theory is employed, and the necessary partition function of a single lipid is obtained via a partial enumeration over a large sample of molecular conformations. The influence of the lipid architecture on the transition between the lamellar and inverted-hexagonal phases is calculated, and qualitative agreement with experiment is found.Comment: to appear in Phys.Rev.