Molecular Dynamics Simulations of Salt Effects On Membranes At Different Thermotropic Phases

FERNANDO E. HERRERA; M. MILAGROS SALES; DANIEL E. RODRIGUES

Buzios, Río de Janeiro

Congreso; II Latin American Federation of Biophysical Societies (LaFeBS) Congress; 2012

Federación de Sociedades de Biofísica Latinoamericanas

Biological membranes are very complex systems since they might present a variety of  structural and dynamic characteristics associated with different conditions such as temperature or the ionic composition of aqueous buffers around them. The temperature determines the thermotropic phases and ordering of the lipids, like the ordered Gel (G) or the Liquid crystalline (LC). The ionic concentration seems to affects the membrane fluidity in a progressive way. A study of the interplay between these two factors it is therefore necessary to understand the molecular mechanisms of their interactions. We have performed Molecular Dynamics simulations of DPPC hydrated bilayers at Gel (T=22°C) and Liquid Crystalline (T=50°C) phases, at different ionic concentration of NaCl in order to rationalize the effect of the ionic forces on different thermotropic phases of the same system. We analyzed the change of several structural and dynamical properties with the ionic concentration (area per lipid, atomic density profiles, thickness fluctuations 2D-maps, ion depth profiles, ion solvation depth profiles, etc).The results show that the ionic absorption and the effects of the ions on many membrane properties depend primarily on the phase of the membranes. The area per lipid and the diffusion coefficients in the LC phase is reduced when the ionic concentration increase while they remain unchanged in G phase. Additionally, the lipid fraction that interacts with ions depends only on the ionic concentration.  Furthermore, the absorbed Na ions interact principally with the carbonyl oxigens in both phases. This work has finally contributed to emphasize that salt concentration and temperature are important factors to take into account in the design of any kind of experiments.