Lipid bilayer fluid phases are not always fluid: a simulation experiment?

PINTO O. A; FRIAS M.A.; DISALVO E.A.

carlos Paz , cordoba

Congreso; XLII Reunión Anual de sociedad Argentina de biofísica; 2013

FTIR measurements showed that the macroscopic disordered state of the lipid phase has, at the molecular level, different arrangements according to the lipid composition. Thus, fluid membranes are not always fluid because membrane has, according to the acyl chain composition, different ratios of connected and isolated methylene populations. In this work, we have reproduced the typical curves of  changes in the asymmetric stretching modes of CH2 of phosphatidylcholines with temperature by modelizing the membrane system as a Lattice gas in which each element of the network (the CH2 residues of the acyl chains) is coordinated with six neighbors. The change of the free energy of each element state is given by modulated by a cooperativity parameter accounting for the interaction with the first neighbours. The Monte carlo simulation was run using the dynamic of  Glauber or single spin flip and the algorithm of Metropolis. The results show that the introduction of water blocks the membrane elements frustrating some of the elements to contribute to the transition. In consequence, the freezing/frustration of some elements changes the number of isolated elements as found by FTIR spectroscopy. It is concluded that the state of the cooperative units in the  so called fluid state are not similar due to the chemical composition of the acyl change and the water amount buried in it.