A Microthermodynamic Interpretation of phase States of lipid membranes by FTIR , monolayer and simulations studies ements in Lipid Membranes: A Monte Carlo Study

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

Bahia Blanca

Congreso; XII Trefemac; 2014

Membranes that surround cells are self-organized assemblies of lipids and proteins, highly selective permeability barriers controlling the flow of information between cells and their environments. It is accepted that the lipid bilayer is the underlying structure of most, if not all, biomembranes.In this work, a critical analysis of the classical paradigms describing membrane properties that should be overcome to have a more realistic structural and thermodynamics picture in correlation to membrane functional response is presented.The present new proposal considers the membrane as a responsive (reactive) structure in which the interfacial free energy is modulated by the water state attached to the lipid molecules. The new approach is backed by monolayer studies complemented with FTIR spectroscopy revealing that water immediately beyond the hydration shell of the phospholipids is sensitive to peptide insertion. This region has particular compressibility properties and excess free energy. Expansion of the monolayer gives place to measurable perturbations produced by solutes present in the aqueous media in particular, peptides, aminoacids and proteins. These perturbations produce discrete amounts of water molecules that reaccomodates in the lipid interphase.Fourier Transform Infrared spectroscopy (FTIR) is usually employed to obtain transition temperatures of lipids and lipid mixtures and the effect on it of several effectors, such as cholesterol. In this paper, we demonstrate that data obtained by means of FTIR spectroscopy measurements contain information about the microscopic thermodynamics of the lipid phase transition, consistent with macroscopic magnitudes such as enthalpy and heat capacitance. By means of Monte Carlo simulation, we have been able to show that the frequency shift of the CH stretching mode of the methylene groups in the lipid acyl chain from low to high values can be taken as a two state transition of molecular constituents in a lattice rearrangement. The elements in each lattice cell are the acyl chain methylene groups. According to the model, at temperatures below Tc all the groups are defined in the lowest energy state defined by the lowest frequency value and therefore they are all connected in a gel lattice. Above Tc, some groups may reach different energy states depending on the restrictions imposed to the groups. Ideally, when all the groups are able to reach the highest frequency, a fully ?fluid? state is reached. All groups in the acyl chain would reach a disordered state. Taken this hypothetical state as reference it is possible to show that the higher states become less accessible when, at constant number of methylene groups, the number of nodes frozen increases or when, at a given number of frozen nodes, the total number of nodes increases. The first case is suitable to describe the effect of cholesterol, which is able to dump the phase transition. The second is congruent with previous data denoting that in the so-called fluid phase the first 4-5 methylene groups remain in the gel state even above Tc. Thus, the frequency value attained above Tc depends on the nature of the lipid acyl chain.The model allows to calculate the main phase transition features as enthalpy and heat capacity.This analysis of the lipid membrane properties as a unique material replaces the classical view to explain the penetration of polar aminoacids into membranes (translocons) by a more realistically one introducing waterons, i.e. defined number of water molecules participating as informational units in the adsorption partition process.