Microthermodynamic interpretation of fluid states from FTIR measurements in lipid membranes: a Monte Carlo study

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. However, no interpretation of the molecular information provided by the frequency shift to higher values observed at Tc is available. In this article, we demonstrate that data obtained by means of FTIR measurements contain information about the microscopic thermodynamics of the lipid-phase transition. By means of Monte Carlo simulation, we have been able to show that the frequency shift from low to high values can be taken as a two-state transition of molecular constituents in a lattice rearrangement. According to the model, at temperatures below Tc all of 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 on the groups. Ideally, when all of the groups are able to reach the highest frequency, a fully "fluid" state is reached, which is a disordered state. If we take this hypothetical state as a reference, it is possible to show that the higher states become less accessible. The model is suitable for describing the effect of cholesterol, which is able to dump the phase transition and is congruent with previous data denoting that in the so-called fluid phase the first four to five methylene groups remain in the gel state even above Tc. The frequency value attained above Tc depends on the nature of the lipid acyl chain.