A microthermodynamic interpretation of fluid states from FTIR measures in lipid membranes: A Monte Carlo Study

PINTO, OSCAR; BOUCHET, ANA; FRÍAS, M DE LOS A.; DISALVO, E ANIBAL

JOURNAL OF PHYSICAL CHEMISTRY B - (Print)

AMER CHEMICAL SOC

Lugar: Washington; Año: 2014 vol. 118 p. 10436 - 10443

1520-6106

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 paper, 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 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, meant as a disordered state.  Taken this hypothetical state as reference  it  is possible to show that the higher states become  less accessible. The model is suitable to describe 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 4-5 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.