Thermodynamics of phospholipid tubules in alcohol/water solutions

Circular dichroism and calorimetry have been used to study the melting of self-assembled phospholipid tubules in alcohol/water solutions. The tubules are found to melt either-continuously or discontinuously, depending on the type of alcohol, the alcohol/water ratio, and the lipid concentration. At high proportions of alcohol and low lipid concentrations, the ellipticity continuously decreases with temperature and a very broad peak in the specific heat is observed. In solutions with lower proportions of alcohol or large lipid concentrations, a large drop in ellipticity is accompanied by a sharp peak in the specific heat when the acyl chains disorder. The CD and specific heat measurements can be explained in terms of two distinct thermodynamic processes: the discontinuous melting is a first-order transition of the lipid bilayers from the ordered L-alpha phase to the disordered L, phase, while the continuous melting reflects an increase in the lipid solubility with temperature. These results show that the solvent has an important effect on the thermodynamics of lipid tubules, and that circular dichroism is a sensitive probe of tubule thermodynamics.