Thermodynamic fluctuations in phospholipid bilayers

A simplified statistical mechanical model of the solid–fluid melting transition of wet lipid bilayers is developed which allows for calculation of thermodynamic fluctuation effects. The model is used to estimate the lowering of the free energy barrier for transbilayer ion permeability due to enhanced lateral compressibility near the melting point. The calculation is used to fit permeability data for dipalmitoyl phosphatidyl choline vesicles and the fit provides evidence that the bilayer is quite close to a critical temperature at which the melting transition would go from first to second order.