Critical effects from lipid-protein interaction in membranes. I. Theoretical description.

The ordered-fluid phase transition in lipid membranes is described within the framework of the Landau theory, a general theory for phase transitions. The long-range order of lipids is characterized by the orientational order of the hydrocarbon chains implying the ordered-fluid transition to be of first order. Approaching the transition from either side, thermodynamic fluctuations and response functions are shown to increase, e.g. the specific heat, the lateral compressibility, the permeability, and the lateral diffusion coefficient. A protein molecule incorporated in the membrane is represented as a boundary condition on the lipid order at the protein surface. In the surrounding lipid the perturbation falls off exponentially with the coherence length. Assuming the protein molecules to be distributed homogeneously in the membrane plane and the boundary condition to be temperature independent, the protein-induced shift of the transition temperature and the latent heat is calculated. The latent heat decreases linearly with the protein concentration until it vanishes at a critical point. The critical protein concentration is determined by the ratios of the coherence length to lipid and protein radii in the membrane plane. At the critical point the lipid specific heat, lateral compressibility, permeability, and lateral diffusion coefficient become maximal.