Theory of biomembrane phase transitions

Experiments on the chain melting thermal transition in simple biological membranes are briefly reviewed and shown to indicate that a microscopic order-disorder model may be appropriate to describe the thermodynamics of the transition. To test this conclusion further a pair of two dimensional lattice models (A and B) are introduced and the statistical mechanics is solved exactly using dimer techniques. The phenomenological parameters required by the models are evaluated from experiments on other systems. The more realistic of the two models (model A) has a second order transition at a temperature of 353°K compared to 315°K for dipalmitoyl-L-α-lecithin membranes. However, the specific heat peaks do not have the same shape and the transition is broader for model A than for the experiment. In comparison, the less realistic model B has a first order transition at 925°K, considerably higher than the experimental transition temperature. From these results it seems likely that the points of disagreement between model A and experiment may be due to the simplified features of model A. It is concluded that the basic order-disorder model is likely to be correct for the main transition. There is also a smaller transition at a lower temperature which is also discussed in terms of an order-disorder model involving the phospholipid head groups. In this case the calculation shows that the simple model is either wrong or that some additional features must be added.