Quantitation of the Förster energy transfer for two-dimensional systems. I. Lateral phase separation in unilamellar vesicles formed by binary phospholipid mixtures.

An analytical solution is presented for the rate of energy transfer in unilamellar vesicles formed by binary mixtures of phospholipids showing lateral phase separation. The analytical approach developed here is mainly based on geometrical considerations and, therefore, is formally different for lateral phase separation phenomena taking place in the gel and in the liquid crystalline states of the lipid system. The rate of energy transfer among donor and acceptor molecules attached to chemically different phospholipids is mathematically correlated to the average cluster size of the less-rich component of the binary mixture, thus allowing its calculation from experimental measurements. Moreover, the equations derived here permit the calculation of the average cluster size as a function of the concentration of each lipid component within certain ranges, and this can be used to improve our knowledge of the thermodynamics of these processes.