Lateral organization and domain formation in a two-component lipid membrane system

The thermodynamic phase behavior and lateral lipid membrane organization of unilamellar vesicles made from mixtures of 1,2-dimyristoyl-sn-glycero-3-ph osphocholine (DMPC) and 1,2 distearoyl-sn-glycero-3-phosphocholine (DSPC) w ere investigated by fluorescence resonance energy transfer (FRET) as a func tion of temperature and composition. This was done by incorporating a headg roup-labeled lipid donor (NBD-DPPE) and acceptor (N-Rh-DPPE) in low concent rations into the binary mixtures. Two instances of increased energy transfe r efficiency were observed close to the phase lines in the DMPC/DSPC phase diagram. The increase in energy transfer efficiency was attributed to a dif ferential preference of the probes for dynamic and fluctuating gel/fluid co existing phases. This differential preference causes the probes to segregat e (S. Pedersen, K. Jorgensen, T. R. Baekmark, and O. G. Mouritsen, 1996, Bi ophys. J. 71:554-560). The observed increases in energy transfer match with the boundaries of the DMPC/DSPC phase diagram, as measured by Fourier tran sform infrared spectroscopy (FTIR) and differential scanning calorimetry (D SC). We propose that the two instances of probe segregation are due to the presence of DMPC-rich and DSPC-rich domains, which form a dynamic structure of gel/fluid coexisting phases at two different temperatures. Monitoring t he melting profile of each lipid component independently by FTIR shows that the domain structure is formed by DMPC-rich and DSPC-rich domains rather t han by pure DMPC and DSPC domains.