MONTE-CARLO SIMULATION OF LATERAL DISTRIBUTION OF MOLECULES IN A 2-COMPONENT LIPID-MEMBRANE - EFFECT OF LONG-RANGE REPULSIVE INTERACTIONS

Recently a series of dips has been observed in the plot of E/M (the ra tio of excimer to monomer fluorescence intensity) vs the mole fraction of 2-(10-pyrenyl)decanoyl-sn-glycero-3-phosphocholine (Pyr-PC) in Pyr -PC/DMPC binary mixtures in the liquid crystalline phase at 30 degrees C (Tang and Chong Biophys. J. 1992, 63, 903-910). In order to reveal connections between the E/M dips and the lateral distribution of the c omponents, the membrane is simulated by means of Monte Carlo methods. The simulations confirm the assumption that pyrene-labeled acyl chains tend to distribute regularly in the membrane at mole fractions where E/M dips are detected. The simulations also show that through the assu mption of long-range repulsive interaction between the pyrene-labeled acyl chains (a) the area of regular arrangements have maxima at the cr itical concentrations if the repulsion is strong enough, (b) in the ca se of weak repulsion the area of regular arrangements is a monotone in creasing function of the concentration of the labeled chains, (c) when an E/M dip is measured at critical concentration X(i), then a cluster of type i regular arrangement of labeled chains is always percolated throughout the membrane, (d) when an E/M kink is measured at X(i), a c luster of type i is rarely percolated, (e) between one critical concen tration X(i) and the next one X(i+1), an (order of type i) --> (disord er) and then a (disorder) --> (order of type i+1) phase transition tak es place.