A METHOD FOR CHARACTERIZING TRANSITION CONCERTEDNESS FROM POLYMER DYNAMICS COMPUTER-SIMULATIONS

A statistical method based on classifying the transitions among a set of dihedral angles within an ''energy, transfer window'' is developed, and used to analyze Brownian (BD) and molecular dynamics (MD) simulat ions of the acyl chains in a lipid bilayer, and MD of neat hexadecane. It is shown for the BD simulation that when a transition of the dihed ral angle in the center of the chain occurs, a transition of a particu lar next nearest neighbor (or angle 2-apart) will follow concertedly w ith a probability of approximately 0.10 within a time window of approx imately 3 ps. The MD bilayer simulations, which are based on a more fl exible model of the hydrocarbon chains, yield corresponding concerted transition probabilities of approximately 0.083 and window sizes of 1- 2 ps. An analysis of angles 4-apart yields concerted transition probab ilities of 0.03 and 0.04 for the ED and MD bilayer simulations, respec tively, and window sizes close to those of the corresponding 2-apart c ases. Statistical hypothesis testing very strongly rejects the asserti on that these follower transitions are occurring at random. Similar an alysis reveals marginal ol no evidence of concertedness between 1-apar t (nearest neighbor) and between 3-apart dihedral angle transitions. T he pattern of concertedness for hexadecane is qualitatively similar to that of the lipid chains, although concertedness is somewhat stronger for the 3-apart transitions and somewhat weaker for those 4-apart. Fi nally, it is suggested that the diffusion of small solute molecules in membranes is better facilitated by nonconcerted transitions, which ar e associated with relatively large displacements of the chains, than b y concerted transitions, which do little to change the chain shape. (C ) 1995 John Wiley & Sons, Inc.