The results of a Brownian dynamics simulation of hydrocarbon chain in a membrane bilayer were used to analyze NMR relaxation experiments to yield information concerning the dynamics and ordering of lioids in vesicles. It was shown that the frequency dependence of the data does not arise from trans-gauche isomerizations or from axial rotation of the entire molecule. Quantative agreement is found using a model in which fast axial rotations (approximately 100ps) and large amplitude (with order parameter approximately 0.6) but slow director fluctuations (approximately 10ns) are superimposed on the internal motions. This work supports a very fluid picture of the interior of the bilayer in contrast to the commonly accepted model in which crankshaft motions predominate. Using the stochastic theory of chemical reactions and the theory of first passage times, a simple analytic expression is derived for the distribution of delay times that has been observed in studies of the polymerization kinetics of sickle hemoglobin under conditions where the polymerization progress curves exhibit stochastic variation. The rate of homogeneous nucleation can be readily extracted from such experiments using this expression. This work constitutes a significant addition to the rather limited number of examples where contact can be successfully made between the stochastic theory of chemical kinetics and experiment. The influence of internal conformational dynamics in the electron transfer reaction between a donor and an acceptor was examined. The steady stare flux resulting from the coupling of two multistate systems was shown to be identical to that calculated from a simple kinetic scheme involving only four states, if the effective rate constants of this reduced scheme are approximately defined in terms of the mean first passage times for moving between various points along the multistate cycles.
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