A theoretical model of phospholipid dynamics in membranes

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Abstract
Static and dynamic properties related to the internal configurational motions have been calculated for the alkyl chains of phospholipid molecules in a membrane environment in the liquid crystal phase. The calculations have been performed for the chain 1 of 1,2-dipalmitoyl 3-sn-phosphatidylcholine (DPPC), a typical constituent of phospholipid membranes. Under the assumption of fixed bond lengths and bond angles, the internal dynamics of the chain is described in terms of 15 dihedral angles. The time evolution of the angular variables is assumed to be diffusional in character, and a master equation for transitions among the stable conformers is constructed from the energetics and hydrodynamics of the chain. This method is an extension to the time domain of the rotational isomeric state (RIS) approximation, which has been widely used to compute static properties of the chains. After calculation of the suitable correlation functions, effective rate constants relevant for spectroscopic and kinetic observables have been computed, and the results have been compared with those obtained by recent Brownian dynamics (BD) calculations. The position dependence of the rate constants along the chain has been examined with special reference to understanding the effects resulting from cooperativity in the conformational transitions. The overall spinning and tumbling of the chain has also been described by a diffusive model. The calculated spectral densities for the composite motional process have been used to rationalize the behavior of the relaxation times T1, T2, and T1ρ measured in deuterium nuclear magnetic resonance (NMR) experiments.

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