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Influence of DPH on the Structure and Dynamics of a DPPC Bilayer

Jarmila Repáková ^* , Juha M. Holopainen , Michael R. Morrow , Mark C. McDonald , Pavla apková ^* and Ilpo Vattulainen 

^* Department of Chemical Physics and Optics, Faculty of Mathematics and Physics, Charles University, Prague, Czech Republic; Laboratory of Physics and Helsinki Institute of Physics, Helsinki University of Technology, Helsinki, Finland; Department of Ophthalmology, and Helsinki Biophysics & Biomembrane Group, Institute of Biomedicine, University of Helsinki, Helsinki, Finland; and Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John's, Newfoundland, Canada

Correspondence: Address reprint requests to Dr. Ilpo Vattulainen, Tel.: 358-9-451-5805; E-mail: ilpo.vattulainen{at}csc.fi.

We have conducted extensive molecular dynamics (MD) simulations together with differential scanning calorimetry (DSC) and nuclear magnetic resonance (NMR) experiments to quantify the influence of free 1,6-diphenyl-1,3,5-hexatriene (DPH) fluorescent probes on the structure and dynamics of a dipalmitoylphosphatidylcholine bilayer. Atomistic MD simulations show that in the membrane-water interface the influence of DPH is minor, whereas in the acyl-chain region DPH gives rise to major perturbations. In the latter case, DPH is found to influence a wide range of membrane properties, such as the packing and ordering of hydrocarbon tails and the lateral diffusion of lipid molecules. The effects are prominent but of local nature, i.e., the changes observed in the properties of lipid molecules are significant in the vicinity of DPH, but reduce rapidly as the distance from the probe increases. Long-range perturbations due to DPH are hence not expected. Detailed DSC and ²H NMR measurements support this view. DSC shows only subtle perturbation to the cooperative behavior of the membrane system in the presence of DPH, and ²H NMR shows that DPH gives rise to a slight increase in the lipid chain order, in agreement with MD simulations. Potential effects of other probes such as pyrene are briefly discussed.

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