Hydration, structure, and molecular interactions in the headgroup region of dioleoylphosphatidylcholine bilayers: an electron spin resonance study |
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Authors: | Ge Mingtao Freed Jack H |
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Institution: | Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853, USA. |
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Abstract: | The relationship between bilayer hydration and the dynamic structure of headgroups and interbilayer water in multilamellar vesicles is investigated by electron spin resonance methods. Temperature variations of the order parameter of a headgroup spin label DPP-Tempo in DOPC in excess water and partially dehydrated (10 wt % water) show a cusp-like pattern around the main phase transition, Tc. This pattern is similar to those of temperature variations of the quadrupolar splitting of interbilayer D2O in PC and PE bilayers previously measured by 2H NMR, indicating that the ordering of the headgroup and the interbilayer water are correlated. The cusp-like pattern of these and other physical properties around Tc are suggestive of quasicritical fluctuations. Also, an increase (a decrease) in ordering of DPP-Tempo is correlated with water moving out of (into) interbilayer region into (from) the bulk water phase near the freezing point, Tf. Addition of cholesterol lowers Tf, which remains the point of increasing headgroup ordering. Using the small water-soluble spin probe 4-PT, it is shown that the ordering of interbilayer water increases with bilayer dehydration. It is suggested that increased ordering in the interbilayer region, implying a lowering of entropy, will itself lead to further dehydration of the interbilayer region until its lowered pressure resists further flow, i.e., an osmotic phenomenon. |
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Keywords: | DH dehydrated DLPC dilauroylphosphatidylcholine DLPE dilauroylphosphatidylethaolamine DMPC dimyristoylphosphatidylcholine DOPC dioleylphosphatidylcholine DPPC dipalmitoylphosphatidylcholine DPPE dipalmitoylphosphatidylethanolamine DPP-Tempo 4-O-(12-dipalmitoyl-sn-glycero-3-phospho) 4-hydroxy-2266-tetramethyl-piperidine-1-oxy DSC differential scanning calorimetry DSPC distearoylphosphatidylchlone ESR electron spin resonance FH fully hydrated MOMD microscopically ordered and macroscopically disordered NLLS nonlinear least-squares PEG polyethylenglycol POPC 1-palmitoyl-2-oleoylphosphatidylcholine PS phospatidylserine 4-PT 4-phosphono-oxy-2266-tetramethyl-piperidine-1-oxy Tc main phase transition temperature of lipid bilayers Tf freezing temperature 10WT 10 wt % of water |
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