Hydration and Order in Lipid Bilayers

The relationship between membrane lipid bilayer hydration and acyl chain order was investigated using time-resolved fluorescence spectroscopy. The degree of hydration in the head group region was assessed from fluorescence lifetime data along with fluorescence intensity measurements in D2O, relative to H2O buffer, using N-(5-dimethylaminonaphthalene-1-sulfonyl)dipalmitoylphosphatidylethan ola mine (dansyl-PE). The degree of hydration in the acyl chain region was estimated from its effect on the fluorescence lifetime of 1-palmitoyl-2-[[2-[4-(6-phenyl-trans-1,3,5-hexatrienyl)phenyl]ethyl] carbonyl]-3-sn-phosphatidylcholine (DPH-PC), and acyl chain order was determined from time-resolved anisotropy measurements of the DPH-PC. Comparisons of sn-2 unsaturation with sn-1,2 diunsaturation in phosphatidylcholine (PC) bilayers with the same number of double bonds/PC revealed a marked difference in interchain hydration and acyl chain order but little difference in terms of head group hydration. For diunsaturated dioleoyl-PC (DOPC) bilayers with two double bonds/PC, the DPH-PC fluorescence lifetime data indicated a greater level of interchain hydration than 1-palmitoyl-2-docosahexaenoyl-PC (PDPC) with six double bonds/sn-2 chain. By contrast, the head group hydration for DOPC was markedly less than for PDPC. A similar lack of correlation of effects on the two regions of the bilayer was found with cholesterol, it having opposite effects on interchain and head group hydration. When DPH-PC fluorescence lifetime data for bilayers composed of a range of different lipids was plotted as a function of acyl chain order, a strong correlation of interchain hydration with acyl chain order was revealed that was independent of lipid composition.