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)dipalmitoylphosphatidylethanol (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. By contrast, there was a complete lack of correlation of acyl chain order with head group hydration of Lipids of varied composition. These results are suggestive of head group and interchain hydration being ''uncoupled'' and may explain why effects on bulk membrane physical parameters, such as acyl chain order or the related parameter membrane fluidity, fail as a reliable predictor of functional effects on membrane proteins.