Gel phase polymorphism in ether-linked dihexadecylphosphatidylcholine bilayers

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Abstract
The structure and properties of the ether-linked 1,2-dihexadecylphosphatidylcholine (DHPC) have been examined as a function of hydration. By differential scanning calorimetry, DHPC exhibits an endothermic (chain melting) transition with the transition temperature (limiting value, 44.2.degree. C) and enthalpy (limiting value, .DELTA.H=8.0 kcal/mol) being hydration dependent. For hydration values > 30 wt % water, DHPC exhibits a pretransition at .apprx. 36.degree. C (.DELTA.H=1.1 kcal/mol) and a subtransition at .apprx. 5.degree. C (.DELTA.H=0.2 kcal/mol). By X-ray diffraction, at 22.degree. C DHPC exhibits a normal bilayer gel structure with the bilayer periodicity increasing from 58.0 to 62.5 .ANG. over the hydration range 9.5-25.4% water. At 30-32% water, two coexisting gel phases are observed with d= 63-64 .ANG. and d=44-45 .ANG.; at higher hydration, only the latter phase is present, reaching a limiting d=47.0 .ANG. at 37.5% water. Two different gel phases clearly exist at low and high hydrations. Electron density profiles at low hydration (9.5-25.4%) show a bilayer thickness dp-p = 46 .ANG., whereas at > 32% water the bilayer thickness is markedly reduced, dp-p = 30 .ANG.. These and other structural parameters indicate a hydration-dependent gel .fwdarw. gel structural transition between a normal bilayer (two chains per polar group) and the chain-interdigitated bilayer (four chains per polar group) arrangement described previously for DHPC [Ruocco, M. J., Siminovitch, D. J., and Griffin, R. G. (1985) Biochemistry 24, 2406-2411]. In contrast, at 65.degree. C the normal liquid-crystalline bilayer structure (d=51.0-60.0 .ANG.; dp-p=38 .ANG.) is present at all hydrations. Comparisons of the hydration-dependent structural behavior of DHPC are made with its ester analogue 1,2-dipalmitoylphosphatidylcholine.