Gel phase polymorphism in ether-linked dihexadecylphosphatidylcholine bilayers.

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 degrees C) and enthalpy (limiting value, delta H = 8.0 kcal/mol) being hydration dependent. For hydration values greater than 30 wt % water, DHPC exhibits a pretransition at approximately 36 degrees C (delta H = 1.1 kcal/mol) and a subtransition at approximately 5 degrees C (delta H = 0.2 kcal/mol). By X-ray diffraction, at 22 degrees C DHPC exhibits a normal bilayer gel structure with the bilayer periodicity increasing from 58.0 to 62.5 A over the hydration range 9.5-25.4% water. At 30-32% water, two coexisting gel phases are observed with d = 63-64 A and d = 44-45 A; at higher hydration, only the latter phase is present, reaching a limiting d = 47.0 A 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 A, whereas at greater than 32% water the bilayer thickness is markedly reduced, dp-p = 30 A. These and other structural parameters indicate a hydration-dependent gel----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., & Griffin, R. G. (1985) Biochemistry 24, 2406-2411].(ABSTRACT TRUNCATED AT 250 WORDS)