Interdigitated lamellar phases in the frozen state: Spin-label CW- and FT-EPR.


Department of Chemistry and Chemical Technologies, University of Calabria, 87036 Rende (CS), Italy. Electronic address: [Email]


Interdigitated lamellar phases composed of dipalmitoylphosphatidylcholine (DPPC) and equimolar content of lyso-palmitoylphosphatidylcholine (Lyso-PPC) or DPPC hydrated in ethanol containing water (60% v/v) have been studied in the frozen state. Electron paramagnetic resonance spectra of labeled lipids at C5 or C16 carbon atom positions along the chain are indicative of segmental librational motion over the temperature range 120-260 K. For any dispersion, the mean-square-angular amplitudes of the librations are comparable for both label positions but are larger in DPPC/etOH than in DPPC/Lyso-PPC interdigitated sample. The temperature dependences of the librational amplitudes of the labels in the lipid matrices show a rapid increase at the dynamical transition at Td ≈ 220 K with an activation energy of 20-30 kJ/mol. Three-pulse electron spin echo envelope modulation by D2O revealed comparable solvent accessibility and fractions of singly and doubly hydrogen-bonded nitroxides to deuterons for both positional isomers in the interdigitated lamellae at 77 K. The overall EPR results indicate that the interdigitated DPPC/etOH sample is more loosened packed compared to DPPC/Lyso-PPC sample. The findings of the present work obtained at cryogenic temperatures point out dynamic and molecular properties of interdigitated lamellae that contribute to the biophysical characterization of membrane model systems.

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