Abstract
VO 2+ was used as a probe of polar headgroup dynamics in fully hydrated dipalmitoylphosphatidylcholine membranes. The ESR spectrum exhibited an anisotropic pattern consisting of g zz and g xx (= g yy ) components, which is characteristic of VO 2+ in a slow motion regime. The VO 2+ probe was capable of detecting the main transition from gel to liquid crystalline phase through a change in spectral characteristics. Liouville lineshape analysis revealed that the rotational diffusion of VO 2+ in the liquid crystalline phase is two orders slower than that of the choline terminal methyls, indicating that VO 2+ experiences the motion of the headgroup back-bone due to a tight binding to the phosphate group. The probe also detected the lack of phase transition in the membrane containing 40 mol% cholesterol, which agrees with the depiction of recent phase diagrams. As viewed using the probe, incorporation of cholesterol enhanced the rotational diffusion of DPPC headgroups in the gel phase by a factor of approximately 6, but produced no significant effect in the liquid crystalline phase. The VO 2+-probing method is applicable for investigating motion with diffusion rates larger than 1×10 6 s −1 in phosphatidylcholine membranes.
Published Version
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