USE OF VO2-PROBE FOR DYNAMICS OF POLAR HEADGROUPS IN PHOSPHATIDYLCHOLINE BILAYERS( AS A SPIN)

VO2+ 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)) compone nts, which is characteristic of VO2+ in a slow motion regime. The VO2 probe was capable of detecting the main transition from gel to liquid crystalline phase through a change in spectral characteristics. Liouv ille lineshape analysis revealed that the rotational diffusion of VO2 in the liquid crystalline phase is two orders slower than that of the choline terminal methyls, indicating that VO2+ experiences the motion of the headgroup back-bone due to a tight binding to the phosphate gr oup. The probe also detected the lack of phase transition in the membr ane containing 40 mol% cholesterol, which agrees with the depiction of recent phase diagrams. As viewed using the probe, incorporation of ch olesterol 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 VO2+-probing method is app licable for investigating motion with diffusion rates larger than 1 x 10(6) s(-1) in phosphatidylcholine membranes. Copyright (C) 1997 Elsev ier Science Ireland Ltd.