On the environment and the rotational motion of amphiphilic flavins in artificial membrane vesicles as studied by electron paramagnetic resonance spectrometry.

This paper continues the studies of vesicle-bound flavins ('anisotropic flavin chemistry'). It is possible to anchor the flavin nucleus in various modes within the lipids/water interface by means of long aliphatic chains and using different saturated lipids, thereby mimicking the specific binding of the coenzyme to the apoprotein in flavoproteins. Based on absorption spectroscopy and EPR spectroscopy studies we explored the rotational mobility and the microenvironment of membrane-bound amphiflavin radicals. N(5)-unsubstituted amphiflavin radicals exhibit a similarly high disproportionation constant as known from isotropic flavin chemistry. However, reasonable stabilization of the radical was achieved by introduction of an alkyl group in position 5 in the reduced state prior to the one-electron oxidation. Adopting the fine structure of the corresponding EPR spectra as assay for the mobility of the semiquinone, we determined rotational relaxation times ranging from 60 ns in the crystalline state down to 10 or 15 ns in the liquid-crystalline state of the membrane. The solvatochromic effect shown by absorption spectra of the membrane-bound flavin radicals reflects a dielectric constant of the microenvironment of c = 30-40, corresponding to the lipid/water interface region. The results obtained in this study are consistent with those obtained previously, from fluorescence analyses, supporting our former conclusions.