On the redox reactions and accessibility of amphiphilic flavins in artificial membrane vesicles

(Photo-)redox reactions of different amphiphilic flavins bound to artificial membrane vesicles made from three different, saturated phospholipids have been investigated and compared with those of isotropically dissolved flavin. By means of C18-hydrocarbon chains, substituted at different positions, the flavin nucleus can be specifically oriented within the membrane, thereby imposing sterically anisotropic environments, which are liable to control flavin (photo-) chemistry. A spectrophotometric setup was designed, permitting photoreduction of flavin and its simultaneous control by fluorescence. The characteristic temperature dependency of the (vesicle-bound) flavin photoreduction by external and internal photosubstrates, as studied for the different lipid/flavin systems, is explained by the displacement of the flavin nucleus from the area of the polar head groups of the lipid into the more hydrophobic parts of the membrane upon phase transition (gel→liquid crystalline). Evidence is presented that this flavin displacement is correlated with the pre-phase transition rather than with the main phase transition, supporting a former hypothesis of the structural nature of the pre-phase transitions. The transport of redox equivalents across flavin-charged membranes is discussed. The accessibility of vesicle-bound flavins by a variety of exogeneous ions (H+, Cs+, EDTA, NTA, BH3CN−, I−, N3−) is explored as a function of temperature, i.e., membrane phase which, in turn, appears to control the permeability of the lipid/water interface. Therefore, it appears indispensable to include the interface as a separate structural entity into any theory on membrane transport.