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

Abstract

Summary. (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-,

Further photophysical and photochemical characterization of flavins associated with single-shelled vesicles

“…Note, photoreduction by both exogeneous EDTA and the intrinsic donor (cf. [41]) follow exactly the same iodide dependence, indicating that the availability of the electron donor is not rate limiting differences in the mechanisms of photoreduction by the intrinsic donor between natural lipids (Fig. 7B) and synthetic lipids (Fig.…”

“…In a previous paper we demonstrated that vesicle-bound amphiphilic flavin could be reduced by an intrinsic donor in the gel state of the membrane as well as by exogeneous EDTA in the liquid crystalline state [41]. Surprisingly, both photoreduction mechanisms for the DPL/AF1 3 system, although obviously different, display precisely the same pattern of quenching by iodide (Fig.…”

“…The pK of the N3-proton of aqueous TARF was determined to be 10 (curve C). The fact that the anionic form shows little or no fluorescence permits its precise determination [15,41]. Clearly, the encapsuled flavin remains fluorescent up to the point when the pH of the bulk solution equals 13.…”

“…Quantum efficiencies of photoreduction and fluorescence are given either in absolute numbers (for details cf. [35,41]), or, for the purpose of comparison (cf. Figs.…”

“…1 In these experiz Abbreviations and Definitions." DML: L-~, y dimyristoyl-~-lecithin, Tc=23 ~ DPL: L-fl, 7 dipalmitoyl-~-lecithin, T~= 41 ~ DSL: L-fl, 7 distearoyl-~-lecithin, Tc=58 ~ AFI 3: 7,8,10 trimethyl-3-octadecyl-isoalloxazin; EDTA: Ethylenediaminetetraacetic acid; Tc: Phase-transition temperature; 3-me-LFI: 3-methyl-lumiflavin; LF1-3-aa: Lumiflavin-3-acetic acid; ments we have determined that the phase of the lipid is a highly significant parameter capable of controlling virtually all basic physico-chemical properties of the flavin nucleus within the lipoid microenvironment [25,33,34,36,37,41].…”