Potential Dependence of Transmembrane Electron Transfer across Phospholipid Bilayers Mediated by Ubiquinone 10

Abstract

Transmembrane electron transfer across phospholipid bilayer membranes (BLMs) mediated by ubiquinone have been investigated. The membranes were prepared by the brush method and patch-clamp micropipet techniques and were used to separate identical aqueous Fe(II)/Fe(III) citrate solution. Electron transfer across the BLMs was observed on applying a potential difference across the system. The current, and thus the transmembrane electron transfer rate, were found to be dependent on the redox potential of the bathing solutions. The overall transfer coefficient was 0.5, and it was concluded that the rate-determining step is the transfer of the semiubiquinone radical anion across the hydrophobic interior of the membrane, instead of charge transfer reactions at the membrane/electrolyte solution boundaries. The results obtained could not be understood in terms of tunneling to the neutral ubiquinone molecule residing in the bilayer.

Charge-Transfer Processes and Redox Reactions in Planar Lipid Monolayers and Bilayers

“…Therefore, if the electron exchange rate is fast enough, the rate‐limiting step, thus governing the overall reaction, is the diffusion rate. However, for the electrode modified as depicted schematically in Figure 1, this situation is very rare and refers mainly to the special situations where the Red/Ox system can penetrate into the bilayer at the electrode surface, and therefore, its diffusion through this bilayer is the rate‐limiting step (45, 46). Most of the monolayer, bilayer, and multilayer (including supported bilayer lipid membranes) systems seem to belong to the group, where the electron transfer is kinetically limited, which means that the rate of the diffusion of the redox species to the electrode (BLM) surface is not the rate‐limiting step.…”

“…Among these, one should mention tetracyano‐ p ‐quinodimethane (TCNQ), tetrathiafulvalene (TTF), pyrrole, redox dyes, porphyrins, redox enzymes, etc. (18–21, 46, 62, 96, 114). Probably one of the most extensively studied modifiers endowing the BLM with desired electron conducting properties is TCNQ, one of a class of compounds forming so‐called “organic metals”, whose molecules and crystals with π‐associations, formed from π‐electron orbitals, constitute an excellent realization of almost one‐dimensional electronic systems and are characterized by exceptionally good electrical properties.…”

“…Since then numerous studies have been carried out with such a system, some of the most recent studies involving doping of BLMs with, for example, azo pyridinium with a large lanthanide complex counterion or metallochlorophylls for photoelectric response (57, 58, 108), spiropyran alkyl chain substituted derivatives for the purpose of photochemical regulation of their electrical properties (59) or azobenzene derivatives for the regulation of “quasi‐channels” incorporated in the membrane (60). The use of BLMs as a sensor for the direct electrochemical detection of some toxins, like aflatoxin M 1 (61), has also been reported, as well as electrochemical studies of electron‐transfer processes across BLMs mediated by the ubiquinone 10 (62) or TCNQ (tetracyanoquidodimethane) (46). The most common application of conventional BLMs to date, however, is their use as matrix for the reconstitution of native membrane fragments, such as, ion channel proteins, photoactive pigments, and components involved in biophysical, biochemical, and physiological studies (64 and references therein).…”

“…For such a case, one should be able to observe the sigmoidal shape of a voltammetric response, with a plateau current value independent of the potential scan rate and with the detection limit of membrane modifier capable of electronic conductivity extending down to 10 -15 -10 -17 mol cm -2 (81). Until now, however, the above approach seems to be applied only to the UQ 10 incorporated in the Langmuir-Blodgett films at the electrode surface [62] and conducting polymer nuclei deposited in the alkanethiol monolayer-modified electrodes (118,122); we are not aware of similar treatment of other systems, like s-BLMs or h-BLMs.…”

Supported Bilayer Lipid Membranes as Ion and Molecular Probes

“…5,8 It is of interest to note that TCNQ-containing BLMs have been investigated by Schriffin and colleagues. Cheng et al 45 reported a potential dependence of transmembrane electron transfer across PC-BLMs mediated by ubiquinone, whereas Cunnane and Schiffrin 45 studied the kinetics of ionic transfer across adsorbed PC planar lipid bilayers. More recently, Wang et al 46 , using CV, have examined the electrical behavior of TCNQ-modified s-BLMs, displaying different peaks as a function of scan rates.…”