Electron transfer, REDOX titrations

For M. capsulatus (Bath), values of +350 and -25 mV, for Ex° and E2°, respectively, vs NHE were obtained from redox titrations by measuring the appearance and disappearance of the EPR signal of Hmv (37). Problems were encountered in producing high yields of Hmv from this organism for use in physical studies, despite the large difference between Ex and E2°, and so the potentials were remeasured. In the latter experiments, two sets of electron-transfer mediators were used,... 

Although electron-transfer reactions occur, the electrode in no way supplies or conducts away the electrons - it is merely a probe of the potentials of the constituent redox couples in solution. Accordingly, potentiometric titrations are always zero-current measurements. 

Mediation is mentioned in Bott, A. W., Redox properties of electron-transfer metalloproteins . Current Separations, 18, 47-54 (1999). In addition, Bott, A. W., Electrochemical titrations, Current Separations, 19, 128-132 (20(X)), and Bott, A. W., Controlled current techniques, Current Separations, 19, 125-127 (2000), are both worth consulting. 

Redox titrations are based on the transfer of electrons between the titrant and the analyte. These types of titrations are usually followed by potentiometry, although dyes which change colour when oxidised by excess titrant may be used. 

This chapter deals with the fundamental aspects of redox reactions in non-aque-ous solutions. In Section 4.1, we discuss solvent effects on the potentials of various types of redox couples and on reaction mechanisms. Solvent effects on redox potentials are important in connection with the electrochemical studies of such basic problems as ion solvation and electronic properties of chemical species. We then consider solvent effects on reaction kinetics, paying attention to the role of dynamical solvent properties in electron transfer processes. In Section 4.2, we deal with the potential windows in various solvents, in order to show the advantages of non-aqueous solvents as media for redox reactions. In Section 4.3, we describe some examples of practical redox titrations in non-aqueous solvents. Because many of the redox reactions are realized as electrode reactions, the subjects covered in this chapter will also appear in Part II in connection with electrochemical measurements. 

In 1972, Ingles reported his studies of Fenton s reagent using redox titration. He found evidence in support of Kremer s complex mechanism theory and concluded that, when suitable complexes are formed, substrates are not oxidized by free radical rather, electron transfer processes might be... 

An oxidation-reduction titration or redox titration is an oxidation-reduction reaction involving a transfer of electron(s) between two substances in solutions. A substance is said to be oxidized when it loses electron(s) and reduced when it gains electron(s). Examples of oxidation-reduction reactions are illustrated below ... 

Redox potentials of the molybdenum centers in several of the enzymes have been obtained by potentiometric titration (Table 3a). Although the substrate reaction chemistry requires the metal center to participate in net two-electron redox reactions, the simple electron-transfer reactions of the active sites occur in one-electron steps involving the MoVI/Mov and Mov/MoIV couples. Several of the molybdenum enzymes studied have MoVI/Mov and Mov/MoIV couples that differ by less than 40 mV. However, in sulfite oxidase the Movl/Mov (38 mV) and Mov/Molv (-239 mV) couples are separated by roughly 275 mV [88], In formate dehydrogenase (D. desulfuricans) the MoVI/Mov (-160 mV) and Mov/MoIV (-330 mV) couples are separated by 170 mV [89], Both the MoVI/Mov and... 

The prepared compounds systematically differed in the distance of the dihydropyridine and the flavin recognition part. Binding between flavin and the NADH model systems was proved by potentiometric pH titrations. Redox reaction between the NADH model systems and flavin was monitored by UV - VIS spectroscopy. The intensity of the long-wave absorption of flavin at 456 nm significantly decreased during the reaction and the decrease was attributed to the reduction of flavin to the fully reduced flavohydroquinone. At the same time, the intensity of the peak around 360 nm decreased as well, because of the reduction of flavin and the concerted oxidation of the 1,4-dihydronicotinamide to the corresponding pyridinium species. Kinetics of the electron transfer was studied and two reasonable kinetic models were proposed. 

The EPR-nondetectable ions in laccase function as a cooperative 2-e" unit (5). With cytochrome oxidase redox titrations based on the heme absorption bands (2) indicate the presence of a high and a low potential site (380 and 220 mV, respectively). On the other hand, the quasi equilibrium established in the rapid initial transfer of electrons from reduced cytochrome c to the primary electron acceptor in the oxidase, cytochrome a, indicates a potential of 285 mV for this site (18). 

Redox titration — A - titration method in which electrons are transferred between the - titrant and the - analyte. Usually, the - end point of oxidation/reduction reactions is measured by chemical or potentiometric methods. The chemical method involves an - indicator that usually has a change in color at the end point, while the other method is a - potentiometric titration [i]. 

The structure determinations did not solve, of course, the molecular mechanisms of the hydrogenase reactions (140). However, spectroscopic investigations and redox titrations of [NiFe] hydrogenases strongly indicated that in these enzymes the nickel-sulfur entity functions as a site of substrate activation and conversion (141). Thus nickel-sulfur complexes that could catalyze reactions 45a and b gained considerable interest. Reaction 45b, which does not require the intermolecular transfer of electrons and thus is simpler than reaction 45a, served as a test reaction for hydrogenase activity. 

The P cluster undergoes redox-dependent structural rearrangement, which can be coupled with the transfer of an electron or a proton to FeMoco. (Peters et al., 1997). The oxidation of the P cluster is accompanied by the coordination of (JSerl88 and the amide nitrogen of aCys88 with the Fe atoms of the P cluster. Redox titration of P-cluster indicates that the redox potential of Pl+ — P2+ transition is pH dependent (0.053 V/pH unit) (Lanzilotta et al., 1998). It was suggested that electron transfer from P-cluster to FeMoco at physiological pH values is accompanied by coupled proton transfer. 

The RDE technique has found widespread use in analytical electrochemistry because of an excellent signal-to-noise ratio resulting from the enhanced mass transport. The RDE method has also been employed for monitoring concentrations in kinetic applications [59], as described for ultramicroelectrodes [60] and in the determination of the stoichiometry for electron-transfer reactions by means of redox titration [61]. The latter procedure will be described next. 

Zaban and co-workers reported the use of chemical redox titrations to measure the potential of sensitizers bound to Ti02 [136], An unexpected result from these studies is that redox couples that are not pH sensitive in fluid solution become pH dependent when bound to the semiconductor surface. The magnitude of the pH-induced shift varied from 21 to 53 mV per pH unit depending on the physical location of the sensitizer. Sensitizers inside the semiconductor double layer track the 59 mV pH shift of the semiconductor. When sensitzers were outside the double layer, their potential was almost independent of the semiconductor. This finding has important implications for the determination of interfacial energetics for dye sensitization and interfacial electron transfer studies [136]. 

---