Model redox titration

Fig. 7. A comparison of the FTIR and EPR redox titrations of the D. gigas hydro-genase. Top panel FTIR titration based on the height of the low-frequency band. For experimental conditions, see (65). Bottom panel Calculated potentials obtained from EPR-monitor titration and the TED model (80). The vertical axis represents the proportion of a redox species.

An almost complete description of both OH radical-mediated and one-electron oxidation reactions of the thymine moiety (3) of DNA and related model compounds is now possible on the basis of detailed studies of the final oxidation products and their radical precursors. Relevant information on the structure and redox properties of transient pyrimidine radicals is available from pulse radiolysis measurements that in most cases have involved the use of the redox titration technique. It may be noted that most of the rate constants implicating the formation and the fate of the latter radicals have been also assessed. This has been completed by the isolation and characterization of the main thymine and thymidine hydroperoxides that arise from the fate of the pyrimidine radicals in aerated aqueous solutions. Information is also available on the formation of thymine hydroperoxides as the result of initial addition of radiation-induced reductive species including H" atom and solvated electron. 

A model for the redox chemistry of the Ni site based on redox titrations of H2ase is summarized in Figure 1. Ni,Fe H2ases are generally isolated in air as a combination of two fully oxidized and inactive forms that can be distinguished by their Ni EPR spectra and their kinetics of activation. Form A (g = 2.31, 2.23, and 2.02) requires extensive incu-... 

Figure 8.25. Redox titration curve of a model groundwater system (a) pe response and (b) pH response. Numbered segments correspond to sequential reduction of (1) 02(aq), (2) N03 (aq), (3) Mn02(s), (4) Fe(OH)3(s), and (5) S04"(aq). (From Scott and Morgan, 1990.)...

The redox status of an aqueous system is described by the concentrations of the oxidized and reduced species of all system components. Redox systems, generally not at equilibrium as the result of kinetically slow redox reactions, are poorly characterized by intensity factors (Ej or pE) alone. Capacity factors, which reflect the total concentration of relevant species, are conservative parameters that can be meaningful guides to the redox status of aqueous systems. Oxidative capacity (OXC) is defined as a conservative quantity that incorporates a comprehensive chemical analysis of the redox couples of an aqueous system into a single descriptive parameter. OXC classifies aqueous systems in terms of well-defined geochemical and microbial parameters (e.g., oxic, sulfidic). Examples of model and actual groundwater systems are discussed to illustrate the concept. A redox titration model is another tool that is useful in describing a redox system as it approaches an equilibrium state. 

Figure 3. Path of redox titration of a model groundwater system in pH-pE space and the effect on the speciation of a trace element, e.g., selenium (Sej = 0.1 im).

An outgrowth of mediated potentiometric redox titrations has been the evolution of various models which attempt to explain why biological molecules do not readily communicate their redox states directly to potential-indicating electrodes. Insulation of the redox site from the electrode by the protein portion of the molecule, the lack of the requisite enzyme character on an electrode surface, and adsorption of the biological molecule on the electrode surface are among the features of models which have been proposed. 

To obtain the Em of each of high-potential heme it is possible to use the computer analysis of the redox titration curve based on some models (see for example [1]). The approach based on spectral decomposition allow us to obtain more detailed information about Eh curves of hemes without any assumptions. 

Redox titrations using EPR and laser flash absorption spectroscopy of the electron acceptor complex in Phormidium laminosum photosystem 2 particles which apparently lack (4) are reported as these preparations offer a model system to determine the identity of the components of the electron acceptor complex. The g=1.6 EPR signal previously described in Synechocystis has also been observed in P. laminosum (5). The redox properties and effect of inhibitors and pH on the g=1.6 signal suggest its appearance reflects the redox behaviour of Qj. ... 

In the practice of potentiometric titration there are two aspects to be dealt with first the shape of the titration curve, i.e., its qualitative aspect, and second the titration end-point, i.e., its quantitative aspect. In relation to these aspects, an answer should also be given to the questions of analogy and/or mutual differences between the potentiometric curves of the acid-base, precipitation, complex-formation and redox reactions during titration. Excellent guidance is given by the Nernst equation, while the acid-base titration may serve as a basic model. Further, for convenience we start from the following fairly approximate assumptions (1) as titrations usually take place in dilute (0.1 M) solutions we use ion concentrations in the Nernst equation, etc., instead of ion activities and (2) during titration the volume of the reaction solution is considered to remain constant. 

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 electrochemical potentials of different states of the [FeNij-hydrogenase enzyme from D. gigas have been probed using oxidative titrations at pH 8.0 [49, 50j. In addition to the redox events of the iron-sulfur clusters the data are best fit to a model involving four different active site redox states between 0 and —400 mV. The potentials of these four redox states are collected in Table 3. 

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