Proton redox state

FIGURE 18.19 The structures and redox states of the nicotinamide coenzymes. Hydride ion (H , a proton with two electrons) transfers to NAD to produce NADH. 

Flavin coenzymes can exist in any of three different redox states. Fully oxidized flavin is converted to a semiqulnone by a one-electron transfer, as shown in Figure 18.22. At physiological pH, the semiqulnone is a neutral radical, blue in color, with a A ax of 570 nm. The semiqulnone possesses a pAl of about 8.4. When it loses a proton at higher pH values, it becomes a radical anion, displaying a red color with a A ax of 490 nm. The semiqulnone radical is particularly stable, owing to extensive delocalization of the unpaired electron across the 77-electron system of the isoalloxazine. A second one-electron transfer converts the semiqulnone to the completely reduced dihydroflavin as shown in Figure 18.22. 

The one-electron reduction of the Ni-C form results in the diamagnetic species Ni-R. From the redox titration studies of Lindahl s group, a plausible catalytic cycle can be postulated where the enzyme in the Ni-Sl state binds H2 (77) and becomes the two-electron more reduced Ni-R state. Sequential one-electron oxidations from Ni-R to Ni-C and then to Ni-Sl will close the cycle (Fig. 6). The various redox states differ not only in the extent of their reduction, but also in their protonation, as shown by the pH dependence of their redox potentials (87). It is remarkable that both EPR (which monitors the magnetic... 

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

Negative values of ° (such as a Na = —2.71 V) indicate that the reduced form of the couple will react with protons to form hydrogen gas, as in Equation (7.35). The more negative the value of e, the more potent the reducing power of the redox state, so for the magnesium couple is —2.36 V, and F K = —2.93. 

Living cells visualization of membranes, lipids, proteins, DNA, RNA, surface antigens, surface glycoconjugates membrane dynamics membrane permeability membrane potential intracellular pH cytoplasmic calcium, sodium, chloride, proton concentration redox state enzyme activities cell-cell and cell-virus interactions membrane fusion endocytosis viability, cell cycle cytotoxic activity... 

Protons are translocated across the membrane by what is described as a proton pnmp . How does the pump operate The change in redox state experienced by the prosthetic gronps of the enzymes in the chain causes conformational changes in the proteins that alter the affinities of some amino acid side-chain gronps for protons. In addition, there is a change in the direction in which these groups face in the membrane. Consequently, oxidation results in an association with a proton on the matrix side of the membrane whereas reduction results in reversal of the direction that the side-chain groups face and an increase in... 

Redox-based biosensors. Noble metals (platinum and gold) and carbon electrodes may be functionalized by oxidation procedures leaving oxidized surfaces. In fact, the potentiometric response of solid electrodes is strongly determined by the surface state [147]. Various enzymes have been attached (whether physically or chemically) to these pretreated electrodes and the biocatalytic reaction that takes place at the sensor tip may create potential shifts proportional to the amount of reactant present. Some products of the enzyme reaction that may alter the redox state of the surface e.g. hydrogen peroxide and protons) are suspected to play a major role in the observed potential shifts [147]. 

In solvents, the complexes of metal ions with organic ligands are more soluble, which makes studies easier moreover, in anhydrous and aprotic media, it is possible to reduce the catalysts and their oxygen complexes out of reach of protons and thus to study the first steps of the catalytic cycle. It has been possible to show by studies in benzonitrile that the mixed-valence Co Co redox state of the... 

There are several demands that must be more or less fulfilled by the mediator before a successfull amperometric detection of NADH with CMEs can be realized. Despite having a E° lower or comparable with the optimal working potential range for amperometric detection, the mediator should exhibit fast reaction rates both with the electrode proper and NADH, and also be chemically stable at any redox state. Furthermore, the redox reaction of the mediator should involve two electrons and at least one proton making possible, at least theoretically, a fast inner sphere hydride transfer in the homogeneous reaction with NADH. 

This has been demonstrated in EQCM studies of PTh film redox switching in HC104 solutions of different concentration. Under conditions where thionine reduction is a 2e/3H+ process, electroneutrality alone predicts uptake of one anion (and three protons) per Th site a film mass increase of 102.5 g molTh"1, independent of solution composition. Experimentally, the mass change is less than 20 g mol 1 in 1 mol dm 3 HC104, decreases as the electrolyte is diluted, and even becomes negative at pH > 2 The variation of mass change with concentration is attributable to activity effects. Hydronium perchlorate is included within the film, to an extent dependent on polymer redox state and solution concentration. 

The paramagnetic Ni-C state is a part of the catalytic cycle and has the redox state Ni(III)Fe(II). Hall et al. proposed that the ligand X in Ni-C is an H atom and one of the cysteine residues is protonated, based on a comparison between their DFT study and experimental data.51,62-64 Other groups have come to the same conclusion.51,52,55 Recently, however, Stein et al. have stated a slightly different opinion. Based on g- and hyperfme-tensor experimental data and their relativistic DFT calculations, Stein et al. conclude that Ni-C has no protonated cysteine residue.38... 

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