Eo’ value

The Eo values for 2-substituted 1,4-benzoquinones (sets 45-4 through 45-7, 45-10) show an average value of pr of 59. Thus the resonance effect predominates. For most of these sets, the Op constants are not the best parameters for correlation. By contrast, the electron reduction potentials (set 45-8) show a Pr value of 39, which indicates predominance of the localized effect. The 2,5-disubstituted 1,4-benzoquinones differ distinctly in their behavior from the 2-substituted 1,4-benzoquinones in that they show an average Pr value of 53. The one-electron reduction potentials of these compounds show about the same composition of the electrical effect, with a value of Pr of 50. The only set of Eq values available for the 2,6-disubstituted 1,4-benzoquinones pve a Pr value of 51, comparable to the values observed for the 2,5-disubsti-tuted 1,4-benzoquinones. The 2,3,5,6-tetrasubstituted 1,4-benzoquinones have... 

A group at the Bach Institute in Moscow was able to isolate a flavine pigment (an isoalloxacine derivative) from the polymer obtained by heating a mixture of three amino acids (glutamic acid glycine lysine = 8 3 1) this exhibited photochemical acivity (e.g., redox reactions such as electron transfer to acceptors with lower Eo values) under both aerobic and anaerobic conditions (Kolesnikov and Kritsky, 1999). 

Considering only the enzyme data, a weakness of the scheme could have been in the mechanism and activity of the thiosulfate reductase. This reductase activity was demonstrable only with methylviologen as the reductant, and no activity was seen with NADPH (Peck 1960). This showed that the reductase required an extremely electronegative electron donor the Eq for methylviologen is -446 mV, whereas that for NADPH is only -320 mV. This requirement for reduced methylviologen is consistent with the Eo value for... 

Oxidation-reduction potential (or redox potential, E) is the potential of compounds to accept electrons and is by convention measured relative to that of hydrogen. Thus E is very negative for NADPH (a strong reductant) but positive for 02 (a strong oxidant). Standard redox potentials (Eo values in volts) refer to standard conditions (1M redox components) at neutral pH (pH 7). The standard free energy change at pH 7 for a particular redox reaction (AGo ) is given by ... 

In the so-called light reactions of photosynthesis electrons (e-) are donated to the chain from H20 and ultimately accepted by NADP+ to yield NADPH. The difference between the Eo values of the NADPH/NADP+ (-0.32 V) and H20/i 02 (+0.82 V) half reactions (electron acceptor minus electron donor) is — 0.32 0.82 V = —1.14V and accordingly AGo = — rcFAEo = +52.6 kcal mol-1 for the overall reaction ... 

These estimates bracket the NASA-JPL and lUPAC recommendations of 6.5x10 and 7.7 x 10 cm molecule s [9,60]. It is therefore possible fo reconcile fhe thermochemistry proposed here with the observed lO + NO2 recombination kinetics while employing reasonable input parameters for the unimolecular model. Nevertheless it must be stressed, as emphasized earlier [16], that there is considerable uncertainty in some of the input parameters to an RRKM analysis, especially the Frot term. It is of interest to compare the present kinetic calculations with the Multiwell [61] Master Equation calculations on this system by Golden [16]. He used a Morse potential to locate the centrifugal maximum, and from the bond extension Frot 2.1 is derived, about 1/7 of fhaf used here. On the other hand, the higher Eo value yields a density of sfafes larger by a facfor of 6, and fhese two factors largely cancel. 

As it follows from (2.1.99), the decay of A s concentration inside the cavity is defined by the exponential where eo is the lowest eigenvalue of (2.1.100). For a given cavity volume V minimal eo value corresponds to the spherical cavity. In this case sq = where I is the sphere radius. 

The Eo value of the cytochrome-c-Fe /cytochrome-c-Fe half-reaction is more positive than that of the FAD/FADHa half-reaction. Thus, half-reaction 3 goes as written, forcing half-reaction 2a to go in reverse—as an oxidation. The overall spontaneous reaction (under standard-state conditions) is in the direction shown in reaction 1. FADH (the reducing agent) is oxidized to FAD by cytochrome-c-Fe" (the oxidizing agent). 

Reaction 1 represents any half-reaction whose Eo value is known (for example, the normal calomel electrode HgaCls2e - 2Hg° +2Cr, or the silver/silver chloride electrode AgCl + le - Ag + Cl )- Reaction 2 represents any other half-reaciion that is H ion dependent (e.g., H ... 

H2). If the Eo value of reaction 1 and the concentrations of X and X " are known, then the E value for half-reaction 1 is known. If the AE value of the overall oxidation-reduction reaction can be measured and the Eo value for reaction 2 and the concentrations of Y and YH2 are known, then the only remaining unknown value (the H ion concentration) can be calculated. In this manner, oxidation-reduction reactions can be used to measure the pH of a solution. 

The EO values determined were compared with the following Nemst equation [8]. 

The static dielectric constant (eo) of the alkali azides (cf. Table 11) is of the order of 6.5 while is 2.3. These values are of the same order as those of the alkali halides. Both the high and low frequency dielectric constants increase in the case of thallous. silver and cuprous azides. This is likely to be due to the increasing polarizibihty of the cations and a reflection of the decreasing ionicity of the lattices. The eo value for thallous azide and silver fulminate are however surprisingly high compared with the other azides. 

Redox potentials have been determined for each of the steps of reduction of the trypsin-solubilized reductase (402) step 1, one electron consumed, Eo = —109 mV step 2, two electrons consumed, Eo = —276 mV and step 3, one electron consumed, Eo = —371 mV at pH 7.0, 25°. As expected, the redox potential of step 3 is more negative than the potential of the NADPH-NADP+ couple and was determined from the dithio-nite titration. The overall potentiometric—spectrophotometric titration curves could be very closely fitted with a computer-generated curve based on the assumptions of four one-electron reduction steps and extinction coefficients of 4.9 and 4.5 mM" cm for the semiquinones, FliH and FI2H the Eo values assumed for steps 2 and 3 were —270 and —290 mV. The precise fit was very sensitive to all of the assumptions (402). 

In accordance with almost universal usage Eo values will be determined in terms of the molality, in, rather than the concentration, C. The reason for this is that the molality scale has the distinct advantage of being independent of the temperature. The relation between Eo values on the two concentration scales will be discussed in Chapter 12. The method used by Brown and Maclnnes 2 for obtaining E values from data on the potentials of cells of type (11) is as follows. If we define... 

When applicable, the extended Debye-Hiickel theory of Gronwail, LaMer and Sandved,21r although rather laborious, furnishes a means for the determination of Eo values. Its use may be illustrated by the work of Cowperthwaite and LaMer 22 on zinc sulphate, using a cell of type (25). Their data are given in Table II. Due to the fact that this... 

As the standard state of bromine we are free to choose the liquid or the gaseous state, of which the former will be selected. Correcting for the liquid junction at a- by the Henderson equation, (23), Chapter 13, and for incomplete saturation with bromine of the solutions used in the cell, Jones and Baekstrom obtain —0.9940 volt at 25° for the Eo value of the combination... 

In measurements of pH, instead of using a reference electrode, such as the tenth normal calomel electrode the Eo value of which is known, it is frequently convenient to establish the value of Eo in the equation... 

Points 5-20 of the specified titration curve belong to the saturated solution section, which is flatter in Fig. 3.6.3. Here, the pP eo values are practically constant apart from some experimental points (points 14-16) located near the... 

Dutton and co-workers 20,4 ) detected, in addition to cytochromes b and 6t, a cytochrome b-like species with the a band at 558 nm at room temperature having the Eo value of about -f-130 mV at pH 7.2 in beef heart mitochondria and submitochondrial particles from pigeon heart. Berden and co-workers (43,46) also reported type b cytochrome with the Eo of -)-154 mV at pH 7.2 in beef heart mitochondria, but the absorption maximum was at 562 nm instead of 558 nm. It is not known whether these two are the same species and/or identical with the reported type b cytochrome in complex II or chromophore 558 in complex III (24). A summary of the spectral and potentiometric properties of type b cytochromes proposed to exist in mitochondria is given in Table III. 

The EO values for the electron carriers in the mitochondrial membrane increase in the same order as the sequence in which they are used in electron transport. The order is consistent with being exergonic for the redox reactions. Figure 15.10 lists the contents of the various multiprotein complexes described below ... 

Note that the value of EO for the ethanol/aldehyde oxidation reaction above is the same magnitude, but opposite sign of the EO value given in Table 15.1. This is because the reactions in Table 15.1 are all written as reductions ... 

In order for electrons to be transferred from the reductant to the oxidant, the oxidant must have a greater affinity for electrons than the reductant does. The standard reduction potential, EO is a measure of the affinity of a molecule (or partial reaction) for electrons measured at pH 7 and 25 C. Table 15.1 shows standard reduction potentials for several reactions/species of interest in biochemistry. Species with a higher standard reduction potential tend to accept electrons from molecules with a lower standard reduction potential. Thus, electrons tend to move from cytochrome c(+2) to Cytochrome a (+3), because EO = 0.25 volts for the cytochrome c(+2) half-reaction whereas EO = 0.29 volts for the cytochrome a(+3) half-reaction. Keep in mind, though, that the movement of electrons from one compound to another based on EO values is only a tendency. If equal concentrations of the various species are present, the EO tells the direction electrons will flow. At other concentrations, the free energy change for the process (see below) must be calculated to determine the direction of electron movement. 

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