Reduction potential,

The reduction potentials of different plastocyanins increase as the pH is decreased below 7 (Fig. 8), due to protonation of His 87 at the Cu(I) active site and resultant redox inactivation. Much of this information has been obtained from rate constants (18, 57, 58). The reduction potential of the basic A. variabilis plastocyanin is noticeably smaller than for plastocyanins from higher plant and green algal 

Variation of reduction potential with pH for the spinach, parsley, and Ana-baena variabilis plastocyanins, the PCu(II)/PCu(I) couple. 

The variation of midpoint potentials ofP. aeruginosa azurin with pH (Fig. 9) has been determined using the [Fe(CN)6] couple (59). From a fitting of values at different [H ] to Eq. (1), 

Variation of reduction potential with pH for the Pseudomonas aeruginosa azurin ACu(II)/ACu(I) couple from titrations with [FelCNlel ( ), and from rate constants for ACu(I) + [FelCNleP and [FelCNle) + ACu(II) ( ). 

Substitution of Se for S in Met 121 results in an increase in reduction potential of azurin fromP. aeruginosa by 30 mV (65). On changing Met 121 to Leu 121 by site-directed mutagenesis, the reduction potential is increased by 70 mV (66). Also by site-directed mutagenesis, replacement of the conserved Met 44 by Lys in the hydrophobic region results in a 40- to 60-mV increase in reduction potential (67). 

The reduction potential is central for the function of electron-transfer proteins, since it determines the driving force of the reaction. In particular, it must be poised between the reduction potentials of the donor and acceptor species. Therefore, electron-transfer proteins normally have to modulate the reduction potential of the redox-active group. This is very evident for the blue copper proteins, which show reduction potentials ranging from 184 mV for stellacyanin to 1000 mV for the type 1 copper site in domain 2 of ceruloplasmin [1,110,111]. 

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The UV-visible absorption spectrum of Ru(2,2 -bipyridine)3 maximum at about 450 nm, from which the energy in volts for process XI-39 may be estimated. The standard reduction potential for the R" /R couple is about 1.26 V at 25°C. Estimate from this information (and standard reduction potentials) the potential in volts for processes XI-40 and XI-41. Repeat the calculation for alkaline solutions. 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

This behaviour also stands for functionalized [60]fullerene derivatives, with, however, a few striking differences. The most obvious parameter is the negative shift of the reduction potentials, which typically amounts to -100 mV. Secondly, the separation of the corresponding reduction potentials is clearly different. Wlrile the first two reduction steps follow closely the trend noted for pristine [60]fullerene, the remaining four steps display an enlianced separation. This has, again, a good resemblance to the ITOMO-LUMO calculations, namely, a cancellation of the degeneration for functionalized [60]fullerenes [31, 116, 117]. 

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

Thennodynamic stability is generally provided for noble metals in most media as tlieir oxidation potential is more anodic tlian tire reduction potential of species commonly occurring in tire surrounding phase. However, for many materials of technological and industrial importance tliis is not tire case. 

Although the reduction process is not always a reversible one, oxidation and reduction potential values can be sometimes related to the Hiickel energies of the highest and lowest filled molecular orbital of the dye (108). 

Name Reduction Potential (30°C) in Volts at Suitable pH Range Color Change Upon Oxidation... 

Since the potential for a single half-reaction cannot be measured, a reference halfreaction is arbitrarily assigned a standard-state potential of zero. All other reduction potentials are reported relative to this reference. The standard half-reaction is... 

Appendix 3D contains a listing of the standard-state reduction potentials for selected species. The more positive the standard-state reduction potential, the more favorable the reduction reaction will be under standard-state conditions. Thus, under standard-state conditions, the reduction of Cu + to Cu E° = -1-0.3419) is more favorable than the reduction of Zn + to Zn (E° = -0.7618). 

Consequently, solutions of Fe + and Fe + are buffered to a potential near the standard-state reduction potential for Fe +. 

The electrochemical potential for the reaction is the difference between the reduction potentials for the reduction and oxidation half-reactions thus,... 

Substituting known values for the standard-state reduction potentials (see Appendix 3D) and the concentrations of Ag+ and gives a potential for the electrochemical cell in Figure 11.5 of... 

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