Electronic processes and redox

Electronic Processes and Redox Reactions in Bilayer Lipid Membranes... 

Basics of Cyclic Voltammetry. Electrochemical techniques such as cyclic voltammetry (CV) and linear sweep voltammetry (LSV) are most appropriate to the study of electronic processes and redox reactions. These techniques are conceptually elegant and experimentally simple thus they are popular for studying redox reactions at the electrode-solution interfaces and have been increasingly employed by electrochemists (2, 7). Several remarks regarding the cyclic voltammograms of electron-conducting BLM should be made. 

In D2O, HD was found instead of 0-H2. It is presently assumed that binding of hydrogen to a metal ion in the bimetallic active site weakens the H-H bond sufficiently to enable this reaction. Oxidation of the hydride is expected to be a two-electron process, and hydrogenases should, therefore, contain a redox unit capable of accepting these two electrons simultaneously. I assume here that the bimetallic center plus the conserved proximal Fe-S cluster perform this task. 

Inasmuch as flavins can accommodate two electrons but possess a relatively stable one-electron intermediate, an obvious question which can be asked of any flavin-mediated two electron redox reaction is whether or not the mechanism includes the radical species on a direct line between reactants and products. The mere observation of semiquinones in a reaction mixture is not sufficient evidence for their intermediacy, due to the existence of side reactions such as comproportionation (F -I- FH2 2 FH-) which can generate radicals rapidly. Bruice has discussed this question from a physical-organic point of view and concluded that there must exist a competition between one-electron and two-electron processes and that the actual mechanism should be determined mainly by the free energy of formation of substrate radical and the nucleophilicity of the substrate. Bruice has analyzed a variety of systems which he feels should proceed via one-electron mechanisms among these are quinone and carbonyl group reduction by FH2... 

The redox properties of dinuclear copper(II) complexes have received extensive attention using cyclic voltammetry measurements, and it was recognized in the early literature that the two copper(II) ions could be reduced to copper(I) at the same potential or at different potentials (Section 53.3.7).30,934,1021,1022 In either case the reduction requires a two electron process and if the E° values are well separated may result in the observation, under favourable circumstances, of a two-peaked cyclic voltammogram (Figure 61b), as in... 

A combination of majority and minority carrier processes has been observed to produce quantum yields in excess of one at both n- and p-type interfaces. In all cases where this has been noted, the redox species employed have been capable of multiple-electron processes. This type of behavior is often seen for the oxidation of carboxylic acids at n-type semiconductors (a two-electron process). It has also been noted for hydrazine oxidation (a four-electron process) and the reduction of hydrogen peroxide. 

Organic Systems. The photooxidation and reduction reactions for most organic compounds require two electron processes and are generally irreversible. However, several phenothiazine dyes, such as Thionine and Methylene Blue, function as reversible two electron redox systems. The reversible photobleaching of chlorophyll may also involve a one or two electron process although the exact mechanism is still in doubt. One electron redox processes for organic molecules are possible... 

It is noteworthy that siroheme is present in the enzymes responsible for catalyzing two out of only three known six-electron processes, and accordingly it is of great interest to try and identify any feature in siroheme that makes it particularly suitable for the mediation of multielectron transfer. A comparison of octaethylporphyrin, octaethylchlorin and octaethylisobacteriochlorin complexes of iron shows that redox potentials and vco of Fe(P)L(CO) and Fe(P)L(CO)2 were nearly independent of the porphyrin. The property that was most dependent upon the macrocycle structure was the potential for ring-based oxidation which increased in the order OEiBC < OEC < OEP.734... 

The oxidation of carbohydrate involves the making and breaking of C-H bonds. This is a two electron process, and the most efficient way of achieving the oxidation will use a two electron oxidant. In many biological systems one of the key redox steps involves the two-electron conversion of a quinone to a hydroquinone (Fig. 10-6). 

As discussed above, a chemical transformation which occurs during the ac electrolysis does not require the intermediate formation of excited states. The chemical reaction may take place in the reduced and/ or oxidized form of a compound. Nevertheless, in this case the electrolysis may still lead to the same products as those of the photolysis due to the obvious relationship between electronic excitation and redox processes. It will be then quite difficult to elucidate the mechanism of electrolysis. This reaction type may apply to the electrochemical substitution of Cr(CO) (59). 

Proton transfer is a particularly important transport process. Beyond acid-base reactions, proton transfer may be coupled to electron transfer in redox reactions and to excited-state chemistry. It is of enormous significance in biochemical processes where it is an essential step in hydrolytic enzyme processes and redox reactions spanning respiration, and photosynthesis where proton motion is responsible for sustaining redox gradients. In relatively recent times, proton transfer in the excited state has undergone significant study, primarily fueled by advances in ultrafast spectroscopy. 

In many cases, synthetic iron porphyrins have been investigated primarily as models of hemes in various proteins, but also as potentially useful magnetic materials, or as metal coordination complexes, since the porphyrin Macrocycle often confers upon the metal unique electronic, magnetic, and redox properties see Redox Properties Processed and chemical reactivities that are interesting in their own right. The properties and chemical reactivities of iron porphyrins will be summarized in this article. Iron complexes of related macrocycles, including reduced hemes (chlorins, isobacteriochlorins, etc.)... 

Table 18.5 lists electrode potentials for common one-electron processes. These redox species are also frequently used as electron transfer mediators in sensors and biosensors. 

---