Voltammetry quasi-reversible redox process

As discussed before in the case of nucleic acids the authors have also considered the incidence of the interfacial conformation of the hemoproteins on the appearance of the SERRS signals from the chromophores. Although under their Raman conditions no protein vibration can be observed, the possibility of heme loss or protein denatura-tion are envisaged to explain a direct interaction of the heme chromophores with the electrode surface in the case of the adsorl Mb. extensive denaturation of Cytc at the electrode appears unlikely to the authors on the basis of the close correspondence of the surface and solution spectra. Furthermore, the sluggish electron transfer kinetics measured by cyclic voltammetry in the case of Cytc is also an argument in favour of some structural hindrance for the accessibility to the heme chromophore in the adsorbed state of Cytc. This electrochemical aspect of the behaviour of Cytc has very recently incited Cotton et al. and Tanigushi et al. to modify the silver and gold electrode surface in order to accelerate the electron transfer. The authors show that in the presence of 4,4-bipyridine bis (4-pyridyl)disulfide and purine an enhancement of the quasi-reversible redox process is possible. The SERRS spectroscopy has also permitted the characterization of the surface of the modified silver electrode. It has teen thus shown, that in presence of both pyridine derivates the direct adsorption of the heme chromophore is not detected while in presence of purine a coadsorption of Cytc and purine occurs In the case of the Ag-bipyridyl modified electrode the cyclicvoltammetric and SERRS data indicate that the bipyridyl forms an Ag(I) complex on Ag electrodes with the appropriate redox potential to mediate electron transfer between the electrode and cytochrome c. 

Vitamin B 2 (cyanocob(III)alamin) is an example of a quasi-reversible redox system that exhibits slow heterogeneous electron-transfer kinetics. Cyclic voltammetry alone suggests that the reduction of vitamin B 2 is a single two-electron process at = -0.93 V vs SCE to the Co(I) redox state (Figure lOA). However, thin-layer spectroelectrochemistry using a... 

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

The electrochemistry of Lu(Pc)(OAc)2H has also been studied (Ni et al. 1987). The complex undergoes a reversible one-electron oxidation at 0.58-0.65 V vs. SCE and a quasi-reversible one-electron reduction at -0.98 to -1.09 V, depending on the solvents. The electron-transfer rate constants for both redox processes in different solvents have also been calculated they range from 0.9 to 3.6 cm s". Similarly, the naphthalocyanine analog Lu(l,2-Nc)(OAc) also exhibits one oxidation (0.36 V vs. ferrocenium/ferrocene) and one reduction process (-1.28 V vs. ferrocenium/ferrocene) as revealed by cyclic voltammetry (Guyon et al. 1998). 

Electrochemistry demonstrates the remarkable existence of a reasonably long-lived monomeric Rh(II) species. Cyclic voltammetry shows that [Rh(9S3)2] undergoes reduction in two quasi-reversible one-electron processes rather than one two-electron process (Table 2) [128-130] (cf. the one-electron redox processes of the congeneric [Co(9S3)2]" (n = 3,2,1) couples) [128,131]. Controlled potential coulometry confirms the one-electron nature of both redox processes, and thus buttresses the existence of Rh(II) as a discrete monomeric species of appreciable lifetime (hours). 

The active site (MMS) redox response, which apparently occurs in a fast, quasi-reversible manner, is usually of low magnitude and overlaps with, and is not easily distinguished from, the capacitive double layer response. Eurthermore, when an electrocatalytic process takes place, mediated by the active site species, the catalytic process usually dominates the dc response, i.e., the redox behavior of the active site is not at all obvious. In such circumstances advantage may be taken of ac voltammetry techniques to investigate the vital role of the active sites at the interface [8]. Large amplitude Fourier transformed ac (LAFT-ac) voltammetry has been explored, at both a theoretical and experimental level [9-11], by Bond and co-workers, and the application of this technique to the study of gold surfaces in aqueous acid and base was described recently [12]. 

---