2,2 -bipyridyl electrochemistry

The electrochemistry of cytochrome c is attracting much attention,690 in particular the development of electrodes for cytochrome c, which either have a redox mediator bound to the surface, or utilize a solution mediator. Examples are a platinum gauze electrode functionalized with 2,3,4,5-tetramethyl-l-(dichlorosilyl)methyl[2]ferrocenophane,691 and 4,4 -bipyridyl, which serves as a bridge between cytochrome c and the electrode.692... 

In another study, which extended the understanding of the behavior of both dissolved and tethered redox probes, the electrochemistry of a structurally related series of outer sphere redox complexes was compared at Au-S(CH2)nOH mono-layers (System 5). As bipyridyls replace cyano ligands in Fe(bpy) (CN)6 2 - "... 

One of the first reports on the quasireversible electrochemistry of redox proteins appeared in 1977 when Eddowes and Hill demonstrated (10) cyclic voltammetry of horse heart cytochrome c at a gold electrode in the presence of 4,4 -bipyridyl (Bipy) in solution. In the voltammo-grams (Fig. 1), the peak-to-peak separations were close to 60 mV and the faradaic currents varied linearly with (scan rate), indicating a quasireversible one-electron transfer process controlled by linear diffusion of redox species to the electrode surface. The midpoint potential... 

Eddowes, M.J., Hill, H.A.O., and Uosaki, K. (1980) The electrochemistry of cytochrome c. Investigation of the mechanism of the 4,4 -bipyridyl surface modified gold electrode. 

Ruthenium, (4,4 -bipyridyl)bis(pentaammine)-equilibrium constant solvent effect, 516 Ruthenium(ll) complexes magnetic behavior, 273 polymerization electrochemistry, 488 reactivity, 300 spectra, 254... 

Chromium(I) complexes, 702 alkyl isocyanides, 704-709 aryl isocyanides, 704-709 2,2 -bipyridyl, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 cyanides, 703 fluorophosphine, 716 isocyanides crystallography, 708 electrochemistry, 709 spectroscopy, 708 synthesis, 707 1,10-phenanthroline, 709 electrochemistry, 713 electronic spectra, 712 ESR spectra, 712 IR spectra, 712 magnetic properties, 710 synthesis, 709 tertiary phosphines dinitrogen, 713 tris(bipyridyl)... 

Vanadium(IV) complexes, 487 acetates, 513 acetylacetonates, 504 electrochemistry, 505 ESR spectra, 505 adenine, 568 adipates, 516 alcohols, 502 amines, 489 amino adds, 544 equilibria, 544 ammonia, 489 arsenates, 513 arsenic ligands, 496 arsines, 498 ascorbic add, 502 benzilato, 522 biguanide, 496 binucleating ligands, 561 bipyridyl, 492,494... 

The detailed mechanism by which 4,4 -bipyridyl and (as subsequently found) a large number of other reagents act to promote cytochrome c electrochemistry at noble metals has posed a very interesting problem. While studies by Hill s group and others have established a number of points, there still remain elements of controversy. Titration of the reversible electrochemical response (using AC voltammetric peak currents) as a function of the concentration of 4,4 -bipyridyl or... 

The bipyridyl reagents were also found to promote reversible electrochemistry of cytochrome c at Ag [66] and Pt [67] electrodes. As determined by SERS [68, 69] and ellipsometry [70], the molecules are adsorbed in end-on i.e. perpendicular fashion, so that one N is bound to surface metal atoms while the other N must be positioned towards the solution. 

A number of questions remain concerning the dynamics of the protein-electrode interaction. Do experimental data give any idea about the rates, relative or otherwise, of the electron-transfer step The clearest result so far has been the determination of ke, for horse heart cytochrome c at 4,4 -bipyridyl-modified Au with the rotating ring-disk technique as mentioned above [65], There have been a number of determinations of compound heterogeneous rate constants for protein electrochemistry, mostly using Nicholson s method [124] for their estimation from CV peak separations. All calculations have assumed that the mass transport can be treated in terms of linear diffusion to a uniform planar electrode surface. Bond and co-workers have pointed out [125,126] that in many instances this is unlikely to be... 

Ru(NH3)4(isn)-cyt. c] (fc) [RufNHjfj-cyt. c]. Solutions were in 0.1 M NaC104, 0.08 M phosphate (pH 7) with 0.01 M 4,4 -bipyridyl to promote electrochemistry at the Au working electrode. Pulse amplitude 25 mV, scan rate 2mVs". Redrawn from Ref. 185, with kind permission of the authors... 

Newkome and coworkers also examined a dendritic system in which bipyridine molecules, incorporated into the branches, were subsequently complexed to yield dendrimers containing ruthenium tris-bipyridyl moieties dangling from the dendritic arms. In the electrochemistry of these compounds, the metal centers, surrounded by a dendritic network, exhibited irreversible kinetic behavior, as is commonly found in such systems. On the other hand, there are three separately identifiable ligand-based reductions, each of the... 

The cytochrome c case is noteworthy becanse at NEEs, weU-resolved CVs are obtained in diluted solutions of the protein both with and without promoters such as 4,4 -bipyridyl typically used for promoting cytochrome c electrochemistry (99-101). These promoters are generally required to avoid adsorption/denaturation (83, 102, 103) of cytochrome c on the Au surface. However, such an adsorption is concentration dependent so that lowering the cytochrome c solution concentration below the adsorption limit (possible at NEEs thanks to their lower detection limit) can overcome adsorption-related problems. A similar situation has been reported for the adsorption of some organic dyes such as the phenothiazines (69). 

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