Cytochrome silver electrodes

D.E. Reed and F.M. Hawkridge, Direct electron transfer reactions of cytochrome c at silver electrodes. Anal. Chem. 59, 2334-2339 (1987). 

Dick, L.A., Haes, A.J., and Van Duyne, R.P. (2000) Distance and orientation dependence of heterogeneous electron transfer a surface-enhanced resonance Raman scattering study of cytochrome c bond to carboxylic acid terminated alkanethiols adsorbed on silver electrodes. Journal of Physical Chemistry B, 104, 11752-11762. 

Cotton, T.M., Schultz, S.G., and Van Duyne, R.P. (1980) Surface-enhanced resonance Raman scattering from cytochrome-c and myoglobin adsorbed on a silver electrode. Journal of the American Chemical Society, 102, 7960-7962. 

Wackerbarth, H., Klar, U., Gunther, W., and Hildebrandt, P. (1999) Novel time-resolved surface-enhanced (resonance) Raman spectroscopic technique for smdying the dynamics of interfadal processes application to the electron transfer reaction of cytochrome c at a silver electrode. Applied Spectroscopy, 53, 283-291. 

SERS has been used for the study of molecules of biological importance, such as nucleic acid components.The adsorption on a silver electrode was considered in some way similar to the adsorption to the charged membranes in biological systems. The adsorption of a nucleic acid itself was also studied with SERS." We have already discussed the work of Cotton et at2i8,2i9,22i who studied Cytochrome C and myoglobin. SERS requires only small quantities of material, which is very suitable for biological studies. 

Reed, D. E., Hawkridge, F. M., Direct Electron Transfer Reactions of Cytochrome C at Silver Electrodes , Anal. Chem. 59 (1987) 2334-2339. 

As in the case of Mb, Cotton et al. have reported SERRS spojtra of highly dilute solutions (10 M) of cytochrome c (Cytc) at two different potentials (E —0.2 V and —0.6 V vs. SCE) using the 514.5 nm excitation wavelength. Findings similar to SERRS investigations of Mb can be outlined. The position of the spin-oxidation state marker bands of Cytc on the silver electrode surface indicates that the heme Fe is in its low spin reduced state at —0.6 V. The Fe(III) bulk Cytc solution RR spectrum is only observed by stepping the electrode potential to a more positive value of—0.2 V vs. SCE, see Table 6. 

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. 

Attempts have been made to assign individual potential values obtained from voltammetry to specific heme groups. In one example, the bulk-solution voltammetry of cytochromes was compared to voltammetry of the proteins adsorbed at silver electrodes and the corresponding SERS potentiometric data [35]. Since the adsorbed-film voltammetry gave potentials very close to the bulk solution values, it was concluded that the proteins were adsorbed essentially in their native forms. The SERS potentials each corresponded more closely to the electrochemical... 

Cotton and coworkers first reported that the surface-bounded horse heart cyt c is reoxidized at 0.04 V versus NHEby SERRS measurements [1], which has a 0.22 V more negative potential than the formal potential of the native one [37]. Niki and coworkers showed from SERRS measurements that the formal potential of cyt. c adsorbed on a silver electrode is —0.05 V [38]. Hildebrandt and Stockburger have extensively studied the conformation and redox properties of horse heart cyt. c adsorbed on a silver electrode by using the SERRS technique [7, 9, 39-42]. Cytochrome c exhibits various conformational states upon adsorption on the silver electrode [39-42]. The RRS of ferro- (cyt. c " ") and ferriaqueous solution at pH 7.0 and SERRS of cyt. and cyt. adsorbed on the silver electrode at —0.16 V versus NHE, and SERRS of cyt. and cyt. c + adsorbed on the silver electrode at 0.34 V are shown in Eig. 2 [41,42]. 

Flildebrandt and Stockburger found that there are two conformational states of cyt. c upon adsorption on a silver electrode [38-40]. Cytochrome c adsorbed on the silver electrode at negative potentials (<0.04 V) exhibits the state I (cyt. C ) conformation and that adsorbed at positive electrode potentials (>0.04 V) exhibits the state II (cyt. cn) conformation. The structure of the RR spectrum of cyt. c in the solution is fully maintained in the state I, which must be the same for the whole cyt. c molecules. In the state II, the spin state of the heme iron is in the mixed 5cHS and 6c LS conformation. The conformational states I and II are at potential-dependent equilibrium. The most stable species of ferro-cyt. c is the reduced form of state I (cyt. ci +) in the electrode potential range, more negative than 0.2 V and that of ferri-cyt. c is the oxidized form of state II (cyt. cn +) in the electrode potential range more positive... 

It was shown that cleavage of the S—S bonding of 4-PySSPy takes place upon adsorption on both gold and silver electrodes, and a stable chemisorbed film is formed through sulfur to these electrodes [63, 65]. Cytochrome c adsorbed on a gold or silver electrode is displaced entirely by 4-PySSPy and the electrode reaction of cyt. c in the solution takes place through the PyS- film on the electrode surface [63, 64]. Added purine partially displaces cyt. c from a silver electrode surface and a mixed adsorbed layer of purine and cyt. c is formed, at which a reversible electrode reaction of cyt. c takes place [63]. 

An adsorbed layer of l,2-Bis(4-pyridyl) ethylene (4-PyCH) on a silver electrode is stable and is hard to displace by cyt. c. Cytochrome c is not immobilized on the PyCH layer, but cyt. c in the solution undergoes a rapid electrode reaction through this layer. 

Cytochrome c is found to exhibit a quasireversible voltammetric response at an iodine-modified silver electrode. S ERRS spectroscopy indicates that cyt. c immobilized on the iodine-modified silver electrode is in the 6cLS state [74, 75]. The shift of the oxidation state marker band was plotted against the electrode potential and the formal potential of cyt. c immobilized on the iodine-modified silver electrode is estimated to be 0.19 V versus NHE, which is somewhat negative relative to that of the native state. A similar shift is also seen when cyt. c interacts with mitochondrial membranes (50-60 mV) [76, 77]. It can be concluded that the SERRS results indicate that the adsorbed cyt. c is structurally similar to the native protein in solution. The SERRS spectrum of cyt. c adsorbed on the iodine-modified gold electrode is different from that on the iodine-modified silver electrode. The SERRS spectrum of cyt. c (ox) excited by 550 nm at open-circuit potential reveals that cyt. c is present in the mixed 5cHS and 6cLS state [78]. 

Electron Transfer Kinetics of Cytochromes c and Cs52 at Silver Electrode... 

The electrode reaction of tetraheme cytochrome C3 adsorbed at the silver electrode surface was monitored by SERRS spectroscopy [104, 105] and compared with the results obtained by voltammetric techniques. The formal potential determined on the basis of the SERS measurement is more positive compared to the potential determined by the voltammetry, but it is in good agreement with the macroscopic formal potential of the heme-1. This is because cytochrome C3 is adsorbed on the silver electrode particularly via lysine residues surrounding heme-1, which is in fact responsible for the SERRS spectrum of this protein [105]. 

Taniguchi, I., Iseki, M., Yamaguchi, H., Yasukouchi, K. (1984) "Surface Enhanced Raman Scattering Study of Horse Heart Cytochrome c at a Silver Electrode in the presence of bis(4-pyrdidyl) disulfide and Purine", J. Electroanal. Chem., 175, 341-8. 

Studies from this laboratory 24-27 suggest that a sulfur atom is effective to immobilize promoters by irreversible adsorption onto gold and silver electrodes. To date, various sulfur containing promoters are known to give effective promoter-modified electrodes for cytochrome c. In the present paper, surface structures of such promoter-modified electrodes and electrochemistry of cytochrome c at these electrodes have been investigated to better understand the surface functions of such electrodes. 

X 10 3 cm s-i (Figure 2). Also, 4-mercaptopyridine (PySH)-modified gold and silver electrodes showed almost reversible electrochemical response of cytochrome c, which was indistinguishable with those observed at PySSPy-Au and PySSPy-Ag electrodes, suggesting that the surface structures of these electrodes are similar to each other. 

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