Cytochrome pyridine derivatives

Four different groups of cytochromes are defined. This classification is based upon the type of porphyrin present, and, for practical purposes, upon the spectra of the pyridine derivatives of the Fem form. 

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. 

SERS STUDIES ON THE ELECTRODE REACTION OF CYTOCHROME c AT SILVER ELECTRODE IN THE PRESENCE OF PYRIDINE DERIVATIVES... 

In the present work, the interfacial behavior of cytochrome c at a silver electrode in the presence of both 4-isomers and 2-isomers of pyridine derivatives are investigated by using voltammetric and SERS techniques. The 4-isomers used are 4,4 -bipyridyl, bis(4-pyridyl) disulfide, and 1,2-bis(4-pyridyl)ethylene and the 2-isomers are 2,2 -bipyridyl, bis(2-pyridyl) disulfide, and 1,2-bis(2-pyridyl) ethylene. 

The present results clearly show that only 2-BIPY among the six pyridine derivatives used inhibits the electrode reaction of cytochrome c in the bulk of the solution. This is probably due to the formation of either stable Ag(2-BIPY)2 or a chemisorbed film of 2-BIPY on the silver electrode and these films inhibit both the adsorption of cytochrome c on the silver electrode and electron exchange between the electrode and cytochrome c in the bulk. 

SERS Studies on the Electrode Reaction of Cytochrome c at Silver Electrodes in the Presence of Pyridine Derivatives... 

Hemochromes are the symmetrical bis(ligand) adducts of the Fe11 porphyrins, e.g. the bis(imidazole) moiety [11] presumably occurring in reduced cytochrome a or bs (1, 47). The bis(pyridine) hemochromes Fe(P)Py2 are all rather labile in solution the same is true for the corresponding hemichrome derivatives, [Fe(P)Py2] , which are the cationic oxidation products of the hemochromes. Reliable spectral and other physicochemical data on these species can only be obtained in solution when an excess of L is present to suppress the dissociation equilibrium (5) (M = Fe) (20, 24), which is the origin of subsequent oxidation (n = 0) or solvolysis reactions (n = 0 or 1). 

The mechanism of the epoxidation of alkenes by the cytochrome P450 model, sodium hypochlorite-manganese(III) tetraarylporphyrins, involves rate-determining formation of an active species 234 from a hypochlorite-manganese complex 233 (Scheme 6) pyridine or imidazole derivatives, as axial ligands, accelerate this step by electron donation, although the imidazoles are destroyed under the reaction conditions368. 

Figure 2 Plots of the logarithm of electron transfer rate vs. the negative of the free energy of the reaction for three ET models and six rate measurements. The data are from Refs. 54, 55, 57, 59, 60 for a Zn-substituted Candida krusei cytochrome c that was also successively substituted at histidine 33 by three Ru(NH3)4L(His 33)3+ derivatives with L = NH3, pyridine, or isonicotinamide. The shortest direct distance between the porphyrin and imidazole carbon atoms was 13 A corresponding to the 10-A edge-to-edge D/A distance. Table 1 presents a summary of the parameters used in the three calculations plotted in this figure. For a (3 of 1.2 A-1, Eq. (5) yields HAB values ( 10 cm-1) of 80 cm-1,50 cm-1, and 75 cm-1, respectively, for Eq. (1), the semiclassical model [Eq. (4)], and the Miller-Closs model at the above D/A separation distance. The s values were calculated using Eq. (6) with the following parameters aD = 10 A, aA = 6 A, and r = 13 A. The kj and H°B parameters were varied independently to produce the plotted curves.

The rate of initial electron transfer from A,7V-dimethylaniline to [Fe(phen)3] + is diffusion-limited. This is followed by the rate-determining proton transfer from the radical cation to pyridine to give the deprotonated a-amino radical which is rapidly oxidized by a second equivalent of [Fe(phen)3] + to yield the product iminium ion. Kinetic isotope effects [kii/kjf) for the proton transfer were determined from the J3/tfo ratios of the products derived from p-substituted A-methyl-A-trideuteromethylanilines. The k /kx) value first increases and then decreases with increasing pAa of p-substituted A,A-dimethylaniline. Such a bell-shaped isotope effect profile is typical of proton-transfer reactions [82, 85]. The maximum kn/fco value is determined as 8.8 which is much larger than the corresponding value for the demethylation of the same substrate by cytochrome P-450 (2.6) [79]. 

N-oxidation can occur in a number of ways to give either hydroxylamines from primary and secondary amines [Eqs. (11) and (12)], hydroxamic acids from amides, or N-oxides from tertiary amines [Eq. (13)]. The enzyme systems involved are either cytochrome P450 or a flavoprotein oxygenase. Hydroxylamines may be further oxidized to a nitro compound via a nitroso intermediate [Eq. (11)]. Oximes can be formed by rearrangement of the nitroso intermediate or N-hydroxylation of an imine, that could in turn be derived by dehydration of a hydroxylamine [Eq. (11)]. N-Oxides may be formed from both tertiary arylamines and alkylamines and from nitrogen in heterocyclic aromatic systems, such as a pyridine ring. 

Higuchi, T. and M. Hirobe (1996). Four recent studies in cytochrome P450 modelings A stable iron porphyrin coordinated by a thiolate ligand a robust ruthenium porphyrin-pyridine N-oxide derivatives system polypeptide-bound iron porphyrin application to drug metabolism smdies. J. Mol. Catal A Chem. 113, 403 22. 

Diaphorase and Cytochrome Reductase. Enzymes have been isolated from animal sources that have many of the properties of the various yeast enzymes. The first, and simplest, was liberated from particulate structures by Straub, who employed dilute ethanol and ammonium sulfate at 43 C. The enzyme could then be purified and was named a diaphorase. Diaphorase was coined to identify a widespread group of enzymes that transfer electrons from DPNH to dyes. Many of the purified flavoproteins have been found to oxidize pyridine nucleotides, and almost all of the flavoproteins can reduce dyes. Straub s diaphorase reduces methylene blue but not cytochrome c. Slight modification of the isolation procedure was found by workers at the Enzyme Institute of the University of Wisconsin to yield a cytochrome reductase. The relation between these preparations is not known, but it is possible that one is derived from the other. Cytochrome reductase contains 4 atoms of iron for each flavin, whereas Straub s preparation contains little iron. Both proteins have molecular weights around 75,000, and contain 1 equivalent of FAD. 

The vast majority of cytochrome P450 monooxygenases catalyze the reductive scission of dioxygen, which requires the consecutive delivery of two electrons to the heme iron. P450s utilize reducing equivalents (electrons in the form of hydride ions) ultimately derived from the pyridine cofactors NADH or NADPH and transferred to the heme via special redox proteins [38, 39]. 

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