Dithionite reductive activation products

Dithionite-mediated reductive activation of mitomycin C has been employed in the study of its DNA alkylation chemistry.6,63 However, dithionite activated mitomycin C possesses different DNA alkylation properties than that activated by catalytic hydrogenation and enzymatic reduction. We postulated that a new alkylating species is produced by dithionite reductive activation resulting in different reactivity than the iminium methide species. To investigate dithionite-mediated reductive activation further, we treated 13 C-labeled analogues of WV-15 with dithionite and carried out spectral and product studies. 

Scheme 7.11 shows the product structures resulting from the dithionite reduction of a simplified version of WV-15. The symmetric sulfite diester was extracted from the reaction mixture with methylene chloride. The isolation and characterization of the sulfite diester confirmed that this species can form in dithionite reductive activation reactions and provided the chemical shift for the 10a-13C center of a mitosene sulfite ester (49.37 ppm). The aqueous fraction of the reaction contained the mitosene sulfonate and trace amounts of Bunte salt, based on their 13C chemical shifts. 

The first key element in the uptake of these two metal ions is that the substrate is the lower valent state species, Cu(I) in the case of copper and Fe(II) in the case of iron (Dancis et al., 1990, 1992 Hassett and Kosman, 1995 Kosman, 1993). Normally, these reduced valence species are provided by the action of plasma membrane metal reductases, an activity in yeast provided predominantly by the product of the FREl gene (Dancis et al., 1992). However, Fe(II) [or Cu(I)] provided exogenously to the cell is equally competent for uptake and, in most experimental regimes, is added directly or generated in situ by the addition of a strong reductant like ascorbate or dithionite. Cu(I) is the direct substrate for uptake, through the Ctrlp copper permease in most yeast strains (Dancis et al., 1994). However, the presence of Fe(II), although required, alone is not... 

The lack of reactivity of the semiquinone per se with either thioredoxin or NADPH shows that it cannot be involved in catalysis. The rapid production of semiquinone by irradiation of partially reduced enzyme is a light-activated disproportionation since it is totally dependent upon the presence of some oxidized enzyme. Enzyme fully reduced by dithionite forms no semiquinone, while enzyme partially reduced by dithionite rapidly forms semiquinone upon irradiation. Furthermore, the light-activated disproportionation of enzyme first reduced with NADPH results in the reduction of NADP. Thus, FAD catalyzes the disproportionation in keeping with the known photosensitizing nature of free flavins. This reaction is reversed slowly (half-time ca. 150 min 25°) in the dark. The semiquinone is rapidly reoxidized by oxygen to yield an enzyme with unaltered spectral and catalytic properties (58). Similar reactions have been very briefly reported for lipoamide dehydrogenase the dark reverse reaction is comparatively rapid, being complete in 30 min (16S). 

Further defluorination to octafluoronapthalene, which is often the product of similar reactions, does not occur. Coupled with terminal reductants such as aqueous dithionite this reaction can be performed under catalytic conditions with respect to (C6H6)2Cr [74]. Together these reactions provide a glimpse of possible future directions for research into the application of C-F bond activation chemistry to synthetic problems. 

Evidence for the [4Fe S] cluster as the active form of lysine aminomutase was obtained by Frey and co-workers, who showed by a combination of EPR spectroscopy and enzyme assays that the [4Fe-4S] -LAM generated in the presence of AdoMet was catalytically active. Unlike aRNR-AE, however, LAM catalyzes a reversible reductive cleavage of AdoMet, and thus methionine production and cluster oxidation could not be monitored as evidence of turnover. It is of interest to note that in the case of LAM, the presence of AdoMet facilitates reduction to the [4Fe-4S] state very little [4Fe-4S]" cluster is produced by the reduction of LAM with dithionite in the absence of AdoMet, while the presence of AdoMet or its analogue S-adenosylhomocysteine dramatically increases the quantity of [dFe-dS] " produced. It is not clear whether the presence of AdoMet affects the redox potential of the cluster or whether some other effect, such as accessibility of the cluster by the reductant, is at work. 

The GC/Nafion/Co-(5-N02)TRP modified electrodes were stable in water/ethanol solution and presented only a reversible Ru(III/II) pair of waves at 0.8 V versus Ag/AgCl in the -1.0 to 1.0 V range. However, an irreversible reduction wave, whose intensity is linearly dependent on sulfite concentration in the 0.4 to 70 mg/L range, appeared at Epc = -0.52 V, confirming that tetramthenated porphyrins continue to be electrocatalytic active for reduction processes. The detection and quantification limits were estimated as 0.4 and 1.2 mg/L, respectively. The electrolysis at -0.65 V for 6 h produced thiosulfate and not dithionite, the conventional reduction product in acidic media. The modified electrodes exhibited high stability, and 4.1 % was the RSD measured for 10 independent electrodes stored for 90 days in open air. Rotating disk voltammetry experiments showed a linear Koutecky-Levich plot and the electrocatalytic oxidation of sulfite was limited by electron diffusion through the film. In fact, because of the presence of Co-(5-N02)TRP dispersed in nonelectroactive Nation, several processes such as (a) the mass transfer at film and electrode surface, (b) the electron and substrate diffusion... 

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