Dithionite reduction

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. 

Dithionite reduction of 13C-labeled WV-15 (structure in Scheme 7.6) was carried out in anaerobic D20 and the resulting products evaluated by quantitative 13C-NMR spectroscopy immediately after the reduction (Fig. 7.8).45 The spectrum indicates that both the sulfite ester and the sulfonate are formed in a 60 40 ratio. Also, no long-lived methide species are observed in this reaction. 

The above studies show that dithionite reduction of mitosenes results in the formation of lOa-sulfite esters as well as sulfonates. The presence of the excellent sulfite-leaving group at the mitosene 1 Oa-position suggests that alkylation reactions at this position could still occur. The subsequent sulfite to sulfonate rearrangement results in loss of alkylation capability by this position. 

SCHEME 7.11 Dithionite reduction of a mitosene bearing an acetate-leaving group at the 10a position (a simplified WV-15 analogue). The I3C labels are designated with asterisks ( ). 

Fig. 8.9 UV—Vis spectra of intercalated biomolecules assembled AMP. Soret band absorptions for (A) oxidized myoglobin (met-Mb) and after dithionite reduction (deoxy-Mb), and (B) after CO (CO-Mb) and 02 bindingto intercalated deoxy-Mb.

After its isolation, the structure of alkaloid deplancheine (7) was unambiguously proved by several total syntheses. In one of the first approaches (14), 1,4-dihydropyridine derivative 161, obtained by sodium dithionite reduction of A-[2-(indol-3-yl)ethyl]pyridinium salt 160, was cyclized in acidic medium to yield quinolizidine derivative 162. Upon refluxing 162 with hydrochloric acid, hydrolysis and decarboxylation took place. In the final step of the synthesis, the conjugated iminium salt 163 was selectively reduced to racemic deplancheine. 

Selected examples of the dithionite reduction of alkane-2,4-dienoic esters... 

The Lys(Nic) and Dap(Nic) residues were incorporated into a simple hexa-peptide structure which contained a constrained type II -turn (Fig. 26). Dithionite reduction of both Lys(Nic) and Dap(Nic) peptides rapidly generated... 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

One of the oldest-known reactions of N-nitrosamines is their reduction to 1,1-disubstituted hydrazines discovered by Fischer (19) The most common method to perform this transformation has been zinc dust in acid, generally acetic acid tetrazenes are sometimes formed as by-products (20) and denitrosation can also occur. Several other reducing methods have been investigated reduction with lithium aluminum hydride and catalytic hydrogenation are sometimes useful. Sodium dithionite reduction of benzyl substituted N-nitrosamines in base can result in fragmentation to... 

More recent evidence indicates that the cured meat pigment is actually a mononitrosyl complex. Killday et a ., (1988) showed that dithionite reduction... 

Killday etal. (1988) also provided evidence for internal autoreduction of ferric nitrosyl heme complexes, as previously proposed by Giddings (1977). Heating of chlorohemin( iron-III) dimethyl ester in dimethyl sulfoxide solution with imidazole and NO produced a product with an infrared spectra identical to that of nitrosyl iron(ll) protoporphyrin dimethyl ester prepared by dithionite reduction. Both spectra clearly showed the characteristic nitrosyl stretch at 1663 and 1665 cm. They thus proposed a mechanism for formation of cured meat pigment which includes internal autoreduction of NOMMb via globin imidazole residues. A second mole of nitrite is proposed to bind to the heat-denatured protein, possibly at a charged histidine residue generated in the previous autoreduction step. 

A Fe2S2 ferredoxin from the hypertherm ophilic bacterium Aquifex aeolicus, expressed in E. coli, studied by Meyer et al. was shown not to be closely similar to plant or mammalian Fe2S2ferrredoxins.83 The combined EPR, MCD, resonance Raman and Mossbauer study identified an S = state from dithionite reduction with ghvalues of 2.05,1.96 and 1.88. 

Both stopped-flow and rapid freeze quench kinetic techniques show that the substrate reduces the flavin to its hydroquinone form at a rate faster than catalytic turnover Reoxidation of the flavin hydroquinone by the oxidized Fe4/S4 center leads to formation of a unique spin-coupled species at a rate which appears to be rate limiting in catalysis. Formation of this requires the substrate since dithionite reduction leads to flavin hydroquinone formation and a rhombic ESR spectrum typical of a reduced iron-sulfur protein . The appearance of such a spin-coupled flavin-iron sulfur species suggests the close proximity of the two redox centers and provides a valuable system for the study of flavin-iron sulfur interactions. The publication of further studies of this interesting system is looked forward to with great anticipation. 

Ethoxycarbonyl-l,2,3,4-tetrahydroquinoxalin-2-ones (86) are obtained either by sodium dithionite reduction of the corresponding quin-oxalinone esters or by direct synthesis from o-phenylenediamines and bromomalonic ester (85).102... 

Ancient dyers also had to maintain the dye in the reduced form. In fermentations of the woad plant a species of thermophilic Clostridium apparently supplied the reducing agent. Padden et al. suggested that such bacterial reduction might be used today to avoid pollution by the by-products of dithionite reductions Use of engineered bacteria to form high yields of indole and indoxyl as a source... 

JF Lawrence, FE Lancaster. Determination of total non-sulphonated aromatic amines in food colour amaranth by dithionite reduction followed by derivatization and high performance liquid chromatography. Food Addit Contam 6(4) 415 -423, 1989. 

Mossbauer spectra of the reduced proteins in the above study are not consistent with subsequent data for these proteins (153,164,165). It is now believed (personal communications, W. H. Orme-Johnson and Graham Palmer) that 1, the samples in these experiments were impure, and 2. the buffers used in these experiments were not strong enough to maintain a buffer pH level during the dithionite reductions. Therefore, the Mossbauer spectra of reduced spinach ferredoxin in the above experiment resulted from a mixture of oxodized protein iron and iron from denatured protein material. 

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