Aldehydes with hexacyanoferrate

There is no valid interpretation for the activation by OJ and by hexacyano-ferrate(III), although they fitted nicely in a reaction scheme with Cu(III) as the active species In the oxidation of an alcohol to an aldehyde Cu(III) would be reduced to Cu(I). In the subsequent reaction of Cu(I) with Oj, Cu(II)Oj was considered an intermediate yielding Cu(III) and H O. This intermediate would be in a reversible equilibrium with OJ and with the resting Cu(II)-enzyme. This resting enzyme would be oxidized by hexacyanoferrate(III) to the active Cu(III) species. There was unfortunately no indication in X-ray absorption measurements for the formation of Cu(III) with hexacyanoferrate(III) and the resting enzyme . EPR measurements indicated that Cu(II) was present in the active enzyme It was not possible, moreover, to detect Oj by the reduction of Fe(III)-cytochrome c in a galactose oxidase — galactose system... 

Dithiocarbamate fungicides inhibit aldehyde dehydrogenase. In order to produce an amperometric biosensor with this enzyme also a bi-enzymatic system was designed with the enzyme diaphorase. Reaction of propiraialdehyde and NAD" in the presence of ADH produced NADH which could be detected via its reaction with hexacyanoferrate(in) by diaphorase. The changes of hexacyanoferrate (11) concentrations were monitored amperometrically with a Pt electrode or bi-amperometrically with two platinum electrodes. A bi-amperometric biosensor was also developed in screen-printed configuration with Pt-sputtered carbon paste In aU these biosensors both enzymes were immobilized in a poly(vinyl alcohol)-styrylpyridinium (PVA-SbQ) layer. 

Ana.lytica.1 Methods. Various analytical methods involve titration with oxidants, eg, hexacyanoferrate (ferricyanide), which oxidize dithionites to sulfite. lodimetric titration to sulfate in the presence of formaldehyde enables dithionite to be distinguished from sulfite because aldehyde adducts of sulfite are not oxidized by iodine. Reductive bleaching of dyes can be used to determine dithionite, the extent of reduction being deterrnined photometrically. Methods for determining mixtures of dithionite, sulfite, and thiosulfates have been reviewed (365). Analysis of dithionite particularly for thiosulfate, a frequent and undesirable impurity, can be done easily by Hquid chromatography (366). 

Although the direct reaction of a lipoyl group with the thiamin-bound enamine (active aldehyde) is generally accepted, and is supported by recent studies,3153 an alternative must be considered.315 Hexacyanoferrate (III) can replace NAD+ as an oxidant for pyruvate dehydrogenase and is also able to oxidize nonenzymatically thiamin-bound active acetaldehyde... 

One group of NADH oxidants, which does not fit the proposed reaction scheme in Fig. 2.4 are the metal complexes. Examples of this type include nickel hexacyanoferrate deposited on porous nickel electrodes [29], gold electrodes modified with cobalt hexacyanoferrate films [30] and adsorbed l,10-phenanthroline-5,6-dione complexes of ruthenium and osmium [31]. It is unclear how these systems work and no mechanism has been proposed to date. It may be worth noting that dihydronicotinamide groups have been shown to reduce aldehydes in a non-enzymatic reaction when the reaction is catalysed by zinc, a metal ion [15]. In a reaction between 1,10-phenanthroline-2-carboxaldehyde and N-propyl-l,4-dihydronicotinamide, no reaction was seen in the absence of zinc but when added to the system, the aldehyde was reduced and the nicotinamide was oxidised. This implies that either coordination to, or close proximity of, the metal ion activates... 

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