Carbohydrates, electrochemical oxidation

Electrochemical detectors, which are based on the electrochemical oxidation or reduction of the analyte, can be applied to the analysis of selected compounds such as phenols. It is physically simple, but is very sensitive for catecholamines. However, the adsorption of reacted molecules on the surface of the electrodes can reduce the conductivity. To overcome this problem a pulsed voltage is applied, which cleans the electrode surface between measurements. This pulsed amperometric detection is also sensitive for carbohydrates. 

In all the above methods for oxidizing carbohydrates a stoichiometric oxidant is added to the reaction mixture. This can be avoided by using an electrochemical oxidation. A nickel hydroxide electrode has been applied for oxidizing isopropylidene-protected carbohydrates in aqueous base [26]. While secondary hydroxy groups fail to react under these conditions, the hemiacetal at the anomeric center is oxidized to the lactone in good yield [26]. 

The electrocatal3rtic oxidation of sucrose has only been the subject of a few investigations. The chemical oxidation of sucrose was firstly mentioned in the works of Bresler (1) and Usch (2). Karabinos (3) analysed the oxidation products of fructose, glucose, glucono-y-lactone and sucrose in 0.5 M NaHCOa. The author concluded that the main reaction products were CO2 and H2O. Bockris et al. (4), investigated the electrochemical oxidation of different carbohydrates at platinum electrodes for their possible use in fuel cells. They noticed that the electroactivity was better in alkaline medium than in acidic medium, and that the reactivity of the molecule decreased with increasing molecular weights. 

A variety of spectroscopies have been employed in a study of the electron transfer photophysics of 5-(l-carboxypyrenyl)- and 5-(l-pyrenyl)-2 -deoxyuridine. The inclusion complexes of a-, P- and y-CDs and 2,6-di-O-methyl and 2,3.6-tri-O-methyl P-CD have been investigated using UV (as well as NMR) spectroscopy. The electrochemical oxidation of 4-0-glucopyranosyl-D-glucose in the presence of bromide ions has been examined using cyclic chronovoltammetry. Un-derivatized oligosaccharides and other carbohydrate types were analysed by semimicrocolumn LC/pulsed amperometric detection. ... 

A review on the use of tetraptt ylammonium pemithenate (TPAP) as a catalytic oxidant in conjunction with NMNO includes examples of its use in preparing fully O-protected aldonolactones by oxidation of hemiacetals, and other workers have also favourably compared the use of TPAP as opposed to other oxidants in similar oxidations leading to 0-ben l- and O-allyl-aldonolactones.3 A kinetic study has been carried out on the electrochemical oxidation of carbohydrates to aldonic acids. ... 

MWCNT electrodes have been developed to monitor the electrochemical oxidation of insulin, a pancreas-produced hormone that plays a key role in the regulation of carbohydrates and fat metabolism in the body. This provides a possible method to evaluate the quality of pancreatic islets before their transplantation. By coating similar MWCNT electrodes with platinum microparticles, thiols containing amino acids can be detected in rat striatal that is the subcortical part of the fore-brain. CNT-based sensors can be incorporated into flexible biocompatible substrates to facilitate in vivo sensing. For instance, free cholesterol in blood can be measured using MWCNT electrodes placed on a biocompatible substrate whereas flexible pH sensors can be formed from polyaniline (a conductive polymer) and nanotube composites. ... 

Pivaloyl-p-D-glucopyranuronic acid has been prepared in order to identify it as a metabolite of pivaloyloxyalkyl-containing prodrugs, the acid grouping being introduced by catalytic oxidation. i Electrochemical oxidation of some carbohydrate primary alcohols at a nickel hydroxide electrode in alkaline solution has been developed as a route to uronic acids. Examples... 

Carbohydrates such as 6-deoxyhexoses [210] and aldopentose [211] have been oxidized electrochemically by using manganese mediators. Manganese mediators are also useful for the oxidation of a-amino acids [212]. Sorbic acid precursors... 

Electrochemical detection of carbohydrates at nickel-copper and nickel-chromium-iron alloy electrodes has been reported for sorbitol, and has been used as a detector for HPLC analysis [36]. Oxidation of various carbohydrates at the electrodes was used for detection, and baseline separation was achieved for mixtures of sorbitol, rhamnose, glucose, arabinose, and lactose. 

Dennis Johnson and co-workers pioneered the area of pulsed electrochemical detection (PAD) [30,31]- In this technique, electrode fouling is circumvented through the use of a triple-pulse waveform. Alcohols, carbohydrates, amines, and sulfur compounds can all be detected using this technique. Alcohols and carbohydrates are determined by direct oxidation at the regenerated electrode surface, while amines and sulfur compounds undergo an oxide-catalyzed oxidation. 

E. M. Belgsir and H. J. Schafer, Selective oxidation of carbohydrates on Nafion -TEMPO-modified graphite felt electrodes, Electrochem. Commun., 3 (2001) 32-35. 

Ni electrodes have also been used in analytical electrochemistry, especially for the electrochemical detection of carbohydrates following separation by HPLC. Both Ni [ii] and Ni alloy [iii] electrodes have been used. The mechanism appears to involve the NiOOH species that oxidizes the carbohydrate in an -> EC catalytic sequence [iii]. 

The electrochemical deprotection of carbonyl compounds proved to be a useful method especially in cases where alternative chemical reactions are unsuccessful, a-Keto- and a-hydroxythioacetals, when oxidized in MeCN-HoO (9 1 v/v) on a Pt anode, are transformed into the corresponding a-diones and a-ketols [142]. Diethyl dithioacetals of sugars were anodically oxidized in MeCN-H20 (5% H2O) on Pt electrode, and the substrates were successfully deprotected producing the correspondent carbohydrates in 65-85% yield [143]. It is noteworthy that protected hydroxy groups as esters or cyclic acetals were not affected. Selective deprotection to carbonyl compounds electrooxidizing mixtures of thioacetals, like a ketone and an aldehyde thioacetal, the former being preferentially deprotected, was described [144]. 

According to the reaction scheme given in Fig. 26, the anaerobic oxidation of l-glycerol 3-phosphate was performed under the conditions of an indirect electrochemical process using a water-soluble ferrocene derivative as mediator in the presence of o-fruc-tose-1,6-diphosphate aldolase from rabbit muscle for the in situ generation of the carbohydrate product. With a mediator concentration fo 1.5 mM using about 80 U of immobilized enzyme, after 25 h a 75% turnover of the substrate L-glycerol phosphate... 

There are many substances which would appear to be good candidates for LC-EC from a thermodynamic point of view but which do not behave well due to kinetic limitations. Johnson and co-workers at Iowa State University used some fundamental ideas about electrocatalysis to revolutionize the determination of carbohydrates, nearly intractable substances which do not readily lend themselves to ultraviolet absorption (LC-UV), fluorescence (LC-F), or traditional DC amperometry (LC-EC) [2], At the time that this work began, the EC of carbohydrates was more or less relegated to refractive index detection (LC-RI) of microgram amounts. The importance of polysaccharides and glycoproteins, as well as traditional sugars, has focused a lot of attention on pulsed electrochemical detection (FED) methodology. The detection limits are not competitive with DC amperometry of more easily oxidized substances such as phenols and aromatic amines however, they are far superior to optical detection approaches. 

In this brief overview, we ve seen that oxygen and carbohydrates are produced during photosynthesis, whereas they are consumed during aerobic oxidation. In both processes, the flow of electrons creates a electrochemical gradient, or proton-motive force, that can power ATP synthesis. As we examine these two processes at the molecular level, focusing first on aerobic oxidation and then on photosynthesis, the striking parallels between them will become evident. 

The imine formed on hydrolysis yields o-phenylenediamine and the corresponding carbonyl compound, and, depending on the pH of the medium, can also undergo further chemical and electrochemical reactions. By this reduction of carbohydrate derivatives of quinoxaline, the corresponding deoxy saccharides would be formed. For 1,2-dihydro-2,3-dimethylquinoxaline in an alkaline medium, the decrease in the height of wave i2 and the appearance of an anodic wave, ia, with increasing pH value prove that only its protonated form is reduced to the corresponding quinoxaline derivative, whereas its nonprotonated form is capable of oxidation to the initial 2,3-dimethylquinoxaline, as shown in Scheme 13. 

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