Catecholamines 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. 

Electrochemical oxidation of some catecholamines such as dopamine, L-dopa, and methyldopa has been studied using cyclic voltammetry. The catecholamines undergo intramolecular cyclization to form the corresponding o-quinone derivatives. The significant differences in the electrochemical behaviour of the catecholamines have been attributed to the effects of the side-chain carboxyl group.253 Electron-transfer reactions of 2,-deoxyguanosine-5,-monophosphate (dGMP) in phosphate buffers by cyclic... 

Scheme 5. Electrochemical oxidation of catecholamines to melanoid pigments (257).

The charge-transfer processes between chlorpromazine cation radicals (CPZ" -) and catecholamines were studied spectrochemically in order to determine the biological function of chlorpromazine (28]). Electrochemical oxidation of the neurotransmitter serotonin (SER, which carries a single phenolic group) produced polyhydroxylated compounds and the corresponding quinones (282) which are the most potent neurotoxins known. 

Precolumn derivatization methods include 1,2-diphenylethylenediamine treatment, dansylation of E, NE and DA, derivatization of NE and DA by o-phthalaldehyde and mercaptoethanol and derivatization of catecholamines with 9-fluorenylmethyloxycarbonyl chloride (FMOC-Cl). Derivatization with o-phthalaldehyde increases the sensitivity of NE and DA, but E is not measured because only primary amines are derivatized. Co-analysis of catecholamines, metanephrines and other related compounds by combined electrochemical oxidation and fluorescence derivatization had also been reported. ° This approach involves sequential chromatographic separation, coulometric oxidation and final chemical derivatization with 1,2-diphenylethylenediamine to fluorescent products. 

Rgure 1 Chromatographic profile of a urinary extract of catecholamines using a C18 reversed-phase column. The catecholamines are extracted into a hexane oclanol mixture by complex formation with diphenylborate in alkaline solution and ion-pair formation with tetrabutylammonium bromide. The catecholamines are back-extracted into hydrochloric acid. Mobile phase phosphate buffer pH 5.5/methanol (87.5 12.5). Flow rate 1.2 ml min . Detection electrochemical oxidation at + 0.65 V vs Ag/AgCI with glassy carbon electrode. Column ODS Hypersil 5 urn, 15 cm X 4.6 mm. 

A Nafion film-coated micro-electrode is very advantageous for in vivo detection of neurotransmitter-related species such as dopamine and catecholamine . The partition and diffusion of the substance in the coating film are generally important factors that influence the response characteristics of the coated electrode. A quinonoid polymer electrode, prepared by the electrochemical oxidation of mercaptohydro-quinone and modified with mercaptide, functioned as a selective ion sensor for heavy metal ions such as Ag", Hg , Cd , and Cu in solution A PVC membrane electrode containing macrocyclic polyethers with sulfur atoms (id) exhibited appreciable selectivity for Ca " " relative to Mg ", alkali metal ions and... 

Maldonado, S., Morin, S., and Stevenson, K. J. 2006. Electrochemical oxidation of catecholamines and catechols at carbon nanotube elechodes. Analyst 131 262-267. 

Each current transient corresponds to the electrochemical oxidation of catecholamine (dopamine in these cells) molecules released from a single intracellular vesicle. No event was recorded over 180 s from Electrode 8, which apparently represents a cold spot on the cell, indicating that differences seen in different electrodes at a given cell reflect the subcellular spatial heterogeneity in the exocytosis process. 

Multiple electrodes have been used to obtain selectivity in electrochemical detection. An early example involved the separation of catecholamines from human plasma using a Vydac (The Separation Group Hesperia, CA) SCX cation exchange column eluted with phosphate-EDTA.61 A sensor array using metal oxide-modified surfaces was used with flow injection to analyze multicomponent mixtures of amino acids and sugars.62 An example of the selectivity provided by a multi-electrode system is shown in Figure 2.63... 

Electrochemical biosensors based on detection of hydrogen peroxide at platinized electrodes were found to be more versatile allowing a decrease in detection limit down to 1 i,mol L 1 [109]. However, all biological liquids contain a variety of electrochemically easily oxidizable reductants, e.g. ascorbate, urate, bilirubin, catecholamines, etc., which are oxidized at similar potentials and dramatically affect biosensor selectivity producing parasitic anodic current [110]. 

Sensitive electrochemical techniques have also been developed to directly measure the release of oxidizable neurotransmitters such as catecholamines (CAs) and serotonin (5-hydroxytryptamine, 5-HT). Current flows in the circuit when the potential of the electrode is positive enough to withdraw electrons from, i.e. oxidize, the released neurotransmitter. The technique is very sensitive and readily detects the release of individual quanta of neuro transmitter resulting from the fusion of single secretory vesicles to the plasmalemma (Fig. 10-2). 

As pointed out by Gorton and co-workers [19,65], this process is usually referred to a direct electron transfer (DET), that is, when an electrode substitutes the electron donor substrates in a common peroxidase reaction cycle (Eqs. (17.8)-(17.10)). When an electron donor (SH), such as phenolic compounds and the catecholamines shown in Tables 17.2 and 17.4, is present in a peroxidase-electrode system, both processes can occur simultaneously [29,30,43,48,52-54] and the oxidized donor S is reduced electrochemically by the electrode as shown in the following reaction ... 

Electrochemical detectors are based upon the volta-metric oxidation or reduction of separated analytes at a micro- or thin-film electrode. A number of pharmacologically active compounds that are aldehydes, ketones, or quinones (such as doxorubicin), or nitro compounds (such as nitrofurantoin) are amenable to reduction at a mercury or platinum electrode electron-rich indole derivatives and catecholamines can be oxidized at these electrodes. An important condition that must be fulfilled for electrochemical detection to be practicable is that the mobile phase must be capable of conducting an electrical current. This makes electrochemical detection particularly useful in reversed-phase liquid chromatography, where buffered water mixed with one or more organic cosolvents is usually the mobile phase. 

Electrochemical Electrochemically measures. oxidized/reduced analyte . Selective pg to ng Detector is useful for catecholamines... 

Electrochemical Detectors. In amperometric electrochemical detectors (see Chapter 4), an electroactive analyte enters the flow cell, where it is either oxidized or reduced at an electrode surface under a constant potential. Electroactive compounds of clinical interest conveniently analyzed by HPLC with electrochemical detection include the urhiary catecholamines (see Chapter 29). In addition, electrochemi-cally active tags (e.g., bromine) are added to compounds such as unsaturated fatty acids or prostaglandins. 

Electrochemical detection using amperometric or coulo-metric measurement is preferred for specific measurement of small quantities of 5-HIAA a modification of the method developed by Chou and Jaynes is is available on this book s accompanying Evolve site. Like serotonin, the oxidation potential for 5-HIAA is below 0.6 V and must be optimized for each apphcation. Very few interfering compounds are electrochemically active at such low voltage potentials. But if detection of 5-HIAA with other indoles and catecholamines is desired, then hydrodynamic voltammograms for each analyte should be studied to select the minimum potential that achieves maximum specificity. Some HPLC systems use fluorometric detection, with or without derivatization, for a less demanding measurement of 5-HIAA. A method combining fluorometric and electrochemical detection has also been described. ... 

Several publications on electrochemical mechanistic studies of the oxidative transformations of catecholamines followed the contribution by R. N. Adam s group (256) and involved a-methyldopamine, a-methylnor-adrenaline, dopamine (257), a-methyldopa, 5,6-dihydroxy-2-methylin-dole (255), and dopa (259). These studies (257) (Scheme 5), which confirmed the validity of the melanization scheme by Mason and Raper (Ref. 7, p. 50), explored the pH effect on the sequence of events that characterize the electrooxidation of catecholamines. Thus, the cyclic voltammogram in I M HCIO4 (pH 0.6) shows only peaks corresponding to the catechol-quinone redox couple as the protonation of the amino group prevents the cyclization step. 

Nagy et al. (1982) employed an ascorbate oxidase membrane to eliminate the oxidation current caused by ascorbic acid during the microelectrochemical mesurement of catecholamines in brain. The membrane was attached to a carbon microelectrode and was able to completely oxidize the penetrating ascorbic acid to electrochemically inert dehydroascorbic acid whereas the catecholamines could diffuse to the electrode. The sensor was called an eliminator electrode . 

MD Hawley, SV Tatawawadi, S Piekarski, RN Adams. Electrochemical studies of the oxidation pathways of catecholamines. J Am Chem Soc 89 447-450, 1967. 

Electrochemical mechanistic studies of melanin are an outcome of melanin research in 1980. Various authors have employed these methods using various catecholamines and related compounds as substrates (52,106, 224, 285, 286, 287). These studies have not only confirmed the validity of Raper-Mason s scheme of melanogenesis (see page 158 Fig. 5) but also provided information regarding the mechanism of the chemical steps that occur in the early stages of the melanization process, the identification of each electron-transfer process, and the determination of the rate constants of non-oxidative reactions. 

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