A COMPARATIVE-STUDY OF SOME 3,7-DIAMINOPHENOXAZINE DERIVATIVES AND RELATED-COMPOUNDS FOR ELECTROCATALYTIC OXIDATION OF NADH

The electrocatalytic oxidation of reduced nicotinamide adenine dinucleotide (NADH) at chemically modified electrodes (CMEs) based on some enlarged heterocyclic aromatic structures is investigated. On the basis of a phenoxazinium structure, the influences on the electrocatalysis are documented for different amino substituents in position 3 and 7, the hetero-atom in position 5, and for a benzene ring in the 1,2 position. From the parent molecules Neutral Red (I), Toluidine Blue O (II), Brilliant Cresyl Blue (III), and Nile Blue A (IV), the homologous series consisting of the 7-diethylamino-2-methyl-3-β-naphthamido-phenazinium (VI), -phenothiazinium (VII), and -phenoxazinium (VIII) ions as well as the 7-dimethylamino-3-β-naphthamido-1,2-benzophenoxazinium (IX) ion were obtained by reacting the amine group in position 3 of the parent molecules with 2-naphthoyl chloride. By amination of Meldola Blue (V) with aniline the 3-anilino-7-dimethylamino-1,2-benzophenoxazinium (X) ion was obtained. The compounds (I-X) were adsorbed on graphite to give CMEs, having formal redox potentials, Eo', ranging from -630 to -135 mV vs. SCE at pH 7. The acid-base properties of the adsorbed compounds were investigated by cyclic voltammetry in 0.25 M phosphate buffers in the pH range 2-11. The reaction rate of the electrocatalytic oxidation of NADH for compounds (VII-X) were evaluated from rotating disk electrode experiments. The dependence of the reaction rate on pH between the adsorbed compounds (VII-X) and NADH was studied in a single channel flow system with the CMEs mounted in a flow-through amperometric cell. The formation of a neutral imino structure of the derivatives at higher pHs, is shown to be the main limiting factor to obtaining stable CMEs with a high sensitivity for NADH. © 1990.