NEW ASPECTS OF THE CHEMISTRY AND MECHANISM OF ELECTROCHEMICAL REDUCTION OF KYNURENINE AT MERCURY-ELECTRODES IN AQUEOUS-MEDIA

New features of the chemistry of kynurenine in aqueous media over the pH range of 0 to 12 have been obtained using a combination of spectroscopic and electrochemical techniques at mercury electrodes. While C-13 nuclear magnetic resonance (NMR) spectroscopic data show exchange averaged spectra over the pH range examined, polarographic techniques reveal the presence of three kynurenine reduction processes which are predominant at close to the 100% level at pH 2, 7 and 10 respectively. Thus, the various forms of kynurenine exchange rapidly on the NMR time scale, but not at a diffusion controlled rate, so that the species present in solution can be studied individually by electrochemical methods. The protonated form(s) of kynurenine present at low pH is adsorbed at mercury electrodes in an orientation which is parallel to the electrode surface and obeys a Frumkin adsorption isotherm. At higher concentrations of the acidic form and at all concentrations of the neutral and basic forms, the reduction processes are diffusion controlled in the limiting current region of polarograms and involve an overall irreversible two electron-two proton reduction process at the carbonyl group, with the chemical steps rather than the charge transfer process being rate determining. NMR, infra-red and mass spectral studies of the products of bulk electrolysis at a mercury pool electrode are consistent with the formation of diastereoisomers of gamma(o-aminophenyl)homoserine and in the analytical context, pH 2 offers the most sensitive response towards kynurenine with limits of detection being 3.1 X 10(-8) M with conventional differential pulse polarography and 1.7 X 10(-8) M with a modified pulse method which discriminates against the overlapping hydrogen ion reduction wave.