Electrochemical behavior and characterization of semiquinone radical anion species of coenzyme PQQ in aprotic organic media

Electrochemical behavior of coenzyme PQQ has been investigated in aprotic organic solvents using the trimethyl ester of PQQ (1(ox)H, trimethyl 4,5-dihyro-4,5-dioxo-1H-pyrrolo[2,3-f]quinoline-2,7,9-tricarboxylate) and its 1-methylated derivative (1(ox)Me, trimethyl 4,5-dihydro-4,5-dioxo-1- methylpyrrolo[2,3-f]quinoline-2,7,9-tricarboxylate). 1(ox)H shows a complicated cyclic voltammogram in organic-media (CH₂Cl₂, MeCN, and DMSO); there are at least three reduction peaks and four oxidation peaks. The first irreversible reduction peak of l(ox)H around -0.9 V vs ferrocene/ferricenium (Fc/Fc⁺) is very close to that of the reversible reduction peak of 1(ox)Me. Addition of an equimolar amount of acetic acid into the solution of 1(ox)Me results in a change of the reversible one-electron redox couple of 1(ox)Me/1(rad)Me^- to an irreversible one as observed in 1(ox)H. This phenomenon indicates that protonation of the semiquinone radical anion causes disproportionation of the resulting neutral semiquinone 1(rad)H₂ into ( 1/4 ) 1(ox)H (quinone) and ( 1/4 ) 1(red)H₃ (quinol). In this case, 1(ox)H itself in bulk solution acts as the proton source to be converted into 1(ox). This is confirmed by the observation of reversible redox peaks due to a 1(ox)^- /1(rad)^2- couple at - 1.30 vs Fc/Fc⁺ in DMSO and by the 2:1 formation of 1(ox)^- and 1(red)H₃ in the thin layer UV-vis spectrum of 1(ox)H electrolyzed at -0.8 V Ag/Ag⁺. Solution ESR spectra of electrochemically generated semiquinone radical anion species, 1(rad)H^-, 1(rad)Me^-, and 1(rad)²⁺, as well as the UV-vis spectrum of 1(rad)Me^- are also reported to explore the electronic structures of the semiquinone radical artion species of coenzyme PQQ. Addition of Ca²⁺, which is another cofactor of quinoprotein alcohol dehydrogenases, into the solution of 1(ox)Me causes ca. 0.57 V positive shift of the one-electron reduction potential of 1(ox)Me, showing the enhancement of the oxidation power of 1(ox)Me by a strong interaction between Ca²⁺ and 1(rad)Me^-. The ESR spectrum of 1(rad)Me^-- Ca²⁺ complex is successfully detected and compared with that of the free semiquinone radical anion 1(rad)Me^-.