Electrochemical Behavior and Characterization of Semiquinone Radical Anion Species of Coenzyme PQQ in Aprotic Organic Media

Abstract

Electrochemical behavior of coenzyme PQQ has been investigated in aprotic organic solvents using the trimethyl ester of PQQ (1 ox H, trimethyl 4,5-dihydro-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 (CH 2 Cl 2 , MeCN, and DMSO); there are at least three reduction peaks and four oxidation peaks. The first irreversible reduction peak of 1 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 H 2 into (1/2) 1 ox H (quinone) and (1/2) 1 red H 3 (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 H 3 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 anion species of coenzyme PQQ. Addition of Ca 2+ , which is another cofactor of quinoprotein alcohol dehydrogenases, into the solution of 1 ox Me causes ca. 0.57 V positive shift of the oneelectron reduction potential of 1 ox Me, showing the enhancement of the oxidation power of 1 ox Me by a strong interaction between Ca 2+ and 1 rad Me -. The ESR spectrum of 1 rad Me --Ca 2+ complex is successfully detected and compared with that of the free semiquinone radical anion 1 rad Me -.

Redox chemistry of coenzyme Q—a short overview of the voltammetric features

“…This has also been confirmed when CoQ10 was studied in glacial acetic acid solution [16]. Similar findings about coenzyme Q's redox conversion are also reported elsewhere in many other works performed in [21] reported a rather complex behavior of plastoquinone, observing three reduction and four oxidation peaks in the cyclic voltammograms recorded in aprotic organic solvents. Some of the signals observed are due to the CoQ dimmers formed during the course of the voltammetric experiment.…”

“…Some of the signals observed are due to the CoQ dimmers formed during the course of the voltammetric experiment. Existence of several semiquinone radicals is reported in this work [21], one of them having potential to bind calcium cations. It is worth mentioning that many of the theoretically predicted CoQ species described in Scheme 2 of this work have never been detected by studying CoQs in organic waterless media.…”

Studies on Photochemical Reactions of Air Pollutants. XV. Photoreactivity of Heptachloro-3'-cyclopentenyldioxy Heptachloro-2-cyclopentene Formed by Exposure of Hexachlorocyclopentadiene to Ultraviolet Light in Air

“…3-1-3-3). Calculations were done with Spartan [33][34][35] '04 Version 1.0.3 (WAVEFUNCTION, INC., U.S.A.) (see Table 1). …”

Correlation of voltammetric behavior of α-hydroxy and α-methoxy quinones with the change of acidity level in acetonitrile

“…The j pa /j pc ratio values and E pc µ v prove the presence of chemical reactions coupled to the electron transfer. Between the reduction peaks I 0 c and II 0 c , it is possible to observe the presence of ill defined shoulders, which were associated with the reduction of the quinone homoconjugated species (hydrogen-bonded complexes) [22,44,45].…”

“…2-hydroxy-3-alkyl-1,4-naphthoquinones and pyrroloquinolinequinone), considering the modification of the medium acidity level was recently reported [22,23].…”