Spectroscopic and electrochemical properties of hydroxylated derivatives of 2,6-dimethoxy-1,4-benzoquinone

Methoxy-substituted 1,4-benzoquinones are interesting models for the biologically important compounds of the coenzyme Q family, which have a key function in electrontransfer and redox processes. Partially substituted 1,4-benzoquinones are known to be hydroxylated at quinoid positions in alkaline media, forming intermediate semiquinone radicals accessible to EPR spectroscopy. Recently, we observed that hydroxylation also occurs at the methoxy-substituted positions of partially (2,6-dimethoxy-1,4-benzoquinone, Q0) and, particularly, of fully substituted 1,4-benzoquinones (Q1, Q10) yielding mono- and di-hydroxylated products. EPR and cyclic voltammetry are utilised to investigate the transformation mechanism, the kinetics of radical formation and the electrochemical properties of intermediates and products, focusing here on 2,6-dimethoxy-1,4-benzoquinone (2,6-bq). The experimental results are complemented with DFT calculations. The transformation of 2,6-bq at pH>12 proceeds via a mono-hydroxylated intermediate inducing a radical on the parental compound. Its spectral EPR signature in liquid state at room temperature is consistent with two OCH3-groups and two equivalent ring protons. DFT methods indicate that hydrogen bonding is essential to obtain the correct isotropic hyperfine couplings. In the time course of the reaction a secondary radical slowly appears which lacks a methoxy-group, and, on the basis of its couplings, is assigned to the di-hydroxylated form of the compound. From several possible structures, DFT calculations clearly favour one configuration. The loss of an OCH3-group should be accompanied by formation of methanol, which was demonstrated by NMR. The spectroscopic changes are also monitored in cyclic voltammetry at high pH values. After neutralizing the solvent, the hydroxylated compound reveals a more negative redox potential, and, importantly, has a strong binding capacity for divalent ions such as Ca2+ and Ba2+ in its reduced state. The 2:1 stoichiometry (Ca2+:2,6-bq) matches with a di-hydroxylated compound and is confirmed by DFT calculations. The detailed information derived from the model transformation reaction of 2,6-bq will be useful to understand possible reactions of the more complex, biologically relevant quinones