Some electrochemical and chemical properties of methoxatin and analogous quinoquinones.

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Abstract
Relationships between structure and reactivity for the pyrroloquinoline and phenanthroline quinones were established. The electrochemical reductions of 1,7- and 1,10-phenanthroline-5,6-quinones, like other quinones, are reversible and occur by 2e- transfer in a single step in aqueous solution and by 2 1e--transfer steps in aprotic media. The electron-withdrawing pyridine moieties both increase their potentials and stabilize their aprotic semiquinones. The electrochemistry of the cofactor methoxatin [a bacterial coenzyme] and its trimethylester derivative is similar to the phenanthroline quinones in aqueous solution. The electrochemical reductions of methoxation and its triester in aprotic solutions are characterized by at least 3 potentials, each accounting for < 1e-. This has been explained by the proposal of semiquinone complexing with itself and with quinone. Despite an electron-donating pyrrole moiety, methoxatin and its trimethylester have relatively high potentials in aprotic solution. This is presumably due to stabilization of radical anions by the aforementioned complexing or by delocalization with carboxylic acid and ester groups. The reduction potential of methoxatin, in both aqueous and aprotic solvent, suggests that oxidation of methanol should be a thermodynamically favorable process. No evidence for an electrochemically reduced state lower than the quinol was found for any of the compounds. Chemical reactivity is influenced by the orientation of the pyridine nitrogen. The 2 quinones with a pyridine nitrogen peri to a quinone carbonyl add and oxidize nucleophiles more readily. [The quinone analogs 1,7-phenanthroline-5,6-quinone and 1,10-phenanthroline-5,6-quinone are of biological and chemical interest because of their high cytotoxicity against 5 mammalian cell lines, simple structure and relative abundance.].