The case for an oxidopyrylium intermediate in the mechanism of quercetin dioxygenases.

The quercetin dioxygenases (QDOs) are unusual metalloenzymes in that they display ring-opening dioxygenase activity with several different first-row transition metal ions which do not undergo redox changes during turnover. The QDOs are also unique in that the substrate binds as an η-flavonolate rather than the ηbidentate mode seen in all reported model complexes. The flavonol substrates were early examples of excited state intramolecular proton transfer (ESIPT) phenomena, in which photoexcitation causes an H-atom exchange between the adjacent hydroxyl and ketone, generating an oxidopyrylium emissive state. These oxidopyryliums undergo ring-opening dioxygenations analogous to the enzymatic reactions. Our hypothesis is that lability of the divalent metal ion may allow access to a reactive oxidopyrylium intermediate via coordination switching from the oxy to ketone position, which allows reaction with O. In this report, we use a straight-forward methylation strategy to generate a panel of flavonol and thioflavonol derivatives modeling several η- and ηcoordination modes. Methylation of 3-hydroxythioflavone generates an air stable η hydroxopyrylium salt, which undergoes rapid ring-opening dioxygenation by deprotonation or photoexcitation. By comparison, the η-methoxyflavonol does not react with O under any condition. We find that any of the studied flavonol derivatives, η or η, which demonstrates ESIPT-like oxidopyrylium emissions undergo QDO-like ring-opening reactions with dioxygen. The implications of these results concerning the mechanism of QDOs and related dioxygenases is discussed.