Catalytic Mechanism of Fatty Acid Photodecarboxylase: on the Detection and Stability of the Initial Carbonyloxy Radical Intermediate.

In fatty acid photodecarboxylase (FAP), light-induced formation of the primary radical product RCOO● from fatty acid RCOO- occurs in 300 ps, upon which CO2 is released quasi-immediately. Based on the hypothesis that aliphatic RCOO● (spectroscopically uncharacterized because unstable) absorbs in the red similarly to aromatic carbonyloxy radicals such as 2,6-dichlorobenzoyloxy radical (DCB●), much longer-lived linear RCOO● has been suggested recently. We performed quantum chemical reaction pathway and spectral calculations. These calculations are in line with the experimental DCB● decarboxylation dynamics and spectral properties and show that in contrast to DCB●, aliphatic RCOO● radicals a) decarboxylate with a very low energetic barrier and on the timescale of a few ps and b) exhibit little red absorption. A time-resolved infrared spectroscopy experiment confirms very rapid, <<300 ps RCOO● decarboxylation in FAP. We argue that this property is required for the observed high quantum yield of hydrocarbons formation by FAP.

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