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. Quantum chemical decarboxylation reaction pathway and spectral calculations of aliphatic and aromatic carbonyloxy radicals demonstrate that they have distinct electronic absorption properties and that the former are much more unstable. These findings pertain to a controversy on the mechanism of the photoenzyme fatty acid photodecarboxylase. They support the here reinforced experimental observation of intrinsic CO2 dissociation in ≪300 ps.