The Mechanism of the Flavin Sensitized Photodestruction of Indoleacetic Acid*

Abstract

Abstract— A detailed in vitro study was made of the flavin sensitized photoinactivation of indoleacetic acid, using primarily riboflavin as sensitizer. The dependence of the quantum yield on reactant concentrations, pH, presence of oxygen, viscosity, temperature, KI concentration, and solvent was determined. The involvement of a limiting dark reaction was demonstrated, using an intermittent light technique. The results are consistent with a mechanism involving a metastable state of riboflavin as the photochemically reactive species. The calculated rate constant for intersystem crossing to this state was found to be 2.5 times 108/sec. Riboflavin, in the metastable state, is believed to oxidize indoleacetic acid to indolealdehyde, with subsequent recovery of riboflavin by autoxidation. The maximum quantum yield of the photoinactivation of IAA is 0.71, indicating a highly efficient process, approaching 100% when energy loss due to riboflavin fluorescence is taken into account. Both carotenoids and pure chlorophyll‐a were found to be inactive as sensitizers.

Spectroscopic Characterization of Flavin Mononucleotide Bound to the LOV1 Domain of Phot1 from Chlamydomonas reinhardtii¶

“…For the steady‐state fluorescence saturation studies presented here, φ S1,T1 , φ S1,Ad and φ T1,Ad are not accessible, but using reasonable values of φ S1,T1 between 0.5 and 0.7 (51,53), φ S1,Ad ∼ 0 and φ T1,Ad ∼ 0.5 (7), we find 115 s ≤ τ Ad ≤ 160 s.…”

“…Intersystem crossing seems to be the dominant process of fluorescence quenching (47). For free FMN in aqueous solution, quantum yields of triplet formation of φ S1,T1 = 0.5 to 0.6 (51) and φ S1,T1 = 0.72 (53) have been reported, giving a quantum yield of S 0 –S 1 internal conversion, φ IC = 1 −φ F −φ S1,T1 , between 0.02 and 0.24. The determined fluorescence spectroscopic parameters of LOV1‐WT, LOV1‐C57S and free FMN in neutral aqueous solution are collected in Table 1.…”

“…Solid curves belong to LOV1‐WT. They are calculated for I sat,tot = 375 μW cm –2 , I sat,S1 = 5.2 × 10 6 W cm –2 , τ S1 = 2.9 ns, τ T1 = 2 μs (assumed, value of FMN in LOV2‐WT domain of A. sativa from reference [7] is used), φ S1,T1 = 0.7 (assumed, values between 0.5 and 0.72 [51,53] have been determined for free FMN) and φ S1,Ad = 0. Dashed curves belong to LOV1‐C57S.…”

Spectroscopic Characterization of Flavin Mononucleotide Bound to the LOV1 Domain of Phot1 from Chlamydomonas reinhardtii¶

“…Intersystem crossing seems to be the dominant process of fluorescence quenching (47). For free FMN in aqueous solution, quantum yields of triplet formation of S1,T1 ϭ 0.5 to 0.6 (51) and S1,T1 ϭ 0.72 (53) have been reported, giving a quantum yield of S 0 -S 1 internal conversion, IC ϭ 1 Ϫ F Ϫ S1,T1 , between 0.02 and 0.24. The determined fluorescence spectroscopic parameters of LOV1-WT, LOV1-C57S and free FMN in neutral aqueous solution are collected in Table 1.…”

“…Solid curves belong to LOV1-WT. They are calculated for I sat,tot ϭ 375 W cm Ϫ2 , I sat,S1 ϭ 5.2 ϫ 10 6 W cm Ϫ2 , S1 ϭ 2.9 ns, T1 ϭ 2 s (assumed, value of FMN in LOV2-WT domain of A. sativa from reference [7] is used), S1,T1 ϭ 0.7 (assumed, values between 0.5 and 0.72 [51,53] have been determined for free FMN) and S1,Ad ϭ 0. Dashed curves belong to LOV1-C57S.…”