Investigation on the photoinduced chemical reaction between p -benzoquinone and tryptophan in homogeneous solution.

Photoinduced electron transfer (PET) and energy transfer between amino acids and bioquinones have become research hotspots, due to the important roles they play in a physiological environment. However, as classic benzoquinones and amino acids, the reaction mechanism of p -benzoquinone (PBQ) and tryptophan (Trp) is still unclear. In this work, the photoinduced chemical reaction of PBQ and Trp was investigated in homogeneous solution using time-resolved electron paramagnetic resonance and laser flash photolysis techniques. Under photoexcitation at 355 nm, the 3 PBQ* produced via intersystem crossing (ISC) in ethylene glycol aqueous (EG-H 2 O) solution followed by the H-atom transfer (HAT) from EG to 3 PBQ* was a significant process in competition with the non-radiative transition of 3 PBQ*, which was clearly observed in the transient absorption spectra and chemically induced dynamic electron polarization spectra. When Trp was added into the PBQ/EG-H 2 O solution, a new decay channel of 3 PBQ* was produced that reacted with Trp to form a p -benzoquinone anion radical (PBQ˙ - ) and a tryptophan cationic radical (Trp˙ + ), indicating that the photoinduced chemical reaction mechanism was the electron transfer. By fitting the decay dynamic curves, the quenching rate constant of 3 PBQ* to Trp in homogeneous solution was determined as 6.8 × 10 8 M -1 s -1 , which was close to the diffusion-controlled rate.