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-H2O) 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-H2O 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 x 10(8) M-1 s(-1), which was close to the diffusion-controlled rate.