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

Abstract

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 ³PBQ* produced via intersystem crossing (ISC) in ethylene glycol aqueous (EG-H₂O) solution followed by the H-atom transfer (HAT) from EG to ³PBQ* was a significant process in competition with the non-radiative transition of ³PBQ*, 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 ³PBQ* 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 ³PBQ* to Trp in homogeneous solution was determined as 6.8 × 10⁸ M⁻¹ s⁻¹, which was close to the diffusion-controlled rate.