H-atom transfer reaction of photoinduced excited triplet duroquinone with tryptophan and tyrosine in acetonitrile-water and ethylene glycol-water homogeneous solutions

Abstract

Laser flash photolysis was used to investigate the photoinduced reactions of excited triplet bioquinone molecule duroquinone (DQ) with tryptophan (Trp) and tyrosine (Tyr) in acetonitrile-water (MeCN-H2O) and ethylene glycol-water (EG-H2O) solutions. The reaction mechanisms were analyzed and the reaction rate constants were measured based on Stern-Volmer equation. The H-atom transfer reaction from Trp (Tyr) to 3DQ* is dominant after the formation of 3DQ* during the laser photolysis. For DQ and Trp in MeCN-H2O and EG-H2O solutions, 3DQ* captures H-atom from Trp to generate duroquinone neutral radical DQH•, carbon-centered tryptophan neutral radical Trp•/NH and nitrogen-centered tryptophan neutral radical Trp/N•. For DQ and Tyr in MeCN-H2O and EG-H2O solutions, 3DQ* captures H-atom from Tyr to generate duroquinone neutral radical DQH• and tyrosine neutral radical Tyr/O•. The H-atom transfer reaction rate constant of 3DQ* with Trp (Tyr) is on the level of 109 L·mol−1·s−1, nearly controlled by diffusion. The reaction rate constant of 3DQ* with Trp (Tyr) in MeCN/H2O solution is larger than that in EG/H2O solution, which agrees with Stokes-Einstein relationship qualitatively.