H-Atom Transfer Reaction of Photoinduced Excited Triplet Duroquinone with Tryptophan and Tyrosine in Acetonitrile-Water and Ethylene Glycol-Water Homogeneous Solutions

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-H_2O) and ethylene glycol-water (EG-H_2O) 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-H_2O and EG-H_2O solutions, ^3DQ^* captures H-atom from Trp to generate duroquinone neutral radical DQH^\bullet, carbon-centered tryptophan neutral radical Trp^\bullet/NH and nitrogen-centered tryptophan neutral radical Trp/N^\bullet. For DQ and Tyr in MeCN-H_2O and EG-H_2O solutions, ^3DQ^* captures H-atom from Tyr to generate duroquinone neutral radical DQH^\bullet and tyrosine neutral radical Tyr/O^\bullet. The H-atom transfer reaction rate constant of ^3DQ^* with Trp (Tyr) is on the level of 10^9 L\cdotmol^-1\cdots^-1, nearly controlled by diffusion. The reaction rate constant of ^3DQ^* with Trp (Tyr) in MeCN/H_2O solution is larger than that in EG/H_2O solution, which agrees with Stokes-Einstein relationship qualitatively.