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(Me CN-H_(2)O)and ethylene glycol-water(EG-H_(2)O)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 ^(3)DQ^(*)is dominant after the formation of^(3)DQ^(*)during the laser photolysis.For DQ and Trp in MeCN-H_(2)O and EG-H_(2)O solutions,^(3)DQ^(*)captures H-atom from Trp to generate duroquinone neutral radical DQH^(·),carboncentered tryptophan neutral radical Trp^(·)/NH and nitrogen-centered tryptophan neutral radical Trp/N^(·).For DQ and Tyr in Me CN-H_(2)O and EG-H_(2)O solutions,^(3)DQ^(*)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^(3)DQ^(*)with Trp(Tyr)is on the level of 109 L·mol^(-1)·s^(-1),nearly controlled by diffusion.The reaction rate constant of^(3)DQ^(*)with Trp(Tyr)in Me CN/H_(2)O solution is larger than that in EG/H2O solution,which agrees with Stokes-Einstein relationship qualitatively.