Identification Of The Tyroh(Center Dot+) Radical Cation In The Flavoenzyme Trmfo

Tyrosine (TyrOH) and tryptophan radicals play important roles as intermediates in biochemical charge-transfer reactions. Tryptophanyl radicals have been observed both 'in their protonated cation form and in.their unprotonated neutral' form-, but to date, tyrosyl radicals 'have only been observed in their unprotonated form. With a genetically modified form of the flavoenzyme TrmFOH(center dot+) as a suitable model system and using ultrafast fluorescence and absorption spectroscopy, we characterize itsprotonated precursor TyrOH(center dot+), and-we show this species to have a distinct visible absorption band and a transition moment that we suggest to lie close to the phenol symmetry axis. TyrOH(center dot+) is formed in similar to 1 ps by electron transfer to excited flavin and decays in similar to 3 ps by charge:recoinbination. These findings imply that TyrOH oxidation does not necessarilyinduce its concerted deprotonation. Our results will allow disentangling 'of photoproduct states in flavoproteins in oftenen.couritered complex situations and more generally are important for 'understanding 'redox chains relying on-tyrosyl intermediates.