Light-induced tryptophan radical generation in a click modular assembly of a sensitiser-tryptophan residue

Click chemistry was used as an efficient method to covalently attach a chromophore to an amino acid. Such easily prepared model systems allow for time-resolved studies of one-electron oxidation reactions by the excitation of the chromophore by a laser flash. The model complex ruthenium-tryptophan (Ru-Trp) has been synthesised and studied for its photophysical and electrochemical properties. Despite a small driving force of less than 100 meV, excitation with a laser flash results in fast internal electron transfer leading to the formation of the protonated radical (Trp(center dot)H(+)). At neutral pH electron transfer is followed by deprotonation to form the neutral Trp(center dot) radical with the rate depending on the concentration of water acting as the proton acceptor. The formation of the tryptophan radical was confirmed by EPR.