Internal electron transfer in multi-site redox enzymes is accessed by laser excitation of thiouredopyrene-3,6,8-trisulfonate (TUPS).

We demonstrate that thiouredopyrene-3,6,8-trisulfonate (TUPS), a photoactivatable reagent, can rapidly inject electrons into complex redox enzymes, enabling studies of the kinetics of internal electron that are not accessible using conventional rapid mixing, stopped-flow methods.