Kinetics and mechanism of the photochemical reaction of 2,2 '-dipyridyl with tryptophan in water: Time-resolved CIDNP and laser flash photolysis study

The mechanism of the reactions between photoexcited 2,2'-dipyridyl and N-ac etyl tryptophan has been studied by laser flash photolysis and time-resolve d CIDNP (Chemically Induced Dynamic Nuclear Polarization). The transient ab sorption spectra obtained at different delays after the laser pulse are att ributed to the triplet state of dipyridyl and to dipyridyl and, tryptophan radicals. Depending on the pH of the solution, all three intermediates can be present in either protonated or deprotonated forms. It is shown that irr espective of pH the primary photochemical step is electron transfer from th e tryptophan to triplet dipyridyl followed by protonation/deprotonation of the radicals so formed. The rate constant of the reaction of triplet dipyri dyl with tryptophan is close to the diffusion-controlled limit and decrease s slightly with increasing pH. The kinetics and the stationary value of the CIDNP are determined by the rates of radical termination, nuclear paramagn etic relaxation, and degenerate electron exchange. The last reaction is imp ortant for the protonated tryptophan radical and determines the CIDNP kinet ics of tryptophan in acidic conditions. The nuclear relaxation times estima ted from the CIDNP kinetics are 44 +/- 9 mu s for all protons in the dipyri dyl radical, 91 +/- 20 mu s for the beta-CH2, 44 +/- 9 mu s for H2,6, and 6 3 +/- 12 mu s for H4 aromatic protons in the tryptophan radical.