Time-Resolved CIDNP as a Tool for Determination of Rate Constants of Triplet Quenching Using the Example of Photo-Induced Oxidation of Histidine-Containing Compounds

We describe the application of time-resolved chemically induced dynamic nuclear polarization (TR CIDNP) in the determination of the bimolecular rate constants of triplet quenching. This method is based on the use of a CIDNP-active reference triplet quencher with a known quenching rate constant. The quenching reaction of the triplet excited photosensitizer, 3,3',4,4'-tetracarboxy benzophenone (TCBP), was considered as the reaction under study. In each experiment, geminate CIDNP spectra were obtained during irradiation of the solution containing: (i) TCBP and a reference quencher; (ii) TCBP, a reference quencher, and a quencher whose triplet quenching rate constant was to be determined. The degree of decrease in the signal intensity of the reference quencher for the case (ii) compared to the case (i) depended on the desired quenching rate constant which made it possible to calculate the latter. Thus, the rate constants were determined for quenching of triplet TCBP by histidine-containing compounds.