The mechanism of the reactions between photoexcited 2,2‘-dipyridyl and N-acetyl tryptophan has been studied by laser flash photolysis and time-resolved CIDNP (Chemically Induced Dynamic Nuclear Polarization). The transient absorption spectra obtained at different delays after the laser pulse are attributed 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 irrespective of pH the primary photochemical step is electron transfer from the tryptophan to triplet dipyridyl followed by protonation/deprotonation of the radicals so formed. The rate constant of the reaction of triplet dipyridyl with tryptophan is close to the diffusion-controlled limit and decreases slightly with increasing pH. The kinetics and the stationary value of the CIDNP are determined by the rates of radical termination, nuclear paramagnetic relaxation, and degenerate electron exchange. The last reaction is important for the protonated tryptophan radical and determines the CIDNP kinetics of tryptophan in acidic conditions. The nuclear relaxation times estimated from the CIDNP kinetics are 44 ± 9 μs for all protons in the dipyridyl radical, 91 ± 20 μs for the β-CH2, 44 ± 9 μs for H2,6, and 63 ± 12 μs for H4 aromatic protons in the tryptophan radical.

The reaction mechanism and details of the formation of CIDNP (chemically induced dynamic nuclear polarization) in the photoreactions of 2,2‘-dipyridyl (DP) and N-acetyl histidine (HisH) in aqueous solution have been studied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state TDP reacts with protonated HisH2 + via hydrogen atom transfer with a rate constant k H = 1.2 × 108 M-1 s-1, and with deprotonated His- via electron transfer with k e = 7.5 × 109 M-1 s-1. No reaction occurs when the histidine imidazole ring is in its neutral state HisH, or when the dipyridyl triplet is protonated, TDPH+. The nuclear spin−lattice relaxation times in the radicals formed in these reactions have been determined from the CIDNP kinetics:  T 1 = 44 ± 9 μs for all DP protons, T 1 = 196 ± 25 μs for the β-CH2 protons of HisH, and T 1 = 16 ± 5 μs for the H-2 and H-4 protons of HisH. Under strongly basic conditions the CIDNP is greatly affected by degenerate electron exchange between the neutral His· radical and His- anion, with rate constant k ex = 1.5 × 108 M-1 s-1.

Time-resolved chemically induced dynamic nuclear polarization (CIDNP) and laser flash photolysis (LFP) techniques have been used to study the kinetics and mechanism of the photochemical reaction between 2,2‘-dipyridyl (DP) and the dipeptide l-tryptophan-l-tyrosine (TrpH-TyrOH) in acidic aqueous solution (pH = 3.8). Analysis of the geminate CIDNP pattern reveals that the quenching of the protonated triplet dipyridyl TDPH+ results in the formation of both tryptophan and tyrosine radicals from the dipeptide with comparative efficiency. The total quenching rate constant of triplet dipyridyl by TrpH-TyrOH was found to be (2.5 ± 0.3) × 109 M-1 s-1. The radical transformation TrpH+•→TyrO• via intramolecular electron transfer (IET) leads to an increasing tyrosyl radical concentration, growth of the tyrosine CIDNP signals, fast decay of the CIDNP signal of TrpH, and inversion of the CIDNP sign from emission to enhanced absorption for DP. The nuclear spin−lattice relaxation times of the radicals formed in the reactions and the rate constant for IET (5 × 105s-1) were determined by quantitative analysis of the CIDNP kinetics at different dipeptide concentrations in D2O. A significant isotope effect (k H/k D = 1.5) was found for the IET rate constant by LFP measurements. In the presence of efficient IET, degenerate electron exchange between the TrpH•+ radical of the dipeptide and the diamagnetic molecule makes a negligible contribution to the decay of tryptophan CIDNP signal.