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 kH = 1.2 × 108 M-1 s-1, and with deprotonated His- via electron transfer with ke = 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: T1 = 44 ± 9 μs for all DP protons, T1 = 196 ± 25 μs for the β-CH2 protons of HisH, and T1 = 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 kex = 1.5 × 108 M-1 S-1. © 2000 American Chemical Society.
