Time-Resolved CIDNP and Laser Flash Photolysis Study of the Photoreactions of N-Acetyl Histidine with 2,2'-Dipyridyl in Aqueous Solution

摘要

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 solutin have beens tudied using laser flash photolysis and time-resolved CIDNP techniques. The triplet state ~TDP reacts with protonated HisH_2~+ via hydrogen atom transfer with a rate constant k_H=1.2×10~8 M~(-1) s~(-1), and with deprotonated His~- via electron transfer with k_e=7.5×10~9 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 #mu#s for all DP protons, T_1=196+-25 #mu#s for the #beta#-CH_2 protons of HisH, and T_1=16+-5 #mu#s for the H-2 and H-4 prtons of HisH. Under strongly basic conditinos the CIDNP is greatly affected by degenerate electron exchange between the neutral His. radical and His~- anion, with rate constant k_ex=1.5×10~8 M~(-1) s~(-1).