Stereoselective and driving-force-dependent photoinduced electron-transfer reactions of zinc myoglobin with optically active N,N′-dimethylcinchoninium and N,N′-dimethylcinchonidinium ions

摘要

In order to understand the detailed mechanism of the stereoselective photoinduced electron-transfer (ET) reactions of zinc-substituted myoglobin (ZnMb) with optically active molecules by flash photolysis, we designed and prepared new optically active agents, such as N,N′-dimethylcinchoninium diiodide ([MCN]I2) and N,N′-dimethylcinchonidinium diiodide ([MCD]I2). The photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [MCN]2+ and [MCD]2+ ions to form the radical pair of ZnMb cation (ZnMb·+) and reduced [MCN]·+ and [MCD]·+, followed by a thermal back ET reaction to the ground state. The rate constants (k q) for the ET quenching at 25 °C were obtained as k q(MCN)=(1.9±0.1)×106 M−1 s−1 and k q(MCD)=(3.0±0.2)×106 M−1 s−1, respectively. The ratio of k q(MCD)/k q(MCN)=1.6 indicates that the [MCD]2+ preferentially quenches 3(ZnMb)*. The second-order rate constants (k b) for the thermal back ET reaction from [MCN]·+ and [MCD]·+ to ZnMb·+ at 25 °C were k b(MCN)=(0.79±0.04)×108 M−1 s−1 and k b(MCD)=(1.0±0.1)×108 M−1 s−1, respectively, and the selectivity was k q(MCD)/k q(MCN)=1.3. Both quenching and thermal back ET reactions are controlled by the ET step. In the quenching reaction, the energy differences of ΔΔH ≠(MCD–MCN) and ΔΔS ≠(MCD–MCN) at 25 °C were obtained as −1.1 and 0 kJ mol−1, respectively. On the other hand, ΔΔH ≠(MCD–MCN)=11±2 kJ mol−1 and TΔΔS ≠(MCD–MCN)=−10±2 kJ mol−1 were given in the thermal back ET reaction. The highest stereoselectivity of 1.7 for [MCD]·+ found at low temperature (10 °C) was due to the ΔΔS ≠ value obtained in the thermal back ET reaction.