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 (kq) for the ET quenching at 25 degrees C were obtained as kq(MCN)=(1.9+/-0.1)x10(6) M-1 s-1 and kq(MCD)=(3.0+/-0.2)x10(6) M-1 s-1, respectively. The ratio of kq(MCD)/kq(MCN)=1.6 indicates that the [MCD]2+ preferentially quenches 3ZnMb)*. The second-order rate constants (kb) for the thermal back ET reaction from [MCN].+ and [MCD].+ to ZnMb.+ at 25 degrees C were kb(MCN)=(0.79+/-0.04)x10(8) M-1 s-1 and kb(MCD)=(1.0+/-0.1)x10(8) M-1 s-1, respectively, and the selectivity was kq(MCD)/kq(MCN)=1.3. Both quenching and thermal back ET reactions are controlled by the ET step. In the quenching reaction, the energy differences of DeltaDeltaH ( not equal)(MCD-MCN) and DeltaDeltaS ( not equal)(MCD-MCN) at 25 degrees C were obtained as -1.1 and 0 kJ mol-1, respectively. On the other hand, DeltaDeltaH (not equal)(MCD-MCN)=11+/-2 kJ mol-1 and TDeltaDeltaS (not equal)(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 degrees C) was due to the DeltaDeltaS ( not equal) value obtained in the thermal back ET reaction.