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. ash photolysis, we designed and prepared new optically active agents, such as N,N'-dimethylcinchoninium diiodide ([MCN] I-2) and N,N'-dimethylcinchonidinium diiodide ([MCD] I-2). 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 degrees C were obtained as k(q)(MCN)=( 1.9 +/- 0.1) x 10(6) M-1 s(-1) and k(q)(MCD) = (3.0 +/- 0.2) x 10(6) 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 degrees C were k(b)(MCN)=(0.79 +/- 0.04) x 10(8) M-1 s(-1) and k(b)(MCD)= (1.0 +/- 0.1) x 10(8) 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 Delta Delta H not equal (MCD - MCN) and Delta Delta S-not equal (MCD - MCN) at 25 degrees C were obtained as - 1.1 and 0 kJ mol(-1), respectively. On the other hand, Delta Delta H-not equal (MCD - MCN)= 11 +/- 2 kJ mol(-1) and T Delta Delta S-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 Delta Delta S-not equal value obtained in the thermal back ET reaction.