Remarkably stereoselective photoinduced electron-transfer reaction between zinc myoglobin and optically active binaphthyl bisviologen

We have designed and synthesized new optically active bisviologens ([BNMV]4+) containing a binaphthyl moiety to examine the stereoselective photoinduced electron-transfer (ET) reactions with zinc-substituted myoglobin (ZnMb) by flash photolysis. The photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [BNMV]4+ ions to form the radical pair of a ZnMb cation (ZnMb·+) and a reduced viologen ([BNMV]·3+), followed by a thermal ET reaction to the ground state. The rate constants (kq) for the ET quenching at 25 °C were obtained as kq(R)=(2.9±0.2)×107 M−1 s−1 and kq(S)=(2.2±0.2)×107 M−1 s−1, respectively. The ratio of kq(R)/kq(S)=1.3 indicates that the (R)-isomer of the chiral viologen preferentially quenches 3(ZnMb)*. On the other hand, the rate constants (k) for the thermal ET reaction from [BNMV]·3+ to ZnMb–+ at 25 °C were k(R)=(1.2±0.1)×108 M−1 s−1 and k(S)=(0.47±0.03)×108 M−1 s−1, respectively, and the ratio remarkably increased to k(R)/k(S)=2.6. The activation parameters, ΔH≠ and ΔS≠, were determined from the kinetic measurements at various temperatures (10–30 °C) to understand the ET mechanisms. In the quenching reaction, the energy differences of ΔΔH≠(R−S) and TΔΔS≠(R−S) at 25 °C were calculated to be −3.9±1.6 and −3.3±0.2 kJ mol−1, respectively, whereas ΔΔH≠(R−S)=7.7±1.9 kJ mol−1 and TΔΔS≠(R−S)=9.9±0.5 kJ mol−1 were found for the thermal ET reaction. Therefore, the thermal ET reaction to the ground state was proved to be dominated by the entropy term, and the large stereoselectivity may arise from the decrease in charge repulsion between donor and acceptor.