Direct Comparison of Chiral Substituent Effects for Viologens in Stereoselective Photoinduced Electron-Transfer Reactions with Zinc Myoglobin

Abstract

Abstract In order to understand the detailed mechanisms of stereoselective photoinduced electron-transfer (ET) reactions of zinc-substituted myoglobin (ZnMb) with optically active viologens by flash photolysis, we prepared new optically active agents, such as (R,R)- and (S,S)-1,1′-bis(3-hydroxy-2-methylpropyl)-4,4′-bipyridinediylium dichloride ([BMV]Cl2). The photo-excited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by (R,R)- and (S,S)-[BMV]2+ ions to form the radical pair of ZnMb cation (ZnMb•+) and (R,R)- and (S,S)-[BMV]•+, followed by a thermal back ET reaction to the ground state. The ratio of the rate constants (kq) for ET quenching at 25 °C, kq(R,R)/kq(S,S) = 1.1, indicates that the (R,R)-[BMV]2+ preferentially quenches 3(ZnMb)*. The selectivity of the rate constants (kb) for the back ET from (R,R)- and (S,S)-[BMV]•+ to ZnMb•+ at 25 °C was kb(R,R)/kb(S,S) = 0.83. The highest stereoselectivity of 1.3 for [BMV]•+ was found at low temperature (10 °C) in the thermal back ET reaction, where ΔΔH≠(R–S) = 6.2 ± 1.9 kJ mol−1 and ΔΔS≠(R–S) = 19 ± 2 J mol−1 K−1 were obtained. The structural differences between [BMV]2+ and the other viologens imply that the bulky aromatic substituent seems to be more important than the distance from the chiral center to the viologen moiety in order to enhance the ET stereoselectivity.