Stereoselective Photoinduced Electron-Transfer Reaction between Zinc Myoglobin and New Chiral Quinolinium Ions

New chiral quinolinium derivatives, 1-[(S)- or (R)-(1-phenylethyl)carbamoylmethyl]-6-methoxyquinolinium hexafluorophosphate ([PMQ]PF6, 2) and 1-[(S)- or (R)(1-phenylethyl)carbamoylmethyl]quinolinium hexafluorophosphate ([PQ]PF6, 3), were synthesized and characterized. A cyclic voltammetry in MeCN shows an irreversible redox behavior at -0.88 V and -0.85 V vs SCE (saturated calomel electrode) for 2 and 3, respectively. Fluorescence from the quinolinium moiety of 2 and 3 was observed at 455 nm and 440 nm, respectively. The fluorescence lifetime of 2 was longer than that of 3: (tau)f = 30 ns (2) and 20 ns (3) in MeCN and (tau)f = 26 ns (2) and 16 ns (3) in H2O). The excited triplet state of zinc-substituted myoglobin (3(ZnMb)*) was quenched by chiral [PMQ]+ and [PQ]+ ions; thereafter, the back electron-transfer (ET) reaction from a quinoline radical (PMQ• or PQ•) to a zinc myoglobin radical cation was detected. The stereoselectivity was observed for both ET quenching and back ET reactions; the (S)-isomers preferentially quench 3(ZnMb)* (kq(S)/kq(R) = 1.3 and 1.4 at 25 °C for [PMQ]+ and [PQ]+, respectively); in contrast, the (R)-isomers react faster than the (S)-isomers in the back ET reaction (kb(R)/kb(S) = 1.3 and 1.4 for PMQ• and PQ•, respectively). From a comparison of the rate constants with those for the previously reported systems we suggest that both the quenching and back reactions are controlled by ET, but not by conformational gating.