Photoinduced electron-transfer reactions of zinc and magnesium myoglobins

Stereoselective Photoinduced Electron-Transfer Reactions of Zinc Myoglobin with Optically Active Viologens

“…Plots of the first-order rate constant of the quenching of 3 (ZnMb)*, k obsd , vs the concentrations of viologens were linear (Figures and ), indicating no appreciable complex formation between 3 (ZnMb)* and viologen. When the reaction was monitored at 680 nm, we observed the formation and decay of the radical cation of ZnMb •+ (Figure b), indicating the ET quenching followed by a thermal back ET reaction, as has been previously observed. , The decay of ZnMb •+ was of second order, indicating that an equimolar amount of ZnMb •+ with a viologen radical cation formed. Therefore, the photoinduced ET reaction of ZnMb with viologen is represented in Scheme .…”

“…Three unknown parameters, A 0 , k b [A] 0 , and A ∞ , were simultaneously determined. The value of k b was determined by using the value of [A] 0 which was estimated from the concentration of 3 (ZnMb)* (Δε 428 = ε(ground) − ε(triplet) = 1.00 × 10 5 M -1 cm -1 ). 5b, The values of k b are listed in Table .…”

“…Metmyoglobin from horse heart muscle (Sigma) was purified as previously described was carried out at 4 °C in the dark using a method that has been published …”

“…Photoinduced electron-transfer (ET) reactions of metalloproteins have received considerable attention in the fields of both chemistry and biochemistry. In the case of zinc-substituted hemoproteins, the excited triplet state is bimolecularly quenched by a variety of quenchers and several mechanisms, including the conformational gating mechanism, have been suggested for zinc cytochrome c , hemoglobin, and myoglobin. , We have recently proposed that the conformational change of the excited triplet state of zinc myoglobin ( 3 (ZnMb)*) to an active form is induced by attacking of external quenchers and that the charge and the steric bulk of the quenchers play an important role in the quenching process . As chirality is an obvious property of reactive sites in proteins, optically active substances must discriminate the environment around the reactive site at the surface of metalloproteins.…”

“…Viologens are diquaternary salts of 4,4‘-bipyridine and have been used extensively as mediators in catalytic photolysis of water under visible light with a sensitizer − ,, Therefore, we chose the optically active viologens and bisviologens ( 1 − 13 ) as quenchers (Chart ), which were newly synthesized, to investigate the ET quenching and back ET reactions of 3 (ZnMb)*. The preliminary communication has been reported elsewhere …”