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

Abstract

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 ( k(q)) for the ET quenching at 25 degrees C were obtained as k(q)( R)=(2.9+/-0.2)x10(7) M(-1) s(-1) and k(q)( S)=(2.2+/-0.2)x10(7) M(-1) s(-1), respectively. The ratio of k(q)( R)/ k(q)( 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 degrees C were k( R)=(1.2+/-0.1)x10(8) M(-1) s(-1) and k( S)=(0.47+/-0.03)x10(8) M(-1) s(-1), respectively, and the ratio remarkably increased to k( R)/ k( S)=2.6. The activation parameters, Delta H(not equal) and Delta S(not equal), were determined from the kinetic measurements at various temperatures (10-30 degrees C) to understand the ET mechanisms. In the quenching reaction, the energy differences of Delta Delta H*(R- S) and T Delta Delta S*( R- S) at 25 degrees C were calculated to be -3.9+/-1.6 and -3.3+/-0.2 kJ mol(-1), respectively, whereas Delta Delta H*( R-S)=7.7+/-1.9 kJ mol(-1 )and T Delta Delta S*( 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.

Photoinduced Electron-Transfer Reactions of Carbonic Anhydrase Inhibitor Containing Tris(2,2′-bipyridine)ruthenium(II) Analogue

“…We next examined the electrochemical property of [Ru(bpy) 2 (bpybs)] 2+ by cyclic voltammetry (CV). The reversible one-electron oxidation wave was presented at +1.38 V (vs Ag/AgClO 4 ) in 0.050 M [Bu 4 N]ClO 4 acetonitrile solution ([Ru III (bpy) 2 (bpybs)] 3+ /[Ru II (bpy) 2 (bpybs)] 2+ ; see Table ) calibrated by 1,1′-dimethyl-4,4′-bipyridinediylium perchlorate, [MV](ClO 4 ) 2 (−0.45 V vs NHE) . For the reduction of the bpy ligand, three peaks appeared at −1.25, −1.50, and −1.74 V (vs Ag/AgClO 4 ).…”

Photoinduced Intramolecular Electron-Transfer Reactions of Reconstituted Met- and Zinc-Myoglobins Appending Acridine and Methylacridinium Ion as DNA-Binders

“…Kinetic Measurements. Procedure for the kinetics was the same as that reported previously. 20b, All the sample solutions were gently purged with Ar gas and then carefully degassed by freeze−pump−thaw cycles. Time-resolved fluorescence spectra were measured by a single-photon counting method using a second harmonic generation (SHG) of a Ti:sapphire laser (Spectra-Physics, Tsunami 3950-L2S, 1.5 ps fwhm) as an excitation source .…”

“…For spectra in the visible region (400−600 nm), a Si−PIN photodiode (Hamamatsu Photonics, S1722−02) was used as a detector . A single flash photolysis was done in the deaerated solutions containing ZnMbs (5.0 × 10 -6 M) and quenchers (0 − 6.0 × 10 -5 M) at 25 °C, pH 7.0 (0.010 M phosphate buffer), and I = 0.020 M, using a Photal RA-412 pulse flash apparatus with a 30 μs pulse-width Xe lamp (λ > 450 nm; a Toshiba Y-47 glass filter) 20b…”

“…A series of typical organic and inorganic quenchers, such as methylviologen (MV 2+ ), quinones, and inorganic complexes were utilized for studying bimolecular photoinduced ET and several mechanisms, including conformational gating, have been suggested for zinc cytochrome c , hemoglobin, and myoglobin (ZnMb). 5,14,16 Along this line, since an obvious property of the protein surface is chirality, stereoselective bimolecular photoinduced ET of ZnMb with chiral organic agents has been discussed to date …”

Synthesis, Structure, and Photochemical Behavior of [5]Heli-viologen Isomers

“…In order to address this gap in the structural landscape of viologens, we report here the synthesis, structural characterization, photophysics, and photochemistry of a novel isomeric pair of π-extended helical viologens, 5,10-dimethyl-5,10-diaza[5]­helicene bis-tetrafluoroborate and 3,8-dimethyl-3,8-diaza[5]­helicene bis-tetrafluoroborate ( 1 2+ and 2 2+ ) . These represent rare examples of chiral viologens ,− and the first examples that we are aware of in which the chirality is due to the polyaromatic backbone. These compounds also complete the link, first established by the fascinating helquats, − between helicenes and viologens.…”