Fate of the excited triplet state of zinc cytochrome c in the presence of iron(III), iron(II), iron-free, and heme-free forms of cytochrome c

Metal ion-dependent fluorescent dynamics of photoexcited zinc-porphyrin and zinc-myoglobin modified with ethylenediaminetetraacetic acid

The reconstituted zinc-myoglobin (ZnMb) dyads, ZnMb-[M(II)(edta)], have been prepared by incorporating a zinc-porphyrin (ZnP) cofactor modified with ethylenediaminetetraacetic acid (H(4)edta) into apo-Mb. In case of the monomeric ZnP(edta) cofactor coordinated by one pyridine molecule, ZnP(py)(edta), a spontaneous 1:1 complex with a transient metal ion was formed in an aqueous solvent, and the photoexcited singlet state of ZnP, (1)(ZnP)*, was quenched by the [Cu(II)(edta)] moiety through intramolecular photoinduced electron-transfer (ET) reaction. The rate constant for the intramolecular quenching ET (k(q)) at 25 degrees C was successfully obtained as k(q) = 5.1 x 10(9) s(-1). In the case of Co(2+), Ni(2+), and Mn(2+), intersystem crossing by paramagnetic effect was mainly considered between (1)(ZnP)* and the [M(II)(edta)] complex. For the ZnMb-[M(II)(edta)] systems, the intramolecular ET reaction between the excited singlet state of (1)(ZnMb)* and the [Cu(II)(edta)] moieties provided the slower quenching rate constant, k(q) = 2.1 x 10(8) s(-1), compared with that of the ZnP(py)(edta) one. Kinetic studies also presented the efficient fluorescence quenching of the (1)(ZnMb)*-[Co(II)(edta)] dyad. Our study clearly demonstrates that wrapping of the ZnP cofactor by the apoprotein matrix and synthetic manipulation at the Mb surface ensure metal ion-sensitive fluorescent dynamics of ZnMb and provides valuable information to elucidate the complicated mechanism of the biological photoinduced ET reactions of hemoproteins.

Order Parameters and Orientation Distributions of Solution Adsorbed and Microcontact Printed Cytochrome c Protein Films on Glass and ITO

The structure of solution adsorbed and microcontact printed (muCP) cytochrome c (cyt c) films on glass and indium tin oxide (ITO) was investigated using attenuated total reflectance (ATR) and total internal reflectance fluorescence (TIRF) spectroscopies to determine the orientation of the heme groups in the films. The second and fourth order parameters of the heme as well as information on the angle between the absorption and emission dipoles of the heme, gamma, were experimentally determined. The order parameters of the heme are related to the order parameters of the protein molecule using the known angle between the heme plane and the electrostatic dipole moment of the cyt c protein. The effect of the surface roughness of the substrates (glass and ITO) was also taken into account quantitatively using AFM data. Physically possible order parameters were obtained for the heme group in both solution adsorbed and muCP films, but not for the electrostatic dipole moment of the protein. In addition, the experimental values of {cos2 gamma} for immobilized zinc-substituted cyt c are greater than the values of {cos2 gamma} determined in viscous solutions, which could be an indication that the environment of the heme groups changes upon adsorption. The electron transfer behavior of solution adsorbed and muCP films on ITO, determined using electrochemical methods, is compared to their orientation distribution and surface coverage as determined by spectroscopic methods.

Stereoselective and driving-force-dependent photoinduced electron-transfer reactions of zinc myoglobin with optically active N,N′-dimethylcinchoninium and N,N′-dimethylcinchonidinium ions

In order to understand the detailed mechanism of the stereoselective photoinduced electron-transfer (ET) reactions of zinc-substituted myoglobin (ZnMb) with optically active molecules by flash photolysis, we designed and prepared new optically active agents, such as N,N'-dimethylcinchoninium diiodide ([MCN]I2) and N,N'-dimethylcinchonidinium diiodide ([MCD]I2). The photoexcited triplet state of ZnMb, 3(ZnMb)*, was successfully quenched by [MCN]2+ and [MCD]2+ ions to form the radical pair of ZnMb cation (ZnMb.+) and reduced [MCN].+ and [MCD].+, followed by a thermal back ET reaction to the ground state. The rate constants (kq) for the ET quenching at 25 degrees C were obtained as kq(MCN)=(1.9+/-0.1)x10(6) M-1 s-1 and kq(MCD)=(3.0+/-0.2)x10(6) M-1 s-1, respectively. The ratio of kq(MCD)/kq(MCN)=1.6 indicates that the [MCD]2+ preferentially quenches 3ZnMb)*. The second-order rate constants (kb) for the thermal back ET reaction from [MCN].+ and [MCD].+ to ZnMb.+ at 25 degrees C were kb(MCN)=(0.79+/-0.04)x10(8) M-1 s-1 and kb(MCD)=(1.0+/-0.1)x10(8) M-1 s-1, respectively, and the selectivity was kq(MCD)/kq(MCN)=1.3. Both quenching and thermal back ET reactions are controlled by the ET step. In the quenching reaction, the energy differences of DeltaDeltaH ( not equal)(MCD-MCN) and DeltaDeltaS ( not equal)(MCD-MCN) at 25 degrees C were obtained as -1.1 and 0 kJ mol-1, respectively. On the other hand, DeltaDeltaH (not equal)(MCD-MCN)=11+/-2 kJ mol-1 and TDeltaDeltaS (not equal)(MCD-MCN)=-10+/-2 kJ mol-1 were given in the thermal back ET reaction. The highest stereoselectivity of 1.7 for [MCD].+ found at low temperature (10 degrees C) was due to the DeltaDeltaS ( not equal) value obtained in the thermal back ET reaction.

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 ( 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.