Kazuyuki Ishii

Electronic Absorption Spectra and Redox Properties of C Type Cytochromes in Living Microbes

Shewanella is an electrogenic microbe that has significant content of c type cytochromes (ca. 0.5 mM). This feature allows the optical absorption spectra of the cell-membrane-associated proteins to be monitored in vivo in the course of extracellular respiratory electron-transfer reactions. The results show significant differences to those obtained in vitro with purified proteins.

Catalytic transformation of dinitrogen into ammonia and hydrazine by iron-dinitrogen complexes bearing pincer ligand

Synthesis and reactivity of iron-dinitrogen complexes have been extensively studied, because the iron atom plays an important role in the industrial and biological nitrogen fixation. As a result, iron-catalyzed reduction of molecular dinitrogen into ammonia has recently been achieved. Here we show that an iron-dinitrogen complex bearing an anionic PNP-pincer ligand works as an effective catalyst towards the catalytic nitrogen fixation, where a mixture of ammonia and hydrazine is produced. In the present reaction system, molecular dinitrogen is catalytically and directly converted into hydrazine by using transition metal-dinitrogen complexes as catalysts. Because hydrazine is considered as a key intermediate in the nitrogen fixation in nitrogenase, the findings described in this paper provide an opportunity to elucidate the reaction mechanism in nitrogenase.

Dual Chromophore-Nitroxides: Novel Molecular Probes, Photochemical and Photophysical Models and Magnetic Materials

Over the last decades scientists have faced growing requirements in novel methods of fast and sensitive analysis of antioxidant status of biological systems, spin redox probing and spin trapping, investigation of molecular dynamics, and of convenient models for studies of photophysical and photochemical processes. In approaching this problem, methods based upon the use of dual chromophore‐nitroxide (CN) compounds have been suggested and developed. A CN consists of two molecular sub‐functionality (a chromophore and a stable nitroxide radical) tethered together by spacers. In the dual compound the nitroxide is a strong intramolecular quencher of the fluorescence from the chromophore fragment. Reduction to hydroxylamine, oxidation of the nitroxide fragment or addition of an active radical yield the fluorescence increase and the parallel decay of the fragment electron spin resonance (ESR) signal. At certain conditions the dual molecules undergo photomagnetic switching and form excited state multi‐spin systems. These unique properties of CN were intensively exploited as the basis for several methodologies, which include molecular probing, modeling intramolecular photochemical and photophysical processes, and construction of new magnetic materials.

Magneto‐Chiral Dichroism of Organic Compounds

The homochirality of life—the biased distribution of l-amino acids and d-sugars—is still unsolved. Rikken and Raupach reported that magneto-chiral dichroism (MChD, that is, the dependence of the absorbance of a chiral molecule on the direction of a magnetic field to which it is exposed) could result in asymmetric photochemical reactions, since the MChD of two enantiomers is opposite in nature. Therefore, MChD became a plausible candidate for explaining the homochirality of life, in addition to the Earth s rotational motion (the Coriolis force) 3,4] and circularly polarized lightinduced asymmetric photochemical reactions. The presence of MChD of compounds which have both strong circular dichroism (CD) and magnetic circular dichroism (MCD) effects has been theoretically predicted. After this prediction and the first observation of MChD in the 5D0!7F1,2 luminescent transition of a europium(III) complex, observations of MChD in several metal compounds have been reported. However, the MChD of organic compounds that are correlated well with living beings has not been reported, which weakened the relationship between MChD and the homochirality of life. This existence of metal compounds showing MChD mainly originates from the fact that MCD, one of the important origins, is intensified by the d (or f) orbital-based degeneracy and angular momentum on the metals. Herein, we report the first observation of MChD in organic compounds using chiral J-aggregates of watersoluble porphyrins. To acquire intense MCD signals in organic compounds, the large orbital angular momentum of aromatic p-conjugated system was employed. Moreover, a very intense CD signal was obtained by the exciton chirality of the twisted configuration between porphyrin constituents, 15] which can provide the MChD of organic compounds (Figure 1). Figure 2a–c shows the UV/Vis, CD, and MCD spectra of the protonated form of meso-tetrakis(4-sulfonatophenyl)porphine (H4TPPS4, Figure 3a) and the chiral J-aggregates of H4TPPS4 (Figure 3b). These aggregates were prepared by adding chiral tartaric acid, which induces Coulomb interactions between the positively charged pyrrole protons and the negatively charged sulfonato groups. This formation of the J-aggregates shifts the Soret band (p–p*) to the red-side (491 nm) considerably, owing to the exciton interaction between the H4TPPS4 constituents. For both the enantiomers of the chiral J-aggregates of H4TPPS4, very intense, reproducible CD signals were acquired by the addition of chiral tartaric acid and subsequent addition of sulfuric acid (Figure 2). 17] Positive/negative and negative/positive CD spectral patterns are induced in the J-band region by the addition of land d-tartaric acids, respectively. This finding shows the enantioselective formation of the chiral J-aggregates of H4TPPS4. According to the method reported by Rib et al., the chiral J-aggregates of H4TPPS3 (Figure S2 in the Supporting Information) were also prepared by concentrating the acidic solution with a rotary evaporator. 16] The chiral Jaggregates of H4TPPS3 could be enantioselectively prepared by changing the rotation directions of the rotary evaporator Figure 1. MChD of organic compounds. The absorption coefficient of a chiral molecule is different for an unpolarized light beam when an externally applied magnetic field is parallel and antiparallel to the propagation direction. In the case of porphyrins, the cyclic perimeter model, C16H16 2 p system, is appropriate for explaining an orbital angular momentum (quantum number = ML), in which an ML =0, 1, 2, 3, 4, 5, 6, 7, and 8 sequence is observed in the molecular orbitals. Here, the ML values are 4 and 5 for the degenerate highest occupied and lowest unoccupied molecular orbitals (HOMOs and LUMOs), respectively. Although the symmetry of porphyrins is lower than that of C16H16 2 , the same basic nodal pattern sequence can still clearly be observed in the porphyrins, which can explain the intense MCD signals. Thus, the present MChD is intensified by the orbital angular momentum of aromatic p-conjugated system, which is different from the previous MChD based on the orbital angular momentum of metal compounds (d orbital: ML = 0, 1, 2, f orbital: ML = 0, 1, 2, 3).

In vivo Electrochemistry of C-Type Cytochrome-Mediated Electron-Transfer with Chemical Marking

In subsurface environments, certain types of microbes employ a unique metabolic strategy to respire on the naturally abundant iron-based minerals. Metal-reducing bacteria, such as those of the genus Shewanella and Geobactor, have a significant quantity of redox proteins located in their outer membrane (OM), and they transfer electrons from the cell surface to attached Fe oxides. 3] This is the terminal step in metabolism, and is an essential process to eliminate the excess electrons generated during the microbial oxidation of organic matter. Fe oxide respiration is considered to play an important role in the global biogeochemical cycling of iron. This process has also gained increasing attention for its potential application in mediatorless microbial fuel cells (MFCs). A great deal of research has been focused on measuring and proving bacterial extracellular electron-transfer (ET) respiration. The decaheme c-type cytochromes (c-Cyts), such as OmcA and MtrC, have been predicted to mediate ET to Fe oxides or electrodes. Up until this point, however, electrochemical and spectroscopic characterization of OM c-Cyts has been strictly limited to studies with purified proteins, although ET in living systems might be different from ET in purified proteins. In the whole-cell system, c-Cyts form a membrane-associated protein complex, and protein–protein and protein– membrane interactions would largely affect the energetics and kinetics of extracellular respiratory ET reactions. The main obstacle for the whole-cell study lies in the shortage of methods to differentiate c-Cyts from a number of uncharacterized biological molecules. Moreover, the extreme complexity and dynamic properties of living cells impede our ability to utilize knowledge developed in studies with purified proteins. Herein we report the control of the electronic states of OM c-Cyts in living cells of Shewanella by using an axial-coordination reaction on the heme groups of c-Cyts. We are the first to electrochemically identify OM c-Cyts and determine their ET kinetics under living conditions by using the specific binding affinity of nitric monoxide (NO). Our results reveal the existence of an respiratory ET chain with unusually high efficiency at the outer cell-membrane/electrode interfaces. Extracellular ET in living cells of Shewanella loihica PV-4 was monitored by using a single-chamber, three-electrode system with lactate as the carbon source and electron donor, as described in our previous studies. A tin-doped indium oxide electrode (ITO) was used as the working electrode and was placed on the bottom surface of the reactor. The cells were inoculated into the reactor under potentiostatic conditions at 0.4 V (versus the standard hydrogen electrode, SHE) for 25 h to prepare an electrode with attached cells. The optical microscopy image showed the formation of a thin layer (approximately 2 mm in length and 0.5 mm in width) that consisted of rod-shaped cells ; most of the cells attached horizontally to the electrode surface, not through the apex of their rod-like shape. Viability of the cells on the ITO electrode was confirmed by the generation of a microbial current (Figure S1 in the Supporting Information). As shown in the cyclic voltammograms (CVs, Figure 1), strain PV-4 exhibited one redox wave with a

Optically Active Oxo(phthalocyaninato)vanadium(IV) with Geometric Asymmetry: Synthesis and Correlation between the Circular Dichroism Sign and Conformation

Oxovanadium(IV) phthalocyanines (VOPcs) with a single-handed rotation have been prepared, and their right- and left-handed enantiomers resolved on a chiral HPLC column. These enantiomers gave circular dichroism (CD) spectra of opposite signs; the correlation between the CD sign and conformation was obtained by time-dependent density functional theory (TDDFT) calculations: an enantiomer showing a negative sign in the Q band was suggested to be the right-handed conformer viewing from the axial oxygen side, whereas that giving a positive CD sign was assigned to the left-handed conformer. Although silicon phthalocyanines (SiPcs) with two different alkoxy axial ligands have been resolved similarly, the absence of a meaningful CD difference probably reflects the flat character of the SiPc plane compared to the VOPc plane. Changes in the Q-band CD, depending on the relative orientation of the peripheral substituents, have been worked out theoretically and the origin of the chiroptical properties is discussed.

Observation of In Vivo Cytochrome-Based Electron-Transport Dynamics Using Time-Resolved Evanescent Wave Electroabsorption Spectroscopy†

As biological processes consist of intricate chains of various chemical reactions, one of the central goals of life science is to clarify these reactions from a molecular-dynamics perspective to better understand the processes of life. Flash photolysis using fast laser pulses is the best technique for studying chemical reactions with minimal limitations related to timeresolution. This technique has provided important insights into the crucial details of several dynamic photoactive processes, such as electron and energy transfer in photosynthesis. To photochemically control photo-inactive proteins, the coordination of CO to hemes has been employed as a photochemical tool, as the iron-bound CO ligand dissociates from hemes upon visible-light irradiation. Using the coordination of CO to isolated heme proteins, such as myoglobin, hemoglobin, cytochromes, and cytochrome c oxidase, realtime studies of protein dynamics have been widely conducted to examine the exchange of axial ligands, protein folding, and electron transfer between active sites. Among the numerous critical metabolic processes, cellular respiration, which includes the electron-transport system, is particularly important for the generation of chemical energy, primarily in the form of ATP. Since in vivo respiratory electron-transport systems are thermodynamically open and consist of many proteins interacting with each other, the direct application of flash photolysis to intact living cells will aid in the understanding of the molecular dynamics of in vivo respiratory electron-transport chains. Herein, we demonstrate that time-resolved evanescent wave electroabsorption (TREWEA) spectroscopy can be applied to the direct observation of respiratory electrontransport dynamics based on c-type cytochromes (c-Cyts) in the metal-reducing bacterium Shewanella loihica PV-4. To accomplish this observation, the following techniques were employed: 1) For direct investigation of in vivo respiratory electron-transport dynamics of bacteria, we applied the artificial photochemical reaction of CO to the bacterium S. loihica PV-4 (Figure 1a). In this system, respiration is inhibited by blockage of the redox activities of hemes upon CO binding but can be subsequently reactivated by photo-

Wavelength-tunable excited-state absorption and optical limiting effects in the Q band region based on silicon phthalocyanine oligomers

Optical limiting effects of silicon phthalocyanine (SiPc) oligomers have been examined in the Q band region in order to demonstrate a novel concept for controlling the wavelength of optical limiting effects.

Helicity transfer in rotary evaporator flow

Mechanical rotation of a magnetic stirrer or a rotary evaporator can induce an enantiomeric excess of supramolecular species. In this study, we investigate the effect of fluid motion in a rotary evaporator on chiral supramolecular species. It is shown theoretically that the twisting effect of fluid motion on cylindrical particles is expressed in terms of helicity dissipation rate. Helicity dissipation can be interpreted as the helicity transfer from helical fluid motion to chiral supramolecular structures. A numerical simulation of flow in a rotary evaporator was carried out to evaluate the helicity and its dissipation rate. The volume integral of the helicity dissipation in the computational domain showed a positive value; its sign agrees with experiment in which the right-handed helical structures of J-aggregates were induced by the counter-clockwise rotation of a rotary evaporator. Furthermore, terms in the transport equation for the helicity were evaluated for investigating the helicity behavior.

Magneto-chiral dichroism of aromatic π-conjugated systems

Magneto-chiral dichroism (MChD) is an interesting phenomenon in which the absorbance of a chiral molecule depends on the magnetic field direction. As the MChD of two enantiomers is opposite in nature, MChD has received a considerable attention not only in magneto-optical devices but also for new asymmetric synthetic methods and as an explanation for the origin of the homochirality of life. Recently, several experimental observations of MChD have been reported in aromatic π-conjugated systems. In this review, we introduce these MChD observations, and discuss the theoretical explanations of the π-electronic properties of aromatic π-conjugated systems, such as the orbital angular momentum and the exciton chirality. Furthermore, the possibility of using MChD for the photoresolution of aromatic compounds is discussed.