Yasuhisa Mizutani

Ruthenium‐Catalyzed Selective and Efficient Oxygenation of Hydrocarbons with Water as an Oxygen Source

The development of methods for the highly selective and efficient conversion of abundant organic resources into valuable products is crucial for a sustainable society. To achieve this goal, extensive studies on the methodology of efficient material conversion with metal complexes as catalysts have been made for a long time. High-valent metal–oxo species are key intermediates in biological oxidations by metalloenzymes (mainly heme and non-heme iron enzymes), which catalyze the oxygenation of hydrocarbons in metabolic and catabolic processes. These oxygenases involve high-valent metal–oxo species as reactive species that arise by reductive activation of molecular oxygen coupled with proton transfer. Peroxides such as hydrogen peroxide can lead to a so-called “peroxide shunt” to perform the catalytic oxygenation; this mechanism is found for cytochrome P450 and methane monooxygenase. Thus, a number of model systems for these enzymatic oxidations have been developed to elucidate the reaction mechanisms and to perform effective catalytic oxygenation of external substrates with metal complexes involving the formation of high-valent metal–oxo species. These systems usually require organic solvents and excess amount of organic or inorganic peroxides as both oxidants and oxygen sources. Moreover, in such cases, the reaction pathways become complicated and give multiple products. Consequently it is difficult to control the product distribution that arises mainly from the inevitably produced radical species. Another strategy to generate a high-valent metal–oxo species has been recognized in the oxygen-evolving complex (OEC) in Photosystem II (PSII) for the photosynthesis to oxidize water to produce dioxygen. At the OEC, a manganese(V)–oxo species has been proposed to be formed by proton-coupled electron transfer (PCET), and the deprotonation of coordinated water and the oxidation of the metal center are thought to occur concertedly. This strategy has been applied to form and isolate high-valent metal–oxo species to perform stoichiometric oxidation reactions; however, it has not been applied to catalytic oxidations with transition-metal complexes as catalysts in water. Inspired by the reactions at the OEC in photosynthesis, we have tried to establish a novel catalytic oxygenation system using water as both the solvent and the oxygen source by virtue of PCET. We report herein the formation of a novel ruthenium(IV)–oxo complex and its reactivity toward highly efficient and selective catalytic oxygenation and oxidation reactions of various hydrocarbons in water, which can be used as an oxygen source. We synthesized a novel bis-aqua Ru complex, [Ru(tpa)(H2O)2](PF6)2 (1; tpa= tris(2-pyridylmethyl)amine) (Figure 1a,b), by the treatment of [RuCl(tpa)]2(PF6)2 [20] with AgPF6 in water. Complex 1 exhibits a reversible twostep deprotonation–protonation equilibrium, and the two pKa values were determined by UV/Vis spectroscopic titration (see Figure S1 in the Supporting Information) in the range of

Resonance Raman spectra of highly oxidized metalloporphyrins and heme proteins

Abstract Studies of resonance Raman (RR) spectra of highly oxidized metalloporphyrins and heme proteins in the past decade are surveyed comprehensively. Following the introduction this article consists of two main sections. In Section 2 the FeIV=O stretching (v(FeIV=O)) vibrations of ferryloxo neutral and /Gv cation radical porphyrins and their porphyrin in-plane modes are discussed and the characters of the FeIV=O bond and environmental effects on it are elucidated. For porphyrin π cation radicals, the RR spectral differences between the a1u and a2u radicals are interpreted in relation to the electronic properties of those orbitals for divalent metalloporphyrins as well as iron porphyrins. Some changes in vibrational characters on oxidation of the porphyrin ring are noted on the basis of isotopic substitution data. Studies of environmental effects on porphyrin π cation radicals permit one to deduce factors for determination of radical types. Current RR studies of nitrido iron and N-oxide iron porphyrins and other metal oxo porphyrins with MIV, MV and MVI ions are also covered. In Section 3 the present state of RR studies of reaction intermediates of heme enzymes containing ferryloxo neutral and π cation radical porphyrins is summarized, and discussion is focused on the v(FeIV=O) RR bands. Attention is paid to compound I of horseradish peroxidase for which complete historical vicissitudes of observed RR spectra are pursued. The most recent results from RR studies of reaction intermediates of cytochrome c oxidase with the FeIII-O-O-H and FeIV=O hemes are explained in detail. Finally RR spectra of reaction intermediates of iron-chlorin containing enzymes as well as those of catalase and thiolate-ligated heme enzymes are reviewed.

Primary protein response after ligand photodissociation in carbonmonoxy myoglobin

Time-resolved UV resonance Raman (UVRR) spectroscopic studies of WT and mutant myoglobin were performed to reveal the dynamics of protein motion after ligand dissociation. After dissociation of carbon monoxide (CO) from the heme, UVRR bands of Tyr showed a decrease in intensity with a time constant of 2 ps. The intensity decrease was followed by intensity recovery with a time constant of 8 ps. On the other hand, UVRR bands of Trp residues located in the A helix showed an intensity decrease that was completed within the instrument response time. The intensity decrease was followed by an intensity recovery with a time constant of ≈50 ps and lasted up to 1 ns. The time-resolved UVRR study of the myoglobin mutants demonstrated that the hydrophobicity of environments around Trp-14 decreased, whereas that around Trp-7 barely changed in the primary protein response. The present data indicate that displacement of the E helix toward the heme occurs within the instrument response time and that movement of the FG corner takes place with a time constant of 2 ps. The finding that the instantaneous motion of the E helix strongly suggests a mechanism in which protein structural changes are propagated from the heme to the A helix through the E helix motion.

Evidence for Displacements of the C-helix by CO Ligation and DNA Binding to CooA Revealed by UV Resonance Raman Spectroscopy

The UV and visible resonance Raman spectra are reported for CooA from Rhodospirillum rubrum, which is a transcriptional regulator activated by growth in a CO atmosphere. CO binding to heme in its sensor domain causes rearrangement of its DNA-binding domain, allowing binding of DNA with a specific sequence. The sensor and DNA-binding domains are linked by a hinge region that follows a long C-helix. UV resonance Raman bands arising from Trp-110 in the C-helix revealed local movement around Trp-110 upon CO binding. The indole side chain of Trp-110, which is exposed to solvent in the CO-free ferrous state, becomes buried in the CO-bound state with a slight change in its orientation but maintains a hydrogen bond with a water molecule at the indole nitrogen. This is the first experimental data supporting a previously proposed model involving displacement of the C-helix and heme sliding. The UV resonance Raman spectra for the CooA-DNA complex indicated that binding of DNA to CooA induces a further displacement of the C-helix in the same direction during transition to the complete active conformation. The Fe-CO and C-O stretching bands showed frequency shifts upon DNA binding, but the Fe-His stretching band did not. Moreover, CO-geminate recombination was more efficient in the DNA-bound state. These results suggest that the C-helix displacement in the DNA-bound form causes the CO binding pocket to narrow and become more negative.

Changes in the hydrogen-bond network around the chromophore of photoactive yellow protein in the ground and excited states.

Changes in the hydrogen-bond (HB) network around the chromophore, p-coumaric acid (pCA), in the ground pG and excited pG* states were investigated for wild type (WT) photoactive yellow protein (PYP) and its mutants using ultraviolet resonance Raman (UVRR) spectroscopy. The intensity depletion of Tyr UVRR bands was observed upon photoexcitation of pCA to the pG* state. The spectral change was ascribed to strengthening of HB between pCA and Tyr42. Comparison of Raman intensities indicated that, in the pG state, the HB between pCA and Tyr42 in WT is a short HB, which is weaker than that in E46Q mutant. In the pG* state, the HB network around pCA of WT is similar to that of E46Q mutant. The present results demonstrate that the HB between pCA and Tyr42 and that between pCA and Glu46 are correlated with each other in the HB network.

Characterization of stimulated Raman scattering of hydrogen and methane gases as a light source for picosecond time-resolved Raman spectroscopy

Stimulated Raman scattering (SRS) in compressed hydrogen and methane gas was characterized in terms of pulse energy, temporal width and spectral width in the range of gas pressures 10‐60 atm to use it as a light source for picosecond time-resolved resonance Raman spectroscopy. SRS was pumped by the second harmonic of a Ti : sapphire oscillator‐regenerative amplifier laser system with pulse energy up to 200 mJ, duration2:5 ps and repetition rate 1 kHz. The output spectral region 421‐657 nm was covered by the first and second Stokes SRS components on tuning of the pump wavelength in the range 375‐425 nm. Energy conversion to the first Stokes SRS component was more than 10% with H 2 and more than 20% with CH4. The temporal width of the SRS pulse (1.1‐2.1 ps) was shorter than that of the pump pulse. The spectral band shape was found to be modulated, since the SRS is generated in a transient regime. When more than 100 pulses were averaged, the temporal and spectral profiles of SRS pulses were sufficiently smooth and energy fluctuations were sufficiently small for spectroscopic applications. On the basis of the results obtained, optimized conditions as a Raman shifter were deduced. The Raman shifter served as a light source for two-color pump‐probe time-resolved resonance Raman (TR 3 ) experiments and to demonstrate its capabilities, picosecond TR 3 spectra of nickel tetraphenylporphyrin in toluene solution were measured.

Direct Observation of Vibrational Energy Flow in Cytochrome c

Vibrational energy flow in ferric cytochrome c has been examined by picosecond time-resolved anti-Stokes ultraviolet resonance Raman (UVRR) measurements. By taking advantage of the extremely short nonradiative excited state lifetime of heme in the protein (<< ps), excess vibrational energy of 20000-25000 cm(-1) was optically deposited selectively at the heme site. Subsequent energy relaxation in the protein moiety was investigated by monitoring the anti-Stokes UVRR intensities of the Trp59 residue, which is a single tryptophan residue involved in the protein that is located close to the heme group. It was found from temporal changes of the anti-Stokes UVRR intensities that the energy flow from the heme to Trp59 and the energy release from Trp59 took place with the time constants of 1-3 and ~8 ps, respectively. These data are consistent with the time constants for the vibrational relaxation of the heme and heating of water reported for hemeproteins. The kinetics of the energy flow were not affected by the amount of excess energy deposited at the heme group. These results demonstrate that the present technique is a powerful tool for studying the vibrational energy flow in proteins.

Picosecond time-resolved ultraviolet resonance Raman spectroscopy of bacteriorhodopsin: primary protein response to the photoisomerization of retinal.

Protein dynamics in the primary processes during the bacteriorhodopsin (BR) photocycle under physiological conditions were investigated by measuring picosecond time-resolved ultraviolet resonance Raman (UVRR) spectra of the BR suspended solution at ambient temperature. We used a 565 nm pump pulse to initiate the BR photocycle and two kinds of probe pulses with wavelengths of 225 and 238 nm to detect spectral changes in the tryptophan and tyrosine bands, respectively. The observed spectral changes of the Raman bands are most likely due to tryptophan and tyrosine residues located in the vicinity of the retinal chromophore, that is, Trp86, Trp182, Tyr57, and Tyr185. The 225 nm UVRR spectra exhibited bleaching of intensity for all the tryptophan bands within the instrumental response, followed by recovery with a time constant of 30 ps and no further changes up to 1 ns. This suggests that the stepwise structural changes in the tryptophan residues proceed in response to the retinal photoreaction. It is concluded that the initial intensity bleach arises from the J-intermediate formation and the 30 ps recovery is associated with the K-KL transition. The 30 ps process in the BR photocycle has been detected for the first time. In the 238 nm UVRR spectra, spectral features attributable to the K and KL intermediates were observed. The observed spectral changes showed that the temporal behaviors of the observed spectral changes in each Raman band of both tryptophan and tyrosine were different. This indicates that the spectral changes originated from structural changes of at least two tryptophan and two tyrosine residues.

Time-resolved resonance Raman study of the primary photoprocesses of nickel(II) octaethylporphyrin in solution

Abstract Two-color pump-probe time-resolved resonance Raman studies with 3 ps time resolution were carried out for nickel(II) octaethylporphyrin in toluene and tetrahydrofuran solutions. Reliable proofs have been obtained for the existence of both conformational changes and vibrational heating/cooling in the primary photoprocess. Conformational changes from ruffled to planar structures, which were monitored by time evolution of the v 10 ∗ bands at 1616–1629 cm −1 , took place in the excited (d,d) state within 15 ps following photoexcitation, while the core expansion of the macrocycle occurs immediately after the excitation energy transfer from the ( π,π ∗ ) to (d,d) states. Intensity changes of the v 4 ∗ and v 7 ∗ bands in the anti-Stokes Raman spectra revealed that the conversion of the electronic excess energy into vibrational heating/cooling occurs with time constants of 5–8 ps and is dependent on pump wavelength.

Observing Vibrational Energy Flow in a Protein with the Spatial Resolution of a Single Amino Acid Residue.

One of the challenges in physical chemistry has been understanding how energy flows in a condensed phase from the microscopic viewpoint. To address this, space-resolved information at the molecular scale is required but has been lacking due to experimental difficulties. We succeeded in the real-time mapping of the vibrational energy flow in a protein with the spatial resolution of a single amino acid residue by combining time-resolved resonance Raman spectroscopy and site-directed single-Trp mutagenesis. Anti-Stokes Raman intensities of the Trp residues at different sites exhibited different temporal evolutions, reflecting propagation of the energy released by the heme group. A classical heat transport model was not able to reproduce the entire experimental data set, showing that we need a molecular-level description to explain the energy flow in a protein. The systematic application of our general methodology to proteins with different structural motifs may provide a greatly increased understanding of the energy flow in proteins.