Yutaka Hitomi

An Iron(III)–Monoamidate Complex Catalyst for Selective Hydroxylation of Alkane CH Bonds with Hydrogen Peroxide

Selective oxidation: the success of the title reaction is caused by the strong electron donation from the amidate moiety of the dpaq ligand to the iron center (dpaq=2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamidate). This process facilitates the O-O bond heterolysis of the intermediate Fe(III)OOH species to generate a selective oxidant without forming highly reactive hydroxyl radicals.

Detection of enzymatically generated hydrogen peroxide by metal-based fluorescent probe.

We developed a metal-based fluorescent probe for H(2)O(2) called MBFh1, which has an iron complex as a reaction site for H(2)O(2) and a 3,7-dihydroxyphenoxazine derivative as the fluorescent reporter unit. The iron complex reacts quickly with H(2)O(2) to form oxidants, and then the oxidants convert the closely appended nonfluorescent 3,7-dihydroxyphenoxazine moiety to resorufin in an intramolecular fashion. The quick response to H(2)O(2) allows us to plot the enzymatic evolution of H(2)O(2). A combination of N-acetyl-3,7-dihydroxyphenoxazine and horseradish peroxidase has been frequently used to detect enzymatically generated H(2)O(2), but this method has interference with phenol derivatives. The use of MBFh1 overcomes this drawback.

Reversible O–O Bond Scission of Peroxodiiron(III) to High-Spin Oxodiiron(IV) in Dioxygen Activation of a Diiron Center with a Bis-tpa Dinucleating Ligand as a Soluble Methane Monooxygenase Model

The conversion of peroxodiiron(III) to high-spin S = 2 oxodiiron(IV) via reversible O-O bond scission in a diiron complex with a bis-tpa dinucleating ligand, 6-hpa, has been characterized by elemental analysis; kinetic measurements for alkene epoxidation; cold-spray ionization mass spectrometry; and electronic absorption, Mössbauer, and resonance Raman spectroscopy to gain insight into the O(2) activation mechanism of soluble methane monooxygenases. This is the first synthetic example of a high-spin S = 2 oxodiiron(IV) species that oxidizes alkenes to epoxides efficiently. The bistability of the peroxodiiron(III) and high-spin S = 2 oxodiiron(IV) moieties is the key feature for the reversible O-O bond scission.

Contribution of heme-propionate side chains to structure and function of myoglobin: chemical approach by artificially created prosthetic groups

Horse heart myoglobin was reconstituted with mesohemin derivatives methylated at the 6- or 7-position to evaluate the role of the heme-6-propionate or heme-7-propionate side chain in the protein. The association and dissociation of the O(2) binding for the deoxymyoglobin with 6-methyl-7-propionate mesoheme are clearly accelerated. Furthermore, the myoglobin with 6-methyl-7-propionate mesoheme shows fast autoxidation from oxymyoglobin to metmyoglobin compared to the myoglobin with 6-propionate-7-methyl heme and the reference protein. These results indicate the 6-propionate plays an important physiological role in the stabilization of oxymyoglobin because of the formation of a salt-bridge with the Lys45. The acceleration of CO binding rate is observed for the myoglobin with 6-propionate-7-methyl mesoheme, suggesting that the replacement of the 7-propionate with a methyl group has an influence on the His93-heme iron coordination. The structural perturbation of His93 imidazole was also supported by 1H NMR spectra of cyanide and deoxy forms of the myoglobin with 6-propionate-7-methyl mesoheme. Thus, it is found that the 7-propionate regulates the hydrogen-bonding network and His93-heme iron coordination in the proximal site.

Facile preparation of stable aqueous titania sols for fabrication of highly active TiO2 photocatalyst films

Highly stable aqueous titania sols were prepared via a facile process using titanium tetraisopropoxide in the co-existence of acetylacetone and acetic acid. Their co-existence efficiently suppressed the hydrolysis and condensation reaction of titanium tetraisopropoxide even in water, retaining the diameter of titania colloidal particles lower than 10 nm (average diameter d = ca. 4 nm). The titania sols possessed significantly high stability for more than 1 year and could be easily coated on various substrates such as quartz glass via a simple spin-coating method, forming highly homogeneous and transparent films. The calcinations of the coated films in the air at 600 °C produced densely packed anatase TiO2 particles; the diameter was retained below 50 nm even after the calcinations at 900 °C without phase transition to rutile, maintaining the fairly good transparency of the films. On the other hand, the TiO2 particles prepared on a quartz substrate from other precursors, such as titanium–peroxo-citrate complexes, were found to transform from anatase into the rutile phase at above 700 °C with a significant increase in the particle size up to ca. 200 nm. The TiO2 films prepared from the present aqueous titania sols exhibited higher activity for photo-induced surface superhydrophilicity under UV light irradiation than those prepared from other precursors.

Electronic Tuning of Iron–Oxo-Mediated CH Activation: Effect of Electron-Donating Ligand on Selectivity

We have reported previously that an iron(III) complex supported by an anionic pentadentate monoamido ligand, dpaq(H) (dpaq(H) =2-[bis(pyridin-2-ylmethyl)]amino-N-quinolin-8-yl-acetamido), promotes selective CH hydroxylation with H2 O2 with high regioselectivity. Herein, we report on the preparation of Fe(III) -dpaq derivatives that have a series of substituent groups at the 5-position of a quinoline moiety in the parent ligand dpaq(H) (dpaq(R) , R: OMe, H, Cl, and NO2 ), and examine them with respect to their catalytic activity in CH hydroxylation with H2 O2 . As the substituent group becomes more electron-withdrawing, both the selectivity and the turnover number increase, but the selectivity of epoxidation shows the opposite trend.

Structures and electronic properties of the catecholatoiron complexes in relation to catechol dioxygenases: chlorocatecholatoiron complexes are compared to the 3,5-di-tert-butylcatecholatoiron complex in the solid state and in solution

Chlorocatecholatoiron complexes, [Fe(TPA)(4Cl[bond]Cat)]BPh(4) and [Fe(TPA)(3Cl[bond]Cat)]BPh(4), (4Cl[bond]Cat and 3Cl[bond]Cat: 4- and 3-chlorocatecholates, respectively; TPA: tris(2-pyridylmethyl)amine) were isolated as intermediates for the oxygenative cleavage of chlorocatechols by nonheme iron complexes. Geometric structures of these complexes together with [Fe(TPA)(DTBC)]BPh(4) (DTBC: 3,5-di-tert-butylcatecholate) as reference were analyzed by X-ray absorption spectroscopy (EXAFS) in the solid state and in solution. Structure of the DTBC complex in the solid state was shown to be noticeably different from the other complexes as seen in the magnetic susceptibility and spectroscopic data. Electronic and magnetic properties of these complexes were studied by X-ray absorption (XANES), electronic (VIS) and ESR spectroscopies, and magnetic susceptibility. Electron transfer from the catecholate ligand to the Fe(III) center was indicated by the Fe[bond]K edge values in XANES spectra and by the LMCT bands in electronic spectra. Magnetic susceptibility and ESR data indicated that at low temperatures the complexes are in equilibrium between the low (S=1/2) and high-spin (S=5/2) ferric states with the latter component increasing with temperature. Remarkable differences between the spin states in solid and in solution were observed with the DTBC complex.

NEW APPROACH TO THE CONSTRUCTION OF AN ARTIFICIAL HEMOPROTEIN COMPLEX

Abstract Modified prosthetic metalloporphyrin, having a total of eight carboxylate groups at the terminal of two peripheral propionate side chains, was inserted into apomyoglobin to yield a new reconstituted myoglobin. The cluster of substituted carboxylates acts as the binding domain for cationic compounds such as methyl viologen and cytochrome c. Fluorescence spectroscopic analysis indicates that the reconstituted myoglobin formed a stable complex with methyl viologen and photoinduced singlet electron transfer (ET) occurred within the complex; k et =2.1×10 9 s −1 and k cr =3.3×10 8 s −1 . Cytochrome c with a positively charged domain also interacted with the reconstituted myoglobin with an association constant of 6.5×10 4 M −1 . The photoinduced triplet ET from the zinc reconstituted myoglobin to ferricytochrome c occurred through diprotein complex with a rate constant of 2.2×10 3 s −1 . Furthermore, compared to native myoglobin, the ferryl state of the reconstituted myoglobin generated by hydrogen peroxide revealed the peroxidase activity with the acceleration of the oxidation of ferrocytochrome c via complex formation. The present approach could be very useful for constructing practical protein–protein complex systems to elucidate the biological ET via noncovalently linked biomolecules.

Multistate CASPT2 Study of Native Iron(III)-Dependent Catechol Dioxygenase and Its Functional Models: Electronic Structure and Ligand-to-Metal Charge-Transfer Excitation

We theoretically investigated the ligand-to-metal charge-transfer (LMCT) excitation of the native iron(III)-dependent catechol dioxygenase and its functional model complexes with multistate complete active space second-order perturbation theory (MS-CASPT2) because the LMCT (catecholate-to-iron(III) charge-transfer) excitation energy is believed to relate to the reactivity of the native enzyme and its functional model complexes. The ground state calculated by the MS-CASPT2 method mainly consists of the iron(III)-catecholate electron configuration and moderately of the iron(II)-semiquinonate electron configuration for both of the enzyme active centers and the model complexes when the active center exists in the protein environment and the model complexes exist in the solution. However, the ground-state wave function mainly consists of the iron(II)-semiquinonate electron configuration for both the enzyme active site without a protein environment and the model complexes in vacuo. These results clearly show that the protein environment and solvent play important roles to determine the electronic structure of the catecholatoiron(III) complex. The LMCT excitation energy clearly relates to the weight of the iron(III)-catecholate configuration in the ground state. The reactivity and the LMCT excitation energy directly relate to the ionization potential of the catecholate (IP(CAT)) in the model complex. This is because the charge transfer from the catecholate moiety to the dioxygen molecule plays a key role to activate the dioxygen molecule. However, the reactivity of the native catechol dioxygenase is much larger than those of the model complexes, despite the similar IP(CAT) values, suggesting that other factors such as the coordinatively unsaturated iron(III) center of the native enzyme play a crucial role in the reactivity.

Synthesis and characterization of a binuclear iron(III) complex bridged by 1-aminocyclopropane-1-carboxylic acid. Ethylene production in the presence of hydrogen peroxide.

A mu-oxo-diiron(III) complex bridged by two molecules of 1-aminocyclopropane-1-carboxylic acid (ACCH) was prepared with the ligand 1,4,7-triazacyclononane (TACN): [(TACN)Fe(2)(mu-O)(mu-ACCH)(2)](ClO(4))(4) x 2 H(2)O (1). This complex was characterized, and its crystal structure was solved. The bridging amino acid moieties were found in their zwitterionic forms (noted as ACCH). Reactivity assays were performed in the presence of hydrogen peroxide, and 1 turned out to be the first example of a well-characterized iron-ACCH complex able to produce ethylene from the bound ACCH moiety. The reaction requires the presence of a few equivalents of base, probably involved in the deprotonation of the amine groups of the ACCH bridges.