Photocatalytic overall water splitting under visible light by TaON and WO3 with an IO3-/I- shuttle redox mediator

Photocatalytic water splitting into H2 and O2 under visible-light irradiation (lambda > 420 nm) is demonstrated using the oxynitride Pt-TaON for H2 evolution and a Pt-WO3 catalyst for O2 evolution in an IO3-/I- shuttle redox-mediated system.     

Catalytic roles of active-site amino acid residues of coenzyme B12-dependent diol dehydratase: protonation state of histidine and pull effect of glutamate

The hydrogen abstraction and the OH migration processes catalyzed by diol dehydratase are discussed by means of a quantum mechanical/molecular mechanical method. To evaluate the push effect of His143 and the pull effect of Glu170, we considered three kinds of whole-enzyme model, the protonated and two unprotonated His143 models. A calculated activation energy for the hydrogen abstraction by the adenosyl radical is 15.6 (13.6) kcal/mol in the protonated (unprotonated) His143 model. QM/MM calculational results show that the mechanism of the OH migration is significantly changed by the protonation of His143. In the protonated His143 model, the OH group migration triggered by the full proton donation from the imidazolium to the migrating OH group occurs by a stepwise OH abstraction/re-addition process in which the water production reduces the barrier for the C-O bond cleavage. On the other hand, the OH migration in the unprotonated His143 model proceeds in a concerted manner, as we previously proposed using a simple model including only K+ ion and substrate. The latter mechanism seems to be kinetically more favorable from the calculated energy profiles and is consistent with experimental results. The activation barrier of the OH group migration step is only 1.6 kcal/mol reduced by the hydrogen-bonding interaction between the O2 of the substrate and unprotonated His143. Thus, it is predicted that His143 is not protonated, and therefore the main active-site amino acid residue that lowers the energy of the transition state for the OH group migration is determined to be Glu170.     

Trinuclear copper(II) complex showing high selectivity for the hydrolysis of 2'-5' over 3'-5' for UpU and 3'-5' over 2'-5' for ApA ribonucleotides

The cooperative action of multiple Cu(II) nuclear centers is shown to be effective and selective in the hydrolysis of 2'-5' and 3'-5' ribonucleotides. Reported herein is the specific catalysis by two trinuclear Cu(II) complexes of L3A and L3B. Pseudo first-order kinetic studies reveal that the L3A trinuclear Cu(II) complex effects hydrolysis of Up(2'-5')U with a rate constant of 28 x 10(-)(4) min(-)(1) and Up(3'-5')U with a rate constant of 0.5 x 10(-)(4) min(-)(1). The hydrolyses of Ap(3'-5')A and Ap(2'-5')A proceed with rate constants of 24 x 10(-)(4) min(-)(1) and 0.5 x 10(-)(4) min(-)(1) respectively. The L3A trinuclear Cu(II) complex demonstrates high specificity for Up(2'-5')U and Ap(3'-5')A. Similar studies with the more rigid L3B trinuclear Cu(II) complex shows no selectivity and yields lower rate constants for hydrolysis. The selectivity observed with the L3A ligand is attributed to the geometry of the ligand-bound diribonucleotide which ultimately dictates the proximity of the attacking hydroxyl and the phosphoester to a Cu(II) center for activation and subsequent hydrolysis.     

Adsorption, Migration and Reactions of Hydrocarbons on Zeolites Observed by Ftir Spectroscopy

The adsorption, migration and reactions of hydrocarbons on zeolites studied by FTIR spectroscopy are briefly reviewed. At low temperatures, alkyl-BAS (Brønsted acid sites) and -BAS complexes were first formed before protonation of olefins. For 1-butene, it underwent double bond migration (DBM) to form cis- and trans-2-butenes. With increasing temperature, oligomerization occurred and resulted in highly branched dimer, e.g., 3,4-dimethyl-3-hexene. Compared with 1-butene, isobutene can oligomerize at much lower temperature and formed dimerized alkoxy species, even on isolated silanols of zeolites. Upon adsorption of cyclic olefins such as 1-methylcyclopentene, alkenyl carbenium ions were formed at temperatures as low as 150 K. The adsorption of butenes on ferrierite indicates the existence of energy barriers for their intercalation into the pores. The H/D isotope exchange reaction of alkanes with acidic hydroxyl groups of zeolites will also be discussed.     

Water reduction and oxidation on Pt-Ru/Y2Ta2O5N2 catalyst under visible light irradiation

Y(2)Ta(2)O(5)N(2) is presented as a novel photocatalyst with high activity for water splitting under visible-light irradiation in the presence of appropriate sacrificial reagents; the activity for reduction to H(2) is increased by the incorporation of Pt or Ru as a co-catalyst, with a significant increase in production efficiency when both Pt and Ru are present.     

Catalytic Reduction of Dinitrogen into Ammonia and Hydrazine by Using Chromium Complexes Bearing PCP-Type Pincer Ligands**

A series of chromium-halide, -nitride, and -dinitrogen complexes bearing carbene- and phosphine-based PCP-type pincer ligands has been newly prepared, and some of them are found to work as effective catalysts to reduce dinitrogen under atmospheric pressure, whereby up to 11.60 equiv. of ammonia and 2.52 equiv. of hydrazine (16.6 equiv. of fixed N atom) are produced based on the chromium atom. To the best of our knowledge, this is the first successful example of chromium-catalyzed conversion of dinitrogen to ammonia and hydrazine under mild reaction conditions.