Mechanism of oxygenative cleavage of catechols by nonheme iron complexes in relevance to catechol dioxygenases studied by quantum chemical calculations

Mechanism of oxygenative cleavage of catechols by nonheme iron complexes was studied by quantum chemical calculations. Calculations based on the density-functional theory indicate that all carbon atoms of the DMC and Cat ligands of [Fe^III(NH₃)₄(DMC)]⁺ (DMC: 3,5-dimethylcatecholate), [Fe^III(NH₃)₄(Cat)]⁺ and [Fe^II(NH₃)₄(Cat)] (Cat: catecholate) are positively charged, that is not favorable for the electrophilic attack. Significant amounts of the spin density, that are greater on oxygen than carbons, appear on the catecholate ligand. The spin density on aromatic carbon atoms is greater in the ferric complex than in the ferrous complex, supporting the Fe(II)-semiquinonate character of the ferric catecholate complexes. Results are obtained to support the probability of the initial binding of molecular oxygen to the iron center rather than to the aromatic carbons. In the step of the oxygen insertion into the C–C bond, formation of an epoxide-like structure is proposed. It is shown that the postulated intermediate can be converted to an oxygen-inserted product in the change of the electronic state from the anionic ligand to the neutral product.     

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.     

Efficient Preparation of Large‐Sized Rings of Single‐Stranded DNA through One‐Pot Ligation of Multiple Fragments

Circular single‐stranded DNA (c‐ssDNA) has significant applications in DNA detection, the development of nucleic acid medicine, and DNA nanotechnology because it shows highly unique features in mobility, dynamics, and topology. However, in most cases, the efficiency of c‐ssDNA preparation is very low because polymeric byproducts are easily formed due to intermolecular reaction. Herein, we report a one‐pot ligation method to efficiently prepare large c‐ssDNA. By ligating several short fragments of linear single‐stranded DNA (l‐ssDNA) in one‐pot by using T4 DNA ligase, longer l‐ssDNAs intermediates are formed and then rapidly consumed by the cyclization. Since the intramolecular cyclization reaction is much faster than intermolecular polymerization, the formation of polymeric products is suppressed and the dominance of intramolecular cyclization is promoted. With this simple approach, large‐sized single‐stranded c‐ssDNAs (e.g., 200‐nt) were successfully synthesized in high selectivity and yield.     

Imaging of cyclotron emission from edge channels in quantum Hall conductors

A local probing technique of cyclotron emission is applied to image nonequilibrium electrons generated along edge channels in quantum Hall conductors. In a lower-magnetic field region of a quantum Hall state plateau (filling factor 2相似文献   

Migration-coalescence of Nanoparticles During Deposition of Au,Ag, Cu,and GaAs on Amorphous SiO2

Nanoparticles formed during the initial period of film growth can migrate, coalesce, and may also melt. Nanoparticles of Au, Ag, Cu, and GaAs ranging from 1 to 15nm in diameter were sputter-deposited on amorphous SiO₂ (a-SiO₂). Transmission electron microscopy was used to analyze the time-dependent change of the dispersion of particles on a thin film. The number density of nanoparticles was nearly constant during the deposition of Ag. For Au, Cu, and GaAs, however, the number density decreased with time during the early deposition period. For example, for Au the number density decreased from 2.8×10¹⁶m^–2 (surface coverage ratio of 0.08) to 1.8×10¹⁶m^–2 (surface coverage ratio of 0.14). The surface coverage increased because the particle size increased as the number density decreased. This decrease suggests that migration followed by coalescence occurred. For Au, although we found evidence of migration of 2-nm particles at 500°C, the migration rate was too slow to account for the results from the deposition experiments. These observations indicate an autocatalytic mechanism that migration followed by coalescence liberates energy by the formation of chemical bonds, heats the coalesced particles, and enhances further migration. The strong dependence of the structure of nanoparticle dispersions on the deposition rate is a direct consequence of the deposition mechanism, which is a nonlinear, kinetically-controlled process.     

Light-driven open-close motion of chiral molecular scissors

The first example of "light-driven chiral molecular scissors" (1), which consists of 1,1',3,3'-tetraarylferrocene as a pivot part and azobenzene as a driving part, was synthesized. Absorption, circular dichroism (CD), and 1H NMR spectral studies on the photoinduced isomerization process of an enantiomer of 1 agreed well with a prediction by a DFT calculation, where a motion of the handles via light-driven contraction/expansion of the connecting azobenzene strap was transformed, through a pivotal motion of the ferrocene unit, into an open-close motion of the blade parts.     

Polarographic study on poly alpha-L-glutamic acid-Cd2+,Co2+ complexes in the helix-coil pH region

Interactions between poly alpha- L-glutamic acid (PGA) and metal ions Cd(2+), Co(2+) were studied by direct current polarography. The diffusion currents of these ions decreased sharply in the presence of PGA in the pH region from 5.0 through neutral. A corresponding increase in the helix content of the PGA-metal ion complex was revealed by CD measurements on the same solutions. Helix contents determined by polarography were in good agreement with those by CD in the neutral pH region. On the contrary, the decrease of current in lower acidic pH regions was independent of helix formation and suggested that metal ions coordinate to sporadically-dissociated carboxylate groups to cause aggregation of the intra and/or inter polymer chains. The diffusion current of the ions, therefore, is a parameter sensitive to the conformational changes of PGA from acidic through neutral pH region.     

Spin-crossover dendrimers: generation number-dependent cooperativity for thermal spin transition

Poly(benzyl ether) dendrons having a focal triazole unit (Gntrz: trz = triazole; n = generation number = 0-2) were found to react with (MeSO(3))(2)Fe to form dendritic coordination polymers ([Fe(Gntrz)(3)](MeSO(3))(2).2H(2)O) that undergo the thermal spin transition. When the generation number of the dendritic unit was larger (n = 0 --> 1 --> 2), the average degree of polymerization (D(p) = 20 --> 10 --> 3) and spin-crossover temperature (T(c) = 335 --> 315 --> 300 K) of the resulting polymer were lower. However, the abruptness of the spin transition was not monotonically dependent on the generation number; (G1trz)Fe exhibited an abrupt spin transition with a temperature width of only 10 K, while the smallest and largest members of the (Gntrz)Fe family both displayed a rather broad spin-transition temperature width (30 (n = 0) and 50 K (n = 2)). X-ray diffraction and calorimetric analyses indicated the presence of a discotic columnar core-shell assembly with a crystal lattice best occupied by a C(3)(v)() symmetric array of medium-sized (G1trz)Fe.