Maturation processes of dinuclear copper proteins such as tyrosinase, catechol oxidase, and hemocyanin have been a long-standing mystery in copper protein chemistry. Until now, several crystal structures have revealed that these copper proteins share a similar dinuclear copper active site, where each copper ion is ligated by three histidine imidazoles, and binds molecular oxygen in a side-on fashion to form a (µ-η2:η2-peroxido)dicopper(II) species not only as the dioxygen-adduct in oxy-hemocyanins but also as the key reactive intermediate for the hydroxylation of phenols to catechols (phenolase reaction) and the oxidation of catechols to o-quinones (catecholase reaction) in tyrosinases and catechol oxidases. Recently, we have succeeded in determining the high-resolution crystal structures of the recombinant pro-form of yellow koji mold tyrosinase to find the existence of a distinct C-terminal domain containing a –CXXC– unit, that is the common sequence motif of the copper chaperons. Thus, the C-terminal domain apparently acts as a copper chaperon, helping construction of the dinuclear copper active site of tyrosinase. Furthermore, we have found that the proteolytic cleavage of the C-terminal domain from the pro-form (inactive-form) of tyrosinase greatly enhances the tyrosinase activity, thus suggesting that the C-terminal domain also acts as a shielding domain to regulate the enzymatic activity. In fact, overall structure of the pro-form resembles the structure of one of the functional units of octopus hemocyanin (oxygen carrier protein), which also has a similar C-terminal domain prohibiting the monooxygenase activity. On the basis of these results together with the detailed kinetic and spectroscopic analyses, the maturation process of the dinuclear copper proteins is discussed to provide new insights into the regulation mechanism of the dicopper protein functions; dioxygen binding and activation. We have also succeeded in evolving phenolase activity from molluscan and arthropod hemocyanins by treating them with a hydrolytic enzyme or an acid, and demonstrated that the reaction mechanism of their phenolase activity is the same to that of tyrosinase itself, that is the electrophilic aromatic substitution mechanism. Furthermore, we have developed an artificial dicopper protein exhibiting catecholase activity using metallo-β-lactamase, a dinuclear zinc enzyme, as a metal binding platform.
The activation process of the coupled dinuclear copper proteins such as tyrosinase remained to be clarified until lately. Thorough understanding of its molecular mechanism as well as the phenolase reaction mechanism is provided on the basis of the crystal structures.
Visible light sensitive heterogeneous photocatalyst was successfully designed by utilization of 4-sulfocalixarene as a linker molecule for stable immobilization of 5,10,15,20-tetrakis(1-methyl-pyridinium-4-yl)porphyrinatozinc(II) ion on Pt-loaded TiO2. This heterogeneous system can act as an efficient photocatalyst for generation of H2 under controlled reaction conditions.
The moment analysis (MA) theory was introduced into the analysis of elution peak profiles measured by affinity capillary electrophoresis (ACE). New moment equations were developed, which were essential for analytically determining the association (ka) and dissociation (kd) rate constants of intermolecular interactions from the ACE data. The combination of the MA theory and ACE procedure leads to a method for the kinetic study of intermolecular interactions (MA-ACE). Because ACE has quite frequently been used for determining binding (association equilibrium) constants of intermolecular interactions, it must be important that ka and kd can also be determined from ACE elution peaks. An attempt was made to analyze ACE elution peaks in a published paper by the MA method to determine the rate constants. The values of ka and kd were analytically determined for the intermolecular interaction between three boronic acids and fructose. They were comparable with those determined by the other method based on the macroscopic approach for studying kinetics at equilibrium. It was demonstrated that the MA method was effective for extracting information about affinity kinetics of intermolecular interactions from elution peaks measured by ACE. This means that the MA method makes it possible to use ACE data previously published as a source for kinetic study of intermolecular interactions.
The present communication incorporates utilization of a simple Schiff base (RO) fruitfully for the turn “off-on” successive fluorescent recognition of Cu2+ and Cys in aqueous media. The single crystal of RO-Cu2+ ensemble revealed a binding pattern which ultimately leads to quenching of the green fluorescence of RO. Interestingly the quenched fluorescence of the RO-Cu2+ ensemble was revived selectively in the presence of Cys via Cu2+ displacement approach. The selective “off-on” behavior of RO towards Cu2+ and Cys was consequently used as inputs to build up an implication (IMP) logic gate. Moreover, the RO exhibited successful bio-imaging of Cu2+ and Cys also in E. coli cells.
We report herein the results of an experimental and computational study of adamantylideneadamantane (1) and a variety of substituted ethylenic hydrocarbons. The standard enthalpy of formation in the gas phase as well as the gas-phase basicity (GA) of 1 were experimentally determined for the first time, respectively by calorimetric techniques and FT-ICR spectrometry. In parallel, computational studies at the MP2/6-311+G(d,p), G3(MP2), and G3 levels were performed on the neutral (1) and protonated (1H+). The agreement with experimental results was very good. The structures of 1 and 1H+ were subject to treatment by “Atoms in Molecules” in order to assess the characteristics of the closest H···H interactions involving both adamantane moieties. Also, the second-order perturbation analysis within the Natural Bond Orbital Theory methodology shows four degenerate charge-transfer interactions between the σ C–H bond of one of the adamantyl subunits towards the σ* C–H of the other adamantyl subunit. The standard enthalpies of formation of new adamantyl compounds were obtained using our experimental data. The computational study of a variety of ethylenic compounds including cyclohexylidenecyclohexane and several alkyl-substituted ethylenes using isodesmic and homodesmotic reactions was carried out. This study was extended to their proton affinities and gas-phase basicities.
Coupled-cluster calculations were performed for cyclobutane-1,3-diylidene dicarbenes 2 at the CCSD(T)//CCSD/cc-pVDZ level of theory, in which the ground-state spin multiplicity and the structures of unique molecules were investigated in detail. The closed-shell singlet state 2(Sσπ) with a bicyclo[1.1.0]but-1(3)-ene (BBE) structure found to be the ground-state was much lower in energy than the corresponding singlet dicarbene structure 2(S**), the quintet state 2(Q), and the triplet state 2(T), suggesting that the hitherto experimentally unknown BBE structure can be synthesized by the intramolecular dimerization of two carbene units. The energy gap between the BBE structures 2(Sσπ) and corresponding quintet states 2(Q) with electron-withdrawing substituents (X = F) at the C2 and C4 positions was found to be larger than that with electron-donating substituents (X = SiH3), i.e., ca. 100 kcal mol−1 for 2b (X = F) > ca. 85 kcal mol−1 for 2a (X = H) > ca. 70 kcal mol−1 for 2c (X = SiH3). Two unique structures, 2(Tσ) with a C1–σ–C3 bond and 2(Tπ) with a C1–π–C3 bond, were found to be the equilibrium structures for the triplet state of cyclobutane-1,3-diylidene dicarbenes 2.
A series of flame-retarded polyisocyanurate–polyurethane (PIR–PUR) foams were prepared using a novel phosphorus-containing flame retardant, 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO)–Mg. The thermal stability and pyrolytic gases of the PIR–PUR foam with and without DOPO–Mg were detected by TG and TGA-FTIR. The results indicated that DOPO–Mg can significantly reduce the combustible gases released during decomposition of the PIR–PUR foams, such as carbonyl group containing, methyl and methylene containing flammable gas. The Py-GC-MS results proved that DOPO–Mg changed the thermal degradation process of the PIR–PUR foam and prevented a highly toxic flammable gas from being generated. The flame retardancy of the PIR–PUR foams was tested using the LOI and cone calorimeter test. The LOI and heat release rate values of the PIR–PUR sample without DOPO–Mg were 21.2% and 190.7 kW m−2, respectively. 25 phr DOPO–Mg incorporation into the PIR–PUR foam resulted in a LOI value of 23.2%. In addition, 5 phr DOPO–Mg incorporation into the PIR–PUR foam resulted in a peak heat release rate of 154.9 kW m−2. The foams with DOPO–Mg showed more compact residues. The SEM test showed that spherical material formed on the surface of the char to hinder flame and heat transmission, which can be used to explain the flame retardant effect.
We demonstrated that demolding in ultraviolet (UV) nanoimprinting was assisted by a nanoscale lubricating fluid layer of a liquefied 1,1,1,3,3-pentafluoropropane (PFP) gas. UV-curable Resin A composed of glycerol 1,3-diglycerolate diacrylate with low PFP absorption showed low surface roughness of 22-nm line-and-space imprint patterns with the smallest line edge roughness (LER) value of 3σ = 1.8 nm. As the molar ratio of 1,6-hexanediol diacrylate with large PFP absorption was increased in the UV-curable resin, the morphological characteristics worsened. Although thin cured films of Resin A possessed a large surface free energy of 57.6 mJ m−2, Resin A could be used for step-and-repeat UV nanoimprinting with bare and fluorinated silica molds. Quartz crystal microbalance measurements suggested that a PFP adsorption layer was formed at a film surface of uncured Resin A upon exposure to PFP gas and functioned as a nanoscale lubricating fluid layer during UV nanoimprinting.
A reusable ionic liquid 1-butyl-3-methylimidazolium bromide/Cu(II) system was proven to be a reusable catalyst for the homocoupling of terminal alkynes at mild temperature using air as oxidant in aqueous phase or under solvent-limited conditions. In most cases, good to excellent yields can be achieved, either aromatic alkynes or aliphatic alkynes were used as substrates. The homocoupling products 1,3-diynes were separated by extraction from the reaction system. And the activity of residue showed it could be reused for 3 and 6 cycles respectively under the aqueous phase conditions and solvent-limited conditions. Furthermore, this reaction can be easily scaled up to gram level.
Based on transient absorption measurements obtained upon electron-pulse radiolysis of aromatic ethers in N,N-dimethylformamide (DMF), mesolysis processes of the radical anions were investigated. We observed transient absorption spectral change due to the formation of the radical anions in the temperature range (220–295 K). The transient absorption of the radical anions decreased with first-order kinetics along with the growth of the absorption spectrum of the corresponding benzylic radical. Arrhenius analysis of the decay rates of the radical anions provided the activation energies and the frequency factors for the mesolysis. These observations indicated that the mesolysis of the radical anions occurs via a stepwise mechanism, resulting in the formation of benzylic radical and phenolate anion due to the C–O bond cleavage.
The proton-transfer reaction mechanism in protonated benzene was examined using ab initio molecular dynamics, paying special attention to cooperative motions. We first examined the lifetime of each stable state of protonated benzene, taken as equal to the time lag between proton-transfer reactions. The theoretically obtained lifetime distribution shows non-statistical behavior, since it contains a large number of short lifetime trajectories. By analyzing the nature of these short trajectories, we found that a transferred proton can be trapped between two carbon atoms. Because carbon atoms adjacent to the transferred proton will change their hybridization during proton-transfer reactions, and since it requires tens of femtoseconds to reform hybrid orbitals, the proton will oscillate between the two carbon atoms without bonding to either. We refer to this trapped state as a “dynamically stable trapped state” and consider that such states may appear in many types of chemical reactions.
The first example of abnormal Michael reaction of an active methylene compound, 2-acylmethyl-4,4-dimethyl-2-oxazoline with acetylenic ketone in acetonitrile is reported. The reaction accompanies 1,3-migration of the acyl group of the substrate to give 2-(3-acyl-1-buten-4-on-1-yl)-2-oxazoline, which was easily cyclized to 5-acyl-2-pyridone derivatives by treatment with silica gel. Selectivity of the reaction depends on bulkiness of all the substituents of both the substrate and the reagent. The selectivity is interpreted in terms of reduced kinetic acidity of an initial anionic adduct intermediate by both steric and electronic factors.
(3-Aminopropyl)trimethoxysilane (APS)-silica gels were prepared by a one-pot process using sodium silicate as a low-cost source of silica. XRD and SEM analyses revealed that the APS-silica gels were particles with diameters of around 100–200 nm. Furthermore, the introduction of amine groups was confirmed by FT-IR spectroscopy and CO2 uptake measurements. The adsorption properties of the gels were improved by addition of the surfactants hexadecyltrimethylammonium bromide (CTAB), Pluronic F127, and Pluronic P123, which acted as pore forming agents. Using Brunauer–Emmett–Teller (BET) analyses, we confirmed that both the surface area and pore volume were improved on addition of these surfactants. The larger surface area resulted in a greater number of amine groups being exposed on the outer surface of the APS-silica gels; therefore, CO2 uptake increased as the amount of surfactant used was increased. The APS-silica gels with F127 and P123 had pores from the porous particles with 5–11 nm diameters and gaps between particles, while the APS-silica gel with CTAB only had smaller pores from gaps between silica particles. These pore distribution differences resulted from the differences in the lengths of the surfactant hydrophobic chains, and the differences in pore sizes affected the CO2 uptake rates of the APS-silica gels. By comparison of the pore and CO2 uptake properties, we confirmed that P123 was a suitable surfactant for the preparation of APS-silica gel by a one-pot process using sodium silicate as the silica source.
The microwave specific effect on the catalytic atropo-enantioselective ring-opening reaction of biaryl lactones was investigated. Under strictly controlled temperature conditions, the reaction was accelerated by microwave irradiation without any loss of the enantioselectivity. Also, the racemization rate of the atropo-optically active biaryl lactone was enhanced by the microwave irradiation.
The microwave specific effect on the catalytic atropo-enantioselective ring-opening reaction of biaryl lactones was investigated. Under strictly controlled temperature conditions, the reaction was accelerated by microwave irradiation without any loss of the enantioselectivity. Also, it was revealed that the racemization rate of the atropo-optically active biaryl lactone was enhanced by the microwave irradiation.
The near-infrared (NIR) absorption properties of a square-planar nickel–dithiolene complex [Ni(iPr2timdt)2] in various organic solvents have been investigated (iPr2timdt denotes a monoanion of 1,3-diisopropylimidazolidine-2,4,5-trithione). In non-coordinating aprotic organic solvents of smaller relative permittivity (εr < 10) such as dichloromethane (εr = 8.9), the neutral complex [Ni(iPr2timdt)2]0 shows a huge and sharp NIR absorption band at ca. 1000 nm having the molar absorptivity of ε = 6–8 × 104 L mol−1 cm−1. In contrast, in strongly coordinating solvents such as pyridine (Gutmann’s donor number, DN = 33.1), the NIR absorption of [Ni(iPr2timdt)2]0 disappears most probably, owing to the coordination of pyridine molecules onto the axial sites of this square-planar complex. In high relative permittivity solvents such as N,N-dimethylformamide (DMF, εr = 36.71), [Ni(iPr2timdt)2]0 (λmax: ca. 1000 nm) undergoes spontaneous reduction reactions yielding the ionic complexes of [Ni(iPr2timdt)2]− (λmax: ca. 1400 nm) and [Ni(iPr2timdt)2]2− (no NIR peaks), which is indeed reconciled with the results of the spectroelectrochemical measurements. It is stressed that the absorption properties of [Ni(iPr2timdt)2] in the NIR region are sensitive to the solvent properties, and these complex systems serve as NIR probes to report solvent properties.