Olefin metathesis has emerged as an indispensable means to create complex natural products by the virtue of its powerful carbon–carbon bond-forming ability, compatibility with a range of functional groups, and ready availability of highly reactive ruthenium carbene catalysts. Furthermore, Grubbs-type ruthenium carbene complexes originally developed for olefin metathesis reactions also mediate a variety of non-metathetic reactions and found their use in tandem metathetic/non-metathetic processes. This account summarizes our recent efforts on total synthesis of oxacyclic natural products by means of ruthenium carbene-catalyzed reactions.
Microbial rhodopsins are photoreceptive seven-transmembrane proteins diversely found in unicellular microorganisms. The chromophore of microbial rhodopsin is all-trans retinal. The retinal isomerizes to 13-cis form upon light-absorption and it triggers the expression of various biological functions by light. We studied the mechanism of various types of microbial rhodopsins (H+ pump, Cl− pump and sensors) by physicochemical spectroscopic methods. On the basis of these studies, we found the importance of the residues on the third helix (helix C) for the function of microbial rhodopsin, and it led to the discovery of a new functional class of microbial rhodopsin, light-driven outward Na+ pump. The mechanism of Na+ pump rhodopsin was studied by several spectroscopic methods and X-ray crystallography. These studies provided new insights very informative for the development of novel functional artificial rhodopsins (K+ and Cs+ pumps). These new natural and artificial rhodopsins are expected to have high potential for applications such as optogenetics.
Aromaticity is very sensitive to the geometry of the π-system. Topological resonance energy (TRE), defined graph-theoretically, represents an aromatic stabilization energy (ASE) arising from cyclic conjugation in the π-system. TRE can be calculated for not only ground-state but also excited-state species. The TRE concept has since been extended analytically to solve many different problems concerning aromaticity and reactivity. Bond resonance energy (BRE), defined in harmony with TRE, is an excellent probe for exploring kinetic stability of cyclic π-systems. It represents the contribution of individual π-bonds to global aromaticity. The well-known isolated pentagon rule (IPR) for fullerenes was verified in terms of BRE. Superaromatic stabilization energy (SSE) defined for macrocyclic π-systems finally confirmed the absence of macrocyclic aromaticity in kekulene. A novel local aromaticity index for polycyclic aromatic hydrocarbons (PAHs) was devised by re-interpreting the definition of SSE. We then found that aromatic properties of some PAHs are not compatible with Clar’s aromatic sextet rule. We now feel that many fundamental problems with regard to aromatic stabilization and related phenomena have been solved conceptually.
The present work is dedicated to study the performance of H2/Air proton exchange membrane (PEM) fuel cells (FC) by using an empirical modeling and experimental results at symmetric and asymmetric relative humidity. The empirical modeling is presented by Kim’s equation which was used in simulating the performance of the H2/Air PEM FC and in fitting the experimental data at different operating conditions. The simulation of the cell performance helps to understand the impacts of different kinetics, ohmic and mass-transfer resistances on the I-E curves and hence on the performance of the FC. By fitting the experimental data with the theoretical calculations, it is possible to extract important kinetics, ohmic and mass-transfer parameters at different temperatures and at different symmetric and asymmetric relative humidity. Although the exact physical meanings of some fitting parameters are quite clear, others are not.
Poly(ethyleneimine) (PEI) was modified to study the effect of amine type and introduced hydroxy groups on its ability to capture carbon dioxide. Amines were impregnated in as-synthesized mesostructured silica (MSU-F) to fabricate solid amines, and the resulting samples were characterized via 13C nuclear magnetic resonance spectroscopy and diffuse reflectance infrared Fourier transform spectroscopy. Nitrogen contents, structural properties, adsorption capacity, and sorbent durability during CO2 adsorption/desorption processes were measured to study the CO2 sorption performances of the sorbents.
Reaction between PEI and propylene oxide (PO) altered the ratio of primary, secondary, and tertiary amines and also produced hydroxy groups on amine molecules. With a specific reactant ratio, a modified amine-impregnated sorbent (M-EP10) showed a CO2 adsorption capacity and amine efficiency of 13.90 wt % and 0.37, respectively, which were higher than that of the pristine PEI-impregnated sorbent. The new adsorbents also showed excellent stability in cyclic adsorption–desorption operations, even under pure CO2 desorption conditions in which commercial PEI adsorbents are known to lose CO2 adsorption capacity owing to urea formation.
We have synthesized the 1,4-diaryl-1-thio-, seleno- and telluro-1,3-butadiene derivatives incorporated in a dibenzobarrelene skeleton and found that the thio and seleno derivatives are highly fluorescent in solution at room temperature. We report here the synthesis of the oxygen congener. The compounds were synthesized by the reaction of anthranolate with 1,4-diaryl-1,3-butadiyne followed by an intramolecular cycloaddition. Their structures were analyzed by X-ray crystallography to reveal high planarity of the 1,4-diaryl-1-oxy-1,3-butadiene moieties compared with the heavier chalcogen congeners. The oxy compounds are fluorescent similarly to the thio and seleno congeners; the diphenyl and bis(4-trifluorophenyl) derivatives exhibited somewhat red-shifted fluorescence spectra with vibrational structure. Fluorescence of the D-π-A type derivatives reached to the red and near infrared regions. The bonding characters in the 1-chalcogeno-1,3-butadiene moieties are discussed on the basis of DFT calculations.
Preparation of 2,2′-bithiophene derivatives bearing ω-alkenyl groups at the 3,3′-positions and ring-closing metathesis reactions of the obtained compound were performed. The reaction of bithiophene bearing 3-butenyl substituents 1 with 5 mol % Grubbs 1st generation catalyst underwent ring-closing metathesis (RCM) to afford the cyclized product 7 showing winding vine-shaped molecular asymmetry in up to 88% yield. Enantioselective RCM was also achieved by the use of chiral Schrock–Hoveyda molybdenum-alkylidene catalyst in up to 87% ee.
The electromagnetophoretic buoyancy exerted on a carboxylated polystyrene particle in an electrolyte solution was utilized for the measurements of the desorption force of the particles from the carboxylated capillary wall. The effects of the pH and the presence of Cu2+, Ni2+, Fe3+ or Al3+ on the desorption dynamics were examined. The observed histogram of the desorption events of the particles as a function of the pulling force was analyzed by the Bell mechanism, which could be used to evaluate the spontaneous desorption rate constant of the particle in the absence of the external force and the critical distance for the breaking of the interaction. The feasibility of the present method as a simple dynamic force measurement technique of chemical interaction was demonstrated.
We here focus on whether a model peptide for Group 3 LEA (G3LEA) proteins can be used for the dry preservation of vesicles whose size and phospholipid compositions resemble those of living cells. For this purpose, we prepared a peptide called PvLEA-22, which consists of two tandem repeats of the 11-mer motif characteristic of G3LEA proteins from an African sleeping chironomid, and giant vesicles (diameter 6–9 µm) prepared with egg phosphatidylcholine. We examined the particle size distribution of the vesicles before and after drying and rehydration in the presence of the peptide. The model peptide suppressed desiccation-induced disruption of the giant vesicles in a concentration-dependent manner with an efficiency comparable to that of trehalose, a well-known desiccation protectant. To elucidate the underlying mechanism of the peptide’s protective function, we performed molecular dynamics (MD) simulations for model systems composed of the peptide PvLEA-22 and a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) bilayer. The results indicate that Lys residues in PvLEA-22 interact with the bilayer surface, where their positively charged side chains, -(CH2)4NH3+, directly hydrogen bond (H-bond) with nearby phospholipid headgroups. On the basis of these results, we discuss the possible mechanisms for the protective effect of the peptide on dried vesicles.
Molecular and crystal structures of dicationic tetrakis(1,3-benzodithiol-2-ylidene)cyclopentanone (1b), (1b2+)2(ReO4−)4(CH3CN)3(H2O)0.7, were successfully determined by X-ray single-crystal diffraction. It was suggested that two 1,3-dithiol-2-ylidenes adjacent to the carbonyl group were effectively conjugated with each other. The remaining two 1,3-dithiol-2-ylidenes, where two positive charges were mainly distributed, were considerably twisted from the central five-membered ring.
Crystallization of neutral 3,5-pyridinedicarboxylic acid N-oxide (H2-3,5-pydcao) molecules in the presence/absence of solvent molecules affords neutral hydrogen bond-supported one-dimensional, two-dimensional, and three-dimensional molecular networks of H2-3,5-pydcao·DMF (1·DMF), H2-3,5-pydcao·H2O (1·H2O), and H2-3,5-pydcao (1), respectively, that have strong intermolecular hydrogen bonds of Nδ+–Oδ−···HOOC. These molecular networks were found to show reversible structural changes among 1, 1·H2O, and 1·DMF driven by solvent release/uptake.
We performed time-resolved X-ray fluorescence (XRF) and position-dependent X-ray absorption near-edge structure (XANES) spectroscopy on gels containing Co2+ and [Fe(CN)6]4−/[Fe(CN)6]3− ions. Sample tubes containing an agar gel mixed with 0.10 M CoCl2 solution and a water-glass gel mixed with 0.05 M K3[Fe(CN)6]/K4[Fe(CN)6] solution were prepared for the analyses. Dark-purple continuous (DPC) bands were observed in the water-glass gel of the two tubes. XRF results demonstrated that the DPC bands contained diffusive Co–Fe complexes. XANES results indicated that the Fe and Co local structures in the DPC bands were independent of the initial Fe oxidation states in [Fe(CN)6] and the measurement positions. It was found that the Fe local structure of the DPC bands was predominantly [Fe(CN)6]. The real-space multiple scattering calculations using FEFF 8.02 code suggested that Co–Fe-based Prussian blue analogs, of which the most likely local structure is Co(OC)4(NCFe)2, co-existed with [Co(H2O)6]2+ ions in the DPC bands.
We propose photobleaching-assisted near-field optical spectroscopy to obtain the absorption properties of nanomaterials. We used bis(3-sulfopropyl)-5,5′,6,6′-tetrachloro-11′-dioctylbenzimidacarbocyanine (C8S3) double-wall tubular J-aggregates for demonstrating this method. We photobleached the J-aggregate by photoirradiation. We obtained the absorption spectrum of the J-aggregates by subtracting the transmission spectra observed before and after the photobleaching. The spectrum showed an intense asymmetric peak near 595 nm, which was assigned to the J-bands. These features were well reproduced by theoretical model calculations. We estimated the oscillator strength per molecule from the observed absorbance, and we found that the determined oscillator strength is comparable to that of the monomer. These results indicate that the proposed method is applicable for obtaining near-field absorption spectra of nanomaterials in a semi-quantitative manner.