Self-replication is one of the essential characteristics of life, therefore, chemical reaction, in which biologically related chiral enantioenriched compounds can promote their own production, is an attractive challenge in broad scientific fields. Here, we found asymmetric Strecker-type synthesis, in which chiral L- and D-α-amino acids enantioselectively induced the formation and amplification of their own chiral intermediates L- and D-α-aminonitriles in solid state, respectively. Thus, after the hydrolysis of aminonitriles, enantioenriched amino acids with the same structure and the same absolute configuration as that of the original compounds could be replicatively produced with improvement of enantiomeric excess. Following our first report on the replication of α-(p-tolyl)glycine, here we found that the enantiomer of α-(1-naphthyl)glycine and α-(o-tolyl)glycine can also replicatively multiply in the Strecker-type synthesis via the amplification of the corresponding aminonitriles. From the viewpoint of the absolute asymmetric Strecker-type amino acid synthesis, spontaneous formation, amplification and multiplication, i.e., enantioselective reactive crystallization of α-aminonitriles will be discussed.
The transition behavior of Gibbs monolayers of biomolecules at the air/water interface, and the sustainability of their three-dimensional structure during heating by adsorption/immobilization on inorganic particle nanosheets were investigated. Lysozyme (enzyme), cytochrome C (protein), trypsin (digestive enzyme), and luciferase (luminescent enzyme) were the biomolecules used in this study. The surface pressure-time isotherms of these biomolecules showed that the crystal transition of the Gibbs monolayer corresponding to denaturation and deactivation was systematically different. The Gibbs monolayers of these biomolecules were observed to become increasingly unstable with an increase in the number of apparent hydrophobic units, and were susceptible to denaturation by crystal transition. These biomolecules were adsorbed/immobilized on a nanosheet of organo-modified magnetic fine particles. After forming a monolayer on the water surface of the organo-magnetic nanoparticles, these biomolecules were introduced into the subphase and electrostatic interaction between the nanoparticle hydrophilic surface and the biomolecules was induced. When the bio-adsorbed single particle layer was transferred onto the solid substrate, an infra-red (IR) band derived from the adsorbed species was confirmed in this multi-particle layer. In addition, shapeless adsorbed matter was observed by atomic force microscopy images. From the IR measurement under heating, it was found that the secondary structure of the adsorbed lysozyme enzyme was maintained up to about 100 °C by substrate adsorption. This is probably because the three-dimensional structure of biomolecules is less likely to be denatured by using inorganic nanosheets with high density and low defects as the templates.
2,7-Substituted 9,10-bis(phenylethynyl)anthracenes with bulky phenol residues were synthesized and investigated for their antiparallel molecular arrangement. UV-vis absorption and photoluminescence properties and molecular mechanics calculation indicate that they formed an antiparallel π-π stacked dimer, which was strongly associated by hydrogen bonding and π-π interaction. This strongly associated dimer structure was supported by AFM images of specimens prepared on mica by spin casting dilute chloroform/hexane (2/8 v/v) solution. However, the absence of intradimer hydrogen bonding resulted in the offset stacking, which was confirmed from the single crystal X-ray analysis.
Flavin analogues are dispersants of carbon nanotubes. We report the finding that riboflavin, which is a nutrient that is much less expensive than a flavin mononucleotide, is a good SWNT dispersant in water. We carried out a temperature dependence test of the solubilization of SWNTs, and propose a possible solubilization mechanism based on a regression analysis. We assume that the linear and lognormal components for the solubilized SWNTs can be explained by nonspecific and specific interactions between the riboflavin and the SWNTs, respectively. We also carried out computational calculations (molecular dynamics simulations) on this SWNT solubilization by which we proposed a suitable complex structure of the SWNT that provided the number of adsorbed flavin molecules via hydrogen bonding on the tubes.
The photophysical properties of three luminescent Cu(I) complexes, [Cu(dmp)(xantphos)]+ (Cu-1; dmp = 2,9-dimethyl-1,10-phenanthroline, xantphos = 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene), [Cu2(dmp)2(µ-dppa)2]2+ (Cu-2; dppa = bis(diphenylphosphino)acetylene), and [Cu2(Ph2dmp)2(µ-dppa)2]2+ (Cu-2ph; Ph2dmp = 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline), were investigated. The luminescence of Cu-1 at room temperature was assigned to the thermally activated delayed fluorescence (TADF) from the singlet metal-to-ligand charge transfer (1MLCT) excited state. However, the emission of Cu-2 and Cu-2ph exhibited lower radiative rate constants than Cu-1 at room temperature, which can be assigned to phosphorescence from the triplet metal-to-ligand charge transfer (3MLCT) excited state at room temperature. In addition, emission of Cu-2 and Cu-2ph at 77 K indicated significant contributions of the 3ππ* excited state. Theoretical calculations suggested that the energy difference between the optimized S1 and T1 states of Cu-1 is relatively smaller than that of Cu-2. As a result, Cu-1 shows TADF and Cu-2 shows phosphorescence.
Accurate dispersion energy calculations with a low computational cost are necessary in molecular mechanics to evaluate the stabilization of large neutral molecules, as observed in self-assembly systems. However, it is difficult to achieve accurate calculations with a low computational cost. To overcome this difficulty, in this paper, we extend upon our previous approach in two important ways: by introducing a spherical transition density and applying a new fitting approach. With this approach, we can reproduce the Hartree-Fock dispersion energy Edisp(20) in the symmetry-adapted perturbation theory (SAPT) with a low computational cost. Moreover, using the coupled perturbed Hartree-Fock method, the potential presented in this study can improve the error arising from the MP2-like sum-over-states dispersion formula used in Edisp(20).
We report that a regression technique, molecular field analysis (MFA), is useful to design molecules in asymmetric catalysis. We found that MFA using intermediate structures in an enantio-determining step enabled extraction and visualization of important 3D-structural information for improving enantioselectivity. Based on the visualized structural information, we designed a catalyst and substrate. DFT calculations indicated that enantioselectivities of the reactions using the compounds would improve significantly. We confirmed experimentally that the designed substrate exhibited higher enantioselectivity than those in the training set.
A CpRu/Brønsted acid-combined catalyst, CpRu(II)/picolinic acid (PyCOOH), acts as an efficient catalyst for the allyl protection/deprotection of alcohols. This discovery has resulted in the development of a new axially chiral ligand, Cl-Naph-PyCOOH (2a; 6-(2-chloronaphthalen-1-yl)-5-methylpyridine-2-carboxylic acid) through an investigation on the ligand structure-reactivity relationship in the CpRu-catalyzed dehydrative cyclization of (E)-hept-2-ene-1,7-diol (5) to 2-vinyltetrahydro-2H-pyran (6). A large-scale synthetic procedure for 2a and the allyl esters 2b has been established. The activation energy ΔG‡ of the stereoinversion and the half-life time of (R)-2b racemization have been determined to be 33.7 kcal mol−1 and 16,000 years at 25 °C, respectively. The CpRu(II)/(R)-Cl-Naph-PyCOOH catalyst exists as a 1:1 diastereomeric mixture of (R,RRu)-3 (AR) and (R,SRu)-3 (AS) because of the axial chirality of 2a and the Ru stereogenic center. The epimerization rate of the Ru center is 19.5 s−1 at 30 °C with an energy barrier ΔG‡ of 16.0 kcal mol−1. Both AR and AS have their own reactivity and enantioselectivity. Nevertheless, an enantiomer ratio of up to >99:1 can be realized in the allylative cyclization of E-allylic alcohols possessing a protic nucleophile, OH, NHCOR, NHSO2R, or COOH, at the terminal position. Questions about the mechanism have been raised as progress is being made towards a mechanistic investigation.
A macrocyclic compound consisting of six anthracene units formed a Saturn-shaped complex with fullerene C70 as the ellipsoidal guest. The association constant of the host-guest complex was determined by the NMR titration method to be 4.6 × 103 L mol−1, twice than observed for the C60 counterpart. X-ray analysis revealed that the guest molecule was included in the center of the cavity in nearly standing orientation, whereas DFT calculations predicted the complexation in various orientations. In any complex structure, CH•••π interactions play an important role in forming the ring–body supramolecular system. The intraannular hydrogen atoms in the ring moiety were deshielded upon complexation, and this phenomenon is discussed on the basis of the NMR shielding of C70 and the calculated structures. In spite of the different relative orientations of C70, the host-guest formation strength remains similar exposing the great versatility of the host capabilities against non-spherical fullerenes.
Heme in the ferric state (heme(Fe3+)) binds to G-quadruplex DNAs to form stable complexes that exhibit enhanced peroxidase activities. The complexes are considered DNAzymes possessing heme as a prosthetic group (heme-DNAzymes), and have been extensively investigated as promising catalysts for a variety of applications. On ESR and stopped-flow measurements, an iron(IV)oxo porphyrin π-cation radical known as Compound I was detected in reaction mixtures of heme-DNAzymes and hydrogen peroxide. This finding not only resolved the long-standing issue of the mechanism underlying the enhancement of the peroxidase activity of heme(Fe3+) in the scaffold of a G-quadruplex DNA, but also provided new insights as to the design of novel heme-DNAzymes.
In the enantioselecitve hydrogenation of (E)-2-methyl-2-butenoic acid (1) over a cinchonidine-modified Pd/C catalyst, the addition of hydrogen preferentially proceeds from the Re-Si enantioface of the C=C double bond of 1 to yield (S)-2-methylbutanoic acid ((S)-3). Double bond migration of 1 takes place under the reaction conditions and is followed by immediate hydrogenation to yield 3 in a poor enantiomeric purity. Deuterium labeling experiments at 0.1 MPa and 1.9 MPa of D2 verified the previous assumption of competitive double bond migration. The combination of isotopic labeling experiments and chiral analysis revealed that the double bond migration of 1 proceeds with the same enantiofacial differentiation as the hydrogenation of 1. Thus, interaction of 1 with cinchonidine adsorbed on the Pd surface may control the configuration of the double bond migration and the hydrogenation.
Two palladium/chiral diphosphine-catalyzed umpolung cyclizations of aldehyde-containing allylic acetates and allenes with arylboronic acid are fully investigated to establish key factors in their high stereoselectivities. Both cyclization reactions afford cis-disubstituted pyrrolidine and tetrahydrofuran. These occur in high diastereo- and enantioselectivities through a common cationic (Z)-η1-allylpalladium, toward which a ring strain generated in the cyclization step leading to trans-isomers biases the equilibrium through η3-η1-η3-complex in the former cyclization. Varied diastereoselectivities were observed in the formation of five-membered carbocycles and six-membered heterocycles. These reflect release of a ring strain generated in the cyclization step leading to trans-isomers and a different distribution of the (Z)- and the (E)-η1-allylpalladium intermediates generated by the oxidative addition of allylic acetates to Pd(0) or carbopalladation of allenes, respectively. A sterically demanding substituent at the center of the allyl moiety is necessary for high diastereo- and enantioselectivity. The enantioselectivity of the former cyclization was lowered by the presence of organometallic reductants or reagents, possibly causing the formation of neutral η1-allylpalladium species. We used a chiral allylic acetate containing (E)-deuterium-labeled alkene to demonstrate that the electrophilic attack of the aldehyde to the allyl ligand occurred on the side where the palladium existed, consistent with the Zimmerman-Traxler transition state.
Magnesium (Mg)-based micromotors have attracted considerable attention as they are capable of moving in water and human blood plasma without external fuels. It has also been demonstrated that they have potential for drug delivery in mouse stomach. However, their biocompatibility and cytotoxicity to human cells have yet to be studied. Therefore, we performed cytotoxicity study of Mg/Pt Janus micromotors with human lung carcinoma epithelial cells (A549), human breast cancer cells (MCF-7), human embryonic kidney cells (HEK-293), human liver carcinoma cells (HepG2) and human cervical cancer cells (HeLa). The highest concentration of micromotors tested, 200 µg mL−1, drastically induced a high toxic effect on the cells and reduced the cell viability to below 60%. This shows while Pt/Au nanomachines were found to be safe previously, this is not the case of the Mg/Pt micromachines.
The pH dependent structure of thymol blue (TB) in solution, a long-standing controversial issue, has been determined by quantum chemistry with an aid of multivariate analysis of electronic absorption spectra. It has been revealed that the TB structure varies from the neutral biprotonated form with open-sulton ring showing red to the monovalent anion of the pure quinoid form showing yellow and then to the quinoid-phenolate form showing blue on raising the pH for a solution.
An artificial signal transduction system has been constructed by employing engineered human immunodeficiency type-1 (HIV-1) protease and Nostoc punctiforme PCC73102 (Npu) DnaE intein. While the truncation of four amino acid residues at the N-terminus of HIV-1 protease diminished its activity, the attachment of the PQIT sequence into the truncated protease by protein trans-splicing (PTS) reconstituted the enzymatic activity. By combining interaction-dependent native chemical ligation (IDNCL) with the PTS reaction, the peptide-protein interaction was clearly detected by measuring HIV-1 protease activity. Src homology domain 2 (SH2) of c-Src (SrcSH2) and phosphopeptides were used as model binding pairs. HIV-1 protease activities were dose-dependently increased after the IDNCL-PTS reaction when the peptides containing pYEEI (pY = phosohotyrosine) and pYEE sequences were used as the input peptides. HIV-1 protease activity generated by IDNCL-PTS might activate several enzymes, and therefore, the artificial signal transduction system might be available in synthetic biology.
Compounds 5-(5-methyl-2-phenyl-4-thiazolyl)-4-(5-methyl-1-phenyl-1-propenyl)-2-phenylthiazole 1a and 5-(2-methyl-5-phenyl-3-thienyl)-4-(2-methyl-1-phenyl-1-propenyl)-2-phenylthiazole 2a having an S-N heteroatom-contact interaction and a CH-N hydrogen bonding were synthesized in an attempt to obtain new photochromic compounds having terarylene and 1-aryl-2-vinylcyclopentene backbones. The S-N interaction and CH-N hydrogen bonding of 1a and 2a can be explained by DFT calculation and temperature dependent 1H NMR spectra. The tethering interactions are expected to stabilize the reactive conformation of 1a and 2a, and to increase the photocyclization quantum yield. Two 5-heteroaryl-4-vinyl-2-phenylthiazoles 1 and 2 underwent reversible photocyclization reactions from the open-ring isomers 1a and 2a to the closed-ring isomers 1b and 2b, respectively. The photocyclization quantum yields of 1a and 2a are 0.57 and 0.63, which are 3 times larger than that of 1-thiazolyl-2-vinylcyclopentene derivatives (0.20). The number of photochromic cycles exhibited by 1b was 50 in the presence of air, which is higher than that of 2b.
Solid-state vibrational circular dichroism (SD-VCD) spectra were measured for the intercalation compounds of layered double hydroxide (LDH) and d- or l-phenylalanine (d- or l-Phe). The investigated LDH was composed of Zn(II) and Al(III) in 2:1 molar ratio. For comparison, the SD-VCD spectra were recorded for enantiopure crystalline samples of Phe. The measured spectra were analyzed with the help of a theoretical simulation calculated by the Gaussian16 program. It was concluded that Phe formed a tetramer in the crystalline state, forming intermolecular hydrogen bonds between –COO− and –NH3+ groups. In the intercalated states, the neighboring Phe molecules oriented vertically to the layer surface in an anti-parallel fashion, forming their –COO− groups hydrogen bonded individually with the OH groups on the surface of LDH. The results demonstrated the utility of the SD-VCD method for obtaining the detailed conformation of a molecule within an inorganic host.
Heterogeneous catalytic reaction at low temperatures (<500 K) has been proposed and investigated by our group. As described in this report, recent trends of low-temperature catalytic reaction for hydrogen production by reforming and ammonia synthesis are summarized. Furthermore, our findings obtained using surface protonics for these two reactions are introduced. Surface protonics occurs by application of an electric field to a heterogeneous catalyst. It makes low-temperature catalytic reactions possible for hydrogen production and ammonia synthesis.