Search

The third PDZ domain of the postsynaptic density protein 95 (PSD95-PDZ3; 11 kDa, 103 residues) has a propensity to form amyloid fibrils at high temperatures. At neutral pH, PDZ3 is natively folded, but it exhibits a peculiar three-state thermal unfolding with a reversible oligomerization (RO) equilibrium at high temperatures, which is uncharacteristic in the unfolding of a small globular protein as PDZ3 is. Here, we examined the RO's role in PDZ3's amyloidogenesis at high-temperature using two variants (F340A and L342A) that suppress the high-temperature RO and five single-alanine-mutated variants, where we mutated surface-exposed hydrophobic residues to alanine. Circular Dichroism (CD), Analytical Ultracentrifuge (AUC), and other spectroscopic measurements confirmed the retention of the native structure at ambient temperature. Differential Scanning Calorimetry (DSC) was used to assess the presence or absence of the high-temperature RO, and the amyloidogenicity of the variants was measured by Thioflavin T (ThT) fluorescence and Transmission Electron Microscopy (TEM). By comparing the fraction of RO and the ThT signal, we found that mutations that suppressed the high-temperature RO strongly inhibited amyloidogenesis. On the other hand, all variants forming RO also formed amyloids under the same conditions as the wild-type PDZ3.

Chiral spiro π-conjugated compounds have emerged as a new class of circularly polarized luminescent organic materials. Here we report the synthesis and (chir)optical properties of a chiral benzo[b]silole-fused 9,9'-spirobi[fluorene] (SBF) and π-extended spiro polycyclic arene. The benzo[b]silole-fused SBF was successfully synthesized by a rhodium-catalyzed intramolecular silylative cyclization. It was further transformed to the chiral π-extended spiro polycyclic arene by an annulative π-extension reaction. Less effective spiroconjugation was observed for these spiro compounds through UV-Vis absorption spectroscopy and theoretical calculations. They exhibit circularly polarized luminescence with the dissymmetry factors of up to 0.76×10-3 . Theoretical calculations demonstrate that emission of the benzo[b]silole-fused SBF occurs from one subunit, the structure of which is slightly different from that in the Frank-Condon state.

Electrode materials can have a significant impact on the course of an electrolysis reaction. Of particular interest is that different electrodes can generate different products from the same substrate. The electrode-material-selective transformations of styrene derivatives with molecular oxygen are reported. Platinum electrodes afford carbonyl products via cleavage of olefins, whereas tetrahydrofuran formation is achieved with carbon electrodes. A variety of different styrenes are available for both reactions. Electrolysis allows straightforward and mild chemical conversions that are metal- and oxidant-free. Electrochemical measurements illuminate the different effects of platinum and carbon electrodes on styrenes. The key to the differing reactions is probably that the oxidation potentials of the substrates are lower (higher HOMO energy) on carbon electrodes than on platinum electrodes. The adsorption of the substrates on carbon electrodes can also promote tetrahydrofuran formation.

Described herein is the enantioselective syntheses of (+)- and (-)-rishirilide B from the corresponding optically active β-substituted tetralones, which were obtained by oxidative kinetic resolution based on α-hydroxylation in the presence of a chiral guanidine-bisurea bifunctional organocatalyst. Benzylic oxidation of the tetralones at C1 followed by regioselective isomerization of the oxabenzonorbornadiene structure led to rishirilide B. Our findings lead to the revision of the previously proposed (2R,3R,4R) absolute configuration of (+)-rishirilide B to (2S,3S,4S).

The effects of bent linkers in diynes on the efficiency and selectivity of intermolecular macrocyclization by the rhodium-catalyzed sequential cross-cyclotrimerization of L-shaped diynes and di-tert-butyl acetylenedicarboxylate was investigated. A bulky triethylsilyl-protected cyclohexa-1,4-dienediol-linked diyne reacted with di-tert-butyl acetylenedicarboxylate to give the corresponding 16-membered macrocycle as well as the 12-membered macrocycle. Aromatization of the thus-obtained 12-membered macrocycle through desilylation followed by reductive aromatization with SnCl2 and HCl afforded a [12]cycloparaphenylenehexacarboxylate. The use of a conformationally flexible methoxymethyl-protected cyclohexanediol-linked diyne increased the yield of the corresponding 16-membered macrocycle, although oxidative aromatization leading to the corresponding cycloparaphenylenes was unsuccessful.

An electron-deficient Cp(E) rhodium(III) complex bearing a cyclopentadienyl ligand with two ethyl ester substituents catalyzes the tandem [2+2+2] annulation-lactamization of acetanilides with two alkynoates via cleavage of adjacent two C-H bonds to give densely substituted benzo[cd]indolones. The reactions of meta-methoxy-substituted acetanilides with two alkynoates also provided benzo[cd]indolones via cleavage of adjacent C-H/C-O bonds. Furthermore, 3,5-dimethoxyacetanilides reacted with two alkynoates to give dearomatized spiro compounds.

Halohydrin hydrogen-halide-lyase (H-Lyase) is a bacterial enzyme that is involved in the degradation of halohydrins. This enzyme catalyzes the intramolecular nucleophilic displacement of a halogen by a vicinal hydroxyl group in halohydrins to produce the corresponding epoxides. The epoxide products are subsequently hydrolyzed by an epoxide hydrolase, yielding the corresponding 1, 2-diol. Until now, six different H-Lyases have been studied. These H-Lyases are grouped into three subtypes (A, B, and C) based on amino acid sequence similarities and exhibit different enantioselectivity. Corynebacterium sp. strain N-1074 has two different isozymes of H-Lyase, HheA (A-type) and HheB (B-type). We have determined their crystal structures to elucidate the differences in enantioselectivity among them. All three groups share a similar structure, including catalytic sites. The lack of enantioselectivity of HheA seems to be due to the relatively wide size of the substrate tunnel compared to that of other H-Lyases. Among the B-type H-Lyases, HheB shows relatively high enantioselectivity compared to that of HheBGP1 . This difference seems to be due to amino acid replacements at the active site tunnel. The binding mode of 1, 3-dicyano-2-propanol at the catalytic site in the crystal structure of the HheB-DiCN complex suggests that the product should be (R)-epichlorohydrin, which agrees with the enantioselectivity of HheB. Comparison with the structure of HheC provides a clue for the difference in their enantioselectivity.

The reaction mechanism of nitrile hydratase (NHase) was investigated using time-resolved crystallography of the mutant NHase, in which βArg56, strictly conserved and hydrogen bonded to the two post-translationally oxidized cysteine ligands, was replaced by lysine, and pivalonitrile was the substrate. The crystal structures of the reaction intermediates were determined at high resolution (1.2-1.3 Å). In combination with FTIR analyses of NHase following hydration in H2 (18) O, we propose that the metal-coordinated substrate is nucleophilically attacked by the O(SO(-) ) atom of αCys114-SO(-) , followed by nucleophilic attack of the S(SO(-) ) atom by a βArg56-activated water molecule to release the product amide and regenerate αCys114-SO(-) .

A cationic gold(I)/axially chiral biaryl bis(phosphine) complex has been employed to catalyze the asymmetric dearomatization reactions of 1-aminonaphthalene derivatives through a C–C bond-forming reaction with electron-rich heterocycles as the nucleophiles. These reactions afford pentacyclic heterocycles in good yields with moderate enantiomeric excess (ee) values.

The title reaction is promoted by BF3 nitrile complexes and proceeds in moderate to good yields with high regioselectivities.

A 1,1′-bitriphenylene-based sila[7]helicene was synthesized with a high ee value by enantioselective double [2+2+2] cycloaddition of a biaryl-linked tetrayne with a silicon-linked bis(propargylic alcohol) as a key step. This sila[7]helicene exhibited a high tolerance toward racemization, which could be explained by the X-ray crystal structure analysis. With respect to the photophysical properties, this sila[7]helicene exhibited a relatively high fluorescence quantum yield and circularly polarized luminescence activity.

The encapsulin nanocompartment from Rhodococcus erythropolis N771 (Reencapsulin) was expressed and purified in wild-type and C-terminally His-tagged forms. Negative-stained transmission electron microscopy, field-flow fractionation combined with multi-angle light scattering and dynamic light scattering analyses showed that 60 Reencapsulin monomers were assembled as a spherical particle with a diameter of 28 nm. Heterogeneous guest proteins such as EGFP and firefly luciferase were packaged into the internal cavity of the Reencapsulin nanocompartment by fusing the C-terminal 37-amino-acid sequence of the R. erythropolis N771 DypB peroxidase to the C-terminus. Reencapsulin has the potential to package target proteins in its internal cavity and/or display them on its external surface, making it a feasible carrier for nanotechnology applications.

The crystal structure of the triple-helical peptide (Pro-Hyp-Gly)3 -Pro-Arg-Gly-(Pro-Hyp-Gly)4 (POG3-PRG-POG4) was determined at 1.45 A resolution. POG3-PRG-POG4 was designed to permit investigation of the side-chain conformation of the Arg residues in a triple-helical structure. Because of the alternative structure of one of three Arg residues, four side-chain conformations were observed in an asymmetric unit. Among them, three adopt a ttg(-) t conformation and the other adopts a tg(-) g(-) t conformation. A statistical analysis of 80 Arg residues in various triple-helical peptides showed that, unlike those in globular proteins, they preferentially adopt a tt conformation for χ1 and χ2 , as observed in POG3-PRG-POG4. This conformation permits van der Waals contacts between the side-chain atoms of Arg and the main-chain atoms of the adjacent strand in the same molecule. Unlike many other host-guest peptides, in which there is a significant difference between the helical twists in the guest and the host peptides, POG3-PRG-POG4 shows a marked difference between the helical twists in the N-terminal peptide and those in the C-terminal peptide, separated near the Arg residue. This suggested that the unique side-chain conformation of the Arg residue affects not only the conformation of the guest peptide, but also the conformation of the peptide away from the Arg residue.

A domino cyclization of α-alkoxyalkyl ortho-alkynylaryl ethers and electron-rich arenes affords benzofurans.

The asymmetric synthesis of C2-symmetric axially chiral biaryls was achieved by the cationic rhodium(I)/1,3-bis(diphenylphosphino)propane (dppp) complex catalyzed diastereoselective double [2+2+2] cycloaddition of (R)-3-butyn-2-ol-derived tetraynes with functionalized monoynes. Interestingly, the use of propiolates afforded biaryls possessing large dihedral angles, and in contrast, the use of propargyl alcohol afforded biaryls possessing small dihedral angles.

It has been established that a cationic rhodium(I)/H8-binap or binap complex catalyzes two different modes of cyclization of γ-alkynyl aldehydes with carboxylic acid anhydrides to give cyclic aldehyde gem-dicarboxylates and cyclic alkenyl esters through cleavage of the carboxylic acid anhydride C–O bond. The reaction of a terminal γ-alkynyl aldehyde with diethyl pyrocarbonate afforded a cyclic allylic carbonate with a high ee value.

The single-crystal structure of the collagen-like peptide (Pro-Pro-Gly)4 -Hyp-Asp-Gly-(Pro-Pro-Gly)4, was analyzed at 1.02 A resolution. The overall average helical twist (θ = 49.6°) suggests that this peptide adopts a 7/2 triple-helical structure and that its conformation is very similar to that of (Gly-Pro-Hyp)9, which has the typical repeating sequence in collagen. High-resolution studies on other collagen-like peptides have shown that imino acid-rich sequences preferentially adopt a 7/2 triple-helical structure (θ = 51.4°), whereas imino acid-lean sequences adopt relaxed conformations (θ < 51.4°). The guest Gly-Hyp-Asp sequence in the present peptide, however, has a large helical twist (θ = 61.1°), whereas that of the host Pro-Pro-Gly sequence is small (θ = 46.7°), indicating that the relationship between the helical conformation and the amino acid sequence of such peptides is complex. In the present structure, a strong intermolecular hydrogen bond between two Asp residues on the A and B strands might induce the large helical twist of the guest sequence; this is compensated by a reduced helical twist in the host, so that an overall 7/2-helical symmetry is maintained. The Asp residue in the C strand might interact electrostatically with the N-terminus of an adjacent molecule, causing axial displacement, reminiscent of the D-staggered structure in fibrous collagens.

It has been established that a cationic rhodium(I)/BINAP complex catalyzes the cycloisomerization of 2-silylethynylphenols, leading to 3-silylbenzofurans, via 1,2-silicon migration. Similarly, the cycloisomerization of 2-silylethynylanilines, leading to 3-silylindoles, via 1,2-silicon migration was catalyzed by a cationic rhodium(I)/H8–BINAP complex.

It has been established that a cationic rhodium(I)/H8-BINAP complex catalyzes the asymmetric [2 + 2 + 2] cycloaddition of 1,6-enynes with cyclopropylideneacetamides to produce spirocyclohexenes in excellent enantioselectivity with retaining cyclopropane rings.

The goal of this work was to improve the bioluminescence-based signaling assay system to create a practical application of a biomimetic odor sensor using an engineered yeast-expressing olfactory receptors (ORs). Using the yeast endogenous pheromone receptor (Ste2p) as a model GPCR, we determined the suitable promoters for the firefly luciferase (luc) reporter and GPCR genes. Additionally, we deleted some genes to further improve the sensitivity of the luc reporter assay. By replacing the endogenous yeast G-protein α-subunit (Gpa1p) with the olfactory-specific Gα(olf), the optimized yeast strain successfully transduced signal through both OR and yeast Ste2p. Our results will assist the development of a bioluminescence-based odor-sensing system using OR-expressing yeast.

The catalytic cascade reactions of dienynes, e.g. I, leading to 1,2-dihydronaphthalenes, naphthalenes, and 1,4-dihydronaphthalenes, e.g. II, including the catalytic enantioselective carboformylation of alkenes with aldehydes or the cycloisomerization of enallenes have been developed by using the cationic rhodium(I)/xyl-binap or binap complex as a catalyst. [on SciFinder(R)]

Herein, we present details of our conformationally flexible, 1,3-diamine-tethered guanidine/bisthiourea organocatalysts for chemo-, regio-, and enantioselective 1,4-type Friedel-Crafts reactions of phenols. These organocatalysts show a unique stereo-discrimination governed by the differential activation entropy (ΔΔS(≠)), rather than by the differential activation enthalpy (ΔΔH(≠)). Extensive kinetic analyses using Eyring plots for a series of guanidine/bisthiourea organocatalysts revealed the key structural motif in the catalysts associated with a large magnitude of differential activation entropy (ΔΔS(≠)). A plausible guanidine-thiourea cooperative mechanism for the enantioselective Friedel-Crafts reaction is proposed.

In the presence of chiral bis(thiourea) catalyst (I), ortho-selective Friedel—Crafts products are formed.

Crystallization of amorphous poly(lactic acid) (PLA) was investigated in various organic solvents, such as acetone, ethylacetate, diethylether, tetrahydrofurane, methanol, hexane, toluene, xylene, and o-dichlorobenene. Most of the solvents, except hexane, induced crystallization of amorphous PLA. Acetone was the most effective solvent to accelerate the crystallization among the solvents used. The crystallization was induced by permeation of acetone into the amorphous phase of PLA, and the permeation obeyed Fick type diffusion. The crystallization rate increased with increasing of conducting temperature. Crystallized PLA formed α crystalline structure. The permeated acetone in the crystallized PLA gradually evaporated as time passes, and the elimination of acetone affected thermal and mechanical properties of the crystallized PLA. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011

A method for the construction of three or four adjacent stereocenters at the tertiary homoallylic alcohols is presented.

Structures of (Pro-Pro-Gly)4-Xaa-Yaa-Gly-(Pro-Pro-Gly)4 (ppg9-XYG) where (Xaa, Yaa) = (Pro, Hyp), (Hyp, Pro) or (Hyp, Hyp) were analyzed at high resolution using synchrotron radiation. Molecular and crystal structures of these peptides are very similar to those of the (Pro-Pro-Gly)9 peptide. The results obtained in this study, together with those obtained from related compounds, indicated the puckering propensity of the Hyp in the X position: (1) Hyp(X) residues involved in the Hyp(X):Pro(Y) stacking pairs prefer the down-puckering conformation, as in ppg9-OPG, and ppg9-OOG; (2) Hyp(X) residues involved in the Hyp(X):Hyp(Y) stacking pairs prefer the up-puckering conformation if there is no specific reason to adopt the down-puckering conformation. Water molecules in these peptide crystals are classified into two groups, the 1st and 2nd hydration waters. Water molecules in the 1st hydration group have direct hydrogen bonds with peptide oxygen atoms, whereas those in the 2nd hydration group do not. Compared with globular proteins, the number of water molecules in the 2nd hydration shell of the ppg9-XYG peptides is very large, likely due to the unique rod-like molecular structure of collagen model peptides. In the collagen helix, the amino acid residues in the X and Y positions must protrude outside of the triple helix, which forces even the hydrophobic side chains, such as Pro, to be exposed to the surrounding water molecules. Therefore, most of the waters in the 2nd hydration shell are covering hydrophobic Pro side chains by forming clathrate structures. © 2009 Wiley Periodicals, Inc. Biopolymers 91: 361–372, 2009. This article was originally published online as an accepted preprint. The “Published Online” date corresponds to the preprint version. You can request a copy of the preprint by emailing the Biopolymers editorial office at biopolymers@wiley.com

It has been established that a cationic rhodium(I)/H8 -binap complex catalyzes the [3+2+2] cycloaddition of 1,6-diynes with cyclopropylideneacetamides to produce cycloheptadiene derivatives through cleavage of cyclopropane rings. In contrast, a cationic rhodium(I)/(S)-binap complex catalyzes the enantioselective [2+2+2] cycloaddition of terminal alkynes, acetylenedicarboxylates, and cyclopropylideneacetamides to produce spiro-cyclohexadiene derivatives which retain the cyclopropane rings.

The title reaction is promoted by BF3 nitrile complexes and proceeds in moderate to good yields with high regioselectivities.

A 1,1′-bitriphenylene-based sila[7]helicene was synthesized with a high ee value by enantioselective double [2+2+2] cycloaddition of a biaryl-linked tetrayne with a silicon-linked bis(propargylic alcohol) as a key step. This sila[7]helicene exhibited a high tolerance toward racemization, which could be explained by the X-ray crystal structure analysis. With respect to the photophysical properties, this sila[7]helicene exhibited a relatively high fluorescence quantum yield and circularly polarized luminescence activity.

It has been established that a cationic rhodium(I)–BINAP complex catalyzes the enantioselective [2 + 2 + 2] cycloaddition of a tosylamide-linked 5-allenal and 5-allenone with internal alkynes, lead...

Based on the fiber diffraction data from native collagen, Rich and Crick proposed the 10/3-helical model with a 28.6 A axial repeat in 1955 (Rich A.; Crick, F. H. C. Nature (Lond) 1955, 176, 915-916). We obtained the 7/2-helical structure with a 20 A axial repeat from the single crystal analysis of (Pro-Pro-Gly)(10). Since the latter structure could explain fiber diffraction patterns from native collagen, we proposed this structure as a new model for collagen in 1977 (Okuyama et al., Polym J 1977, 9, 341-343). These two structural models were refined against observed continuous intensity data from native collagen using a linked-atom least-squares method. It was found that the diffraction data from native collagen could be explained by the 7/2-helical model better than, or at least the same as, the prevailing 10/3-helical model. Together with the evidence that recent single crystal analyses of many model peptides have supported the 7/2-helical model and there was no such active support for the 10/3-helical model, it was concluded that the average molecular structure of native collagen seems to be closer to the 7/2-helical symmetry than the other one.

It has been established that a cationic Rh(I)/(S)-Segphos or (S)-DTBM-Segphos complex and benzoic acid catalyze the enantioselective cycloisomerization of 1,6-enynes, possessing carbonyl groups at the enyne linkage, to 2-alkylidenebicyclo[3.1.0]hexanes. The present cycloisomerization may involve site selective γ-hydrogen elimination. The one-pot enantioselective cycloisomerization and lactonization of 1,6-enynes, leading to bicyclic lactones, has also been accomplished.

The enantioselective synthesis of completely ortho-fused [9]- and [11]helicene-like molecules has been achieved through a rhodium-mediated, intramolecular, double [2+2+2] cycloaddition of phenol- or 2-naphthol-linked hexaynes. Crystal structures and photophysical properties of these [9]- and [11]helicene-like molecules have also been disclosed.

Triple-helical structures of (Pro-Hyp-Gly)n (n = 10, 11) at 100 K and room temperature (RT) were analyzed at 1.26 A resolution by using synchrotron radiation data. Totals of 49 and 42 water molecules per seven triplets in an asymmetric unit were found for the structures at 100 K and RT, respectively. These water molecules were classified into two groups, those in the first and second hydration shells. Although there was no significant difference between water molecules in the first shell at 100 K and those at RT, a significant difference between those in the second shell was observed. That is, the number of water molecules at RT decreased to one half and the average distance from peptide chains at RT became longer by about 0.3 A. On the other hand, of seven triplets in an asymmetric unit, three proline residues at the X position at 100 K clearly showed an up-puckering conformation, as opposed to the recent propensity-based hypothesis for the stabilization and destabilization of triple-helical structures by proline hydroxylation. This puckering was attributed to the interaction between proline rings and the surrounding water molecules at 100 K, which is much weaker at RT, as shown by longer average distance from peptide chains.

It has been established that a cationic rhodium(I)/(R)-tol-binap complex catalyzes the cross-cyclotrimerization of silylacetylenes, di-tert-butyl acetylenedicarboxylates, and acrylamides with excellent chemo-, regio-, and enantioselectivities. Unsymmetrical alkynoates can also be employed in place of di-tert-butyl acetylenedicarboxylate for this process, but with reduced chemoselectivity.

Significant amounts (20-45% NMR) of undesired regioisomers are also generated in some cases.

Three novel lettuce hypocotyl and root growth regulating compounds, armillariols A (I), B (II), and C (III), are isolated from the culture broth of Armillaria sp.

It has been established that a cationic rhodium(I)/H8-binap or binap complex catalyzes two different modes of cyclization of γ-alkynyl aldehydes with carboxylic acid anhydrides to give cyclic aldehyde gem-dicarboxylates and cyclic alkenyl esters through cleavage of the carboxylic acid anhydride C–O bond. The reaction of a terminal γ-alkynyl aldehyde with diethyl pyrocarbonate afforded a cyclic allylic carbonate with a high ee value.

Two members of a family of pyrrole-imidazole marine alkaloids, (+)-dibromophakellin and the nonnatural congener (+)-phakellin, were synthesized enantioselectively from 4-hydroxy-L-proline. The chiral aminal at C10 was constructed efficiently by means of an Overman-type [3,3] sigmatropic rearrangement of an enamide (see scheme).