Can a consecutive double turn conformation be considered as a peptide based molecular scaffold for supramolecular helix in the solid state?

Helices and sheets are ubiquitous in nature. However, there are also some examples of self-assembling molecules forming supramolecular helices and sheets in unnatural systems. Unlike supramolecular sheets there are a very few examples of peptide sub-units that can be used to construct supramolecular helical architectures using the backbone hydrogen bonding functionalities of peptides. In this report we describe the design and synthesis of two single turn/bend forming peptides (Boc-Phe-Aib-Ile-OMe 1 and Boc-Ala-Leu-Aib-OMe 2) (Aib: α-aminoisobutyric acid) and a series of double-turn forming peptides (Boc-Phe-Aib-Ile-Aib-OMe 3, Boc-Leu-Aib-Gly-Aib-OMe 4 and Boc-γ-Abu-Aib-Leu-Aib-OMe 5) (γ-Abu: γ-aminobutyric acid). It has been found that, in crystals, on self-assembly, single turn/bend forming peptides form either a supramolecular sheet (peptide 1) or a supramolecular helix (peptide 2), unlike self-associating double turn forming peptides, which have only the option of forming supramolecular helical assemblages.     

Syntheses and Br…π supramolecular interactions in the crystal structure of ipriflavone bromides

Supramolecular forces, including electrostatic in- teraction, hydrogen bond, hydrophobic interaction and aromatic stacking interaction, are the important re- search area in supramolecular chemistry and crystal engineering[1]. Aromatic stacking interaction…     

Syntheses and Br...π supramolecular interactions in the crystal structure of ipriflavone bromides

Two novel compounds, 3,8-dibromo-2-methoxyl-7-isopropoxyisoflavanone (1) and 3,6-dibromo-2-methoxyl-7-isopropoxyisoflavanone (2), are synthesized via bromination reaction of their precursor ipriflavone in methanol. The mixture of 1 and 2 co-crystallizes in ethanol and forms two different crystal shapes, the shape of 1 being block and 2 prism, they can be separated manually. 1 and 2 are characterized by IR, ¹H NMR and single crystal X-ray diffraction. The mechanism of bromination reaction is also discussed. 1 crystallizes in the monoclinic space group P2¹/n with cell parameters a = 1.3569(3) nm, b = 0.6706(2) nm, c = 2.0704(4) nm, β = 97.50(2)°, V = 1.8678(6) nm³, Z = 4, Dc = 1.672 Mg/m³, R = 0.0455, wR = 0.0779, F(000) = 936. 2 crystallizes in the monoclinic space group P 2₁/c with cell parameters a =1.3854(17) nm, b = 1.1215(14) nm, c = 1.3038(17) nm, β = 103.71(2)°, V = 1.968(4) nm³, Z = 4, Dc = 1.587 Mg/m³, R = 0.0306, wR = 0.0619, F(000) = 936. The Br...π interactions, hydrogen bonds, aromatic hydrogen bonds and aromatic stacking interactions exist in the crystal structures of 1 and 2, which lead them into supramolecular compounds with a three-dimensional network structure, respectively. 1 and 2 are the isoflavanone derivatives halogenated at C3 and this kind of isoflavanone halides has not been reported yet.     

The further crystallographic refinement of trichosanthin at 2.7A resolution and the comparison among the three molecular structures in two crystal forms

The three-dimensional structure of trichosanthin at 2.7A resolution has been improved further, by refitting one of the C-terminal tails, adjusting 16 residues in the molecular surface regions, discarding some water molecules with high B values, and adjusting weights during the further refinement. The R-factor has been reduced to 18.5% and the r.m.s deviations from ideal geometry are also improved. The structures of the two molecules in the monoclinic asymmetric unit and the only molecule in the orthorhombic asymmetric unit are compared with one another. The main-chain structures for most of the residues in the three molecules are substantially the same. However, the courses of the three C-terminal tails are completely different, and the intermolecular interactions resulting from the particular packing of the molecules in the crystals account for the differences. The strand Be-2 and the preceding B-turn in small domain show large r.m.s. deviations among the three molecules and they are also involved in i     

Zinc catalyst recycling in the preparation of (all-rac)-α-tocopherol from trimethylhydroquinone and isophytol

Recycling of ZnCl₂ as a catalyst in the cyclocondensation of trimethylhydroquinone and isophytol to all-rac-α-tocopherol was studied. ZnCl₂ was recycled at over 98 % efficiency in required purity by repeated extraction of the reaction streams with water, followed by switching of solvent to butyl acetate and then back to water. Recycled ZnCl₂ had no negative effect on the yield of all-rac-α-tocopherol (maintaining over 90 %) at nearly total conversion of isophytol and only a 3 % molar excess of trimethylhydroquinone.     

Tautomerism in acyl-pyrazolones and in a novel photolysis product—importance and impact of the accurate localization of hydrogen atoms in crystal structures

Acyl-pyrazolones exist in four different tautomeric forms (two keto and two enol) in crystal structures. Routine crystal structure refinements using an independent atom model and routine isolated-molecule calculations fail in locating the mobile hydrogen atoms accurately in 22 investigated acyl-pyrazolone examples. However, a combination of both within the framework of quantum crystallography represented by the method of Hirshfeld atom refinement accurately locates the mobile hydrogen atom in a resonance-assisted hydrogen bond of title compound 3, a novel photolysis product. The impact of the hydrogen atom position on the resonance system of the non-hydrogen framework of the various tautomers is discussed, and the importance of intermolecular interactions for the positioning of the hydrogen atom is highlighted.     

High pressure synthesis and crystal structure of two forms of a new tellurium–silicon clathrate related to the classical type I

Two new clathrate-type structures have been identified in the samples obtained by high pressure–high temperature treatment of appropriate mixtures of elemental silicon and tellurium at 5 GPa and 1200 °C for 60 min reaction time. They are both related to the classical type I silicon clathrate, G₈Si₄₆ (G=guest species). The corresponding structures have been solved by X-ray single crystal diffraction analysis. They proved to correspond to a cubic and rhombohedral forms of the same compound, Te₁₆Si₃₈ (or more precisely Te₈@(Si₃₈Te₈)), in which eight extra tellurium atoms are substituted for silicon ones in the 16i crystallographic sites of the parent structure. In the cubic form, the space group is reduced from Pm-3n to P-43n, and the formation of strong bonds between the Te atoms at the centre of the tetrakaidecahedral cages and one or two silicon atoms of the surrounding cage is clearly observed, which is followed by a decrease of the coordination number of the Te atoms in substitutional position from 4 to 3. In the more distorted rhombohedral form, the 16i and 24k sites of the parent structure are both split in four sites. The formation of strong bonds involving the Te atoms at the centre of the tetrakaidecahedral cages is confirmed, but the main characteristic comes from the formation of another kind of strong bonds involving the Te atoms at the centre of the dodecahedral cages. These bonds are at the origin of the elongation of the structure along the [111] direction, which corresponds to the polar axis.     

Synthesis,Crystal Structure and Cytotoxic Activity of a Zinc（Ⅱ） Complex of the Schiff Base Derived from S-benzyldithiocarbazate

A novel zinc(Ⅱ) complex of empirical formula,ZnL2(L=anionic forms of S-benzyl-β-N-(2,4-dichlorobenzylidene) hydrazine carbodithioate),has been synthesized and characterized by elemental analysis,IR spectra and single-crystal X-ray diffraction.The crystal belongs to the triclinic system,space group P with a = 9.3168(9),b = 13.0353(13),c = 14.7702(15) ,α = 71.2860(10),= 87.5140(10),γ = 79.4480(10)o,V = 1670.0(3) 3,Z = 2,Mr = 773.93,Dc = 1.539 g/cm3,μ = 1.334 mm-1,F(000) = 784,the final R = 0.0403 and wR = 0.0800 for 4060 observed reflections with I>2(I).In the crystal structure,the zinc(Ⅱ) complex has a distorted tetrahedral geometry in which the zinc ion is coordinated by the nitrogen and sulfur atoms from two Schiff base ligands,respectively.The preliminary bioassay indicates that the Schiff base and its zinc complex exhibit inhibitory activity against the human gastric cancer cell lines(MKN45) and hepatoma cell lines(HepG2).     

Hierarchical chiral expression from the nano- to mesoscale in synthetic supramolecular helical fibers of a nonamphiphilic C3-symmetrical π-functional molecule

The controlled preparation of chiral structures is a contemporary challenge for supramolecular science because of the interesting properties that can arise from the resulting materials, and here we show that a synthetic nonamphiphilic C(3) compound containing π-functional tetrathiafulvalene units can form this kind of object. We describe the synthesis, characterization, and self-assembly properties in solution and in the solid state of the enantiopure materials. Circular dichroism (CD) measurements show optical activity resulting from the presence of twisted stacks of preferential helicity and also reveal the critical importance of fiber nucleation in their formation. Molecular mechanics (MM) and molecular dynamics (MD) simulations combined with CD theoretical calculations demonstrate that the (S) enantiomer provides the (M) helix, which is more stable than the (P) helix for this enantiomer. This relationship is for the first time established in this family of C(3) symmetric compounds. In addition, we show that introduction of the "wrong" enantiomer in a stack decreases the helical reversal barrier in a nonlinear manner, which very probably accounts for the absence of a "majority rules" effect. Mesoscopic chiral fibers, which show inverted helicity, i.e. (P) for the (S) enantiomer and (M) for the (R) one, have been obtained upon reprecipitation from dioxane and analyzed by optical and electronic microscopy. The fibers obtained with the racemic mixture present, as a remarkable feature, opposite homochiral domains within the same fiber, separated by points of helical reversal. Their formation can be explained through an "oscillating" crystallization mechanism. Although C(3) symmetric disk-shaped molecules containing a central benzene core substituted in the 1,3,5 positions with 3,3'-diamido-2,2'-bipyridine based wedges have shown peculiar self-assembly properties for amphiphilic derivatives, the present result shows the benefits of reducing the nonfunctional part of the molecule, in our case with short chiral isopentyl chains. The research reported herein represents an important step toward the preparation of functional mesostructures with controlled helical architectures.     

Synthesis of a New Chiral Ligand Containing 1,5—Diazacyclooctane and the Application in Diethyl Zinc Addition to Benzaldehyde

A new chiral tetradentate ligand (S,S)-1,5-bis (4-benzyloxazolin-2-yl-methyl)-1,5-diazacyclo-octane I has been synthesized and the application of 1 as catalyst in the enantioselective addition of diethyl zinc to benzaldehyde is also described.     

X-ray crystal structure of arsenite-inhibited xanthine oxidase: μ-sulfido,μ-oxo double bridge between molybdenum and arsenic in the active site

Xanthine oxidoreductase is a molybdenum-containing enzyme that catalyzes the hydroxylation reaction of sp(2)-hybridized carbon centers of a variety of substrates, including purines, aldehydes, and other heterocyclic compounds. The complex of arsenite-inhibited xanthine oxidase has been characterized previously by UV-vis, electron paramagnetic resonance, and X-ray absorption spectroscopy (XAS), and the catalytically essential sulfido ligand of the square-pyrimidal molybdenum center has been suggested to be involved in arsenite binding through either a μ-sulfido,μ-oxo double bridge or a single μ-sulfido bridge. However, this is contrary to the crystallographically observed single μ-oxo bridge between molybdenum and arsenic in the desulfo form of aldehyde oxidoreductase from Desulfovibrio gigas (an enzyme closely related to xanthine oxidase), whose molybdenum center has an oxo ligand replacing the catalytically essential sulfur, as seen in the functional form of xanthine oxidase. Here we use X-ray crystallography to characterize the molybdenum center of arsenite-inhibited xanthine oxidase and solve the structures of the oxidized and reduced inhibition complexes at 1.82 and 2.11 ? resolution, respectively. We observe μ-sulfido,μ-oxo double bridges between molybdenum and arsenic in the active sites of both complexes. Arsenic is four-coordinate with a distorted trigonal-pyramidal geometry in the oxidized complex and three-coordinate with a distorted trigonal-planar geometry in the reduced complex. The doubly bridged binding mode is in agreement with previous XAS data indicating that the catalytically essential sulfur is also essential for the high affinity of reduced xanthine oxidoreductase for arsenite.     

Isolation of a bis-iodurated tetra-THF as a trace product from the oxidation of squalene with RuO₄ and its double ring expansion to a novel bis-THF-bis-THP compound

A novel bis-iodurated polyether compound, based on an unprecedented tetra-THF backbone, has been isolated as a trace by-product of the oxidation of squalene with the catalytic system RuO?(cat.)/NaIO?. The double erythro configuration of the central portion of the molecule furnishes the first indirect support of the previously postulated pathway operating in the oxidative pentacyclization of the isoprenoid substrate. A bidirectional double oxidative bis-cyclization is invoked to explain the formation of this compound. The isolated substance was successfully subjected to a double rearrangement-ring expansion to give a novel bis-THF-bis-THP compound.     

How strong can the bend be on a DNA helix from cisplatin? DFT and MP2 quantum chemical calculations of cisplatin-bridged DNA purine bases

The B3LYP/6-31G(d) level of theory was used for the optimization of [Pt(NH(3))(4)](2+), [Pt(NH(3))(3)(H(2)O)](2+), cis-[Pt(NH(3))(2)(H(2)O)(2)](2+), and related platinum complexes. In addition, water or ammonium ligands were replaced by DNA purine bases so that finally cis-diammineplatinum with two bases (Pt-bridged complexes) is obtained. Single point calculations using the MP2/6-31+G(d) method were performed on the obtained reference geometries and were utilized for estimating bond dissociation energies (BDEs) and stabilization energies, and for electron density analyses. After reoptimization, IR spectra were determined from HF second derivatives. It was found that replacement of both water and ammonium by the DNA base is an exothermic process (20-50 kcal/mol depending on the ligands present in the complex). Asymmetric structures with one interbase H-bond were obtained for cis-diammine[bond](N(7),N(7)'-diadenine)[bond]platinum and mixed cis-diammine[bond](N(7)-adenine)[bond](N(7)-guanine)[bond]platinum complexes. In the case of the diguanine Pt-bridge, a symmetrical complex with two ammonium...O(6) H-bonds was found. The higher stabilization energy of the di-guanine complex is linked to a larger component of the Coulombic interaction. However, the BDE of Pt[bond]N(7)(G) is smaller in this complex than the BDE of Pt[bond]N(7)(G) from the mixed Pt[bond]AG complex. Also, steric repulsion of the ligands is about 10 kcal/mol smaller for the asymmetrical Pt[bond]AA and Pt[bond]AG bridges. The influence of the trans effect on DBE can be clearly seen. Adenine exhibits the largest trans effect, followed by guanine, ammonium, and water. The strength of the H-bond can be determined from the IR spectra. The strongest H-bond is the interbase H-bridge between adenine and guanine in the mixed Pt[bond]AG complex; otherwise, the H-bonds of adenine complexes are weaker than in guanine complexes. BDE can be traced in the guanine-containing complexes. The nature of the covalent bonding is analyzed in terms of partial charges and MO. A general explanation of the lower affinity of transition metals to oxygen than nitrogen can be partially seen in the less favorable geometrical orientation of lone electron pairs of oxygen.     

Is a catalyst always beneficial in plasma catalysis? Insights from the many physical and chemical interactions

Plasma-catalytic dry reforming of CH₄(DRM) is promising to convert the greenhouse gasses CH₄ and CO₂ into value-added chemicals, thus simultaneously providing an alternative to fossil resources as feedstock for the chemical industry. However, while many experiments have been dedicated to plasma-catalytic DRM, there is no consensus yet in literature on the optimal choice of catalyst for targeted products,because the underlying mechanisms are far from understood. I...     

Information on the location of carbon-carbon double bonds in C6−C23 linear alkenes from carbon addition reactions in a quadrupole ion trap equipped with a pulsed sample-inlet system

Ion-molecule reactions of a number of alkene molecular ions with different neutral alkenes were studied in a quadrupole ion trap equipped with a pulsed sample-inlet system. The molecules studied include several isomeric unbranched hexenes, heptenes, octenes, and nonenes, as well as representative alkenes with ten, twelve, fourteen, and twenty-three carbon atoms. Transfer of structurally characteristic number of methylene units between the ionic and neutral reactants dominates the product distributions for all the alkenes studied, with the exception of 1-alkenes. Isomeric alkenes can be readily distinguished on the basis of their products from reactions with neutral alkenes. It is suggested that distonic intermediates are generated in these reactions, and that they fragment by alkene elimination after 1,2- and 1,5-hydride shifts. The ability to vary the reaction time, pressure of the neutral reagents, and the type of ions and neutral molecules present in the reaction chamber during each stage of the experiment sequence makes it possible to maximize the amount of structural information obtained for alkenes in these experiments. Use of CS ₂ ^+? to generate the alkene molecular ion by charge exchange yields the same information without the need to carry out a mass-selection step for the ionized alkene.     

β-Cyclodextrin as a supramolecular catalyst for the synthesis of 2Hindazolo[2,1-b]phthalazine-trione derivatives in water and their antimicrobial activities

An efficient and green method has been developed for the synthesis of 2H-indazolo[2,1-b]phthalazinetriones derivatives by employing 15 mol%β-cyclodextrinvia a one-pot multicomponent reaction of aldehyde,dimedone,hydrazine hydrate with succinic anhydride/phthalic anhydride in water at 80 ℃ for first time.The catalyst could be recovered and reused for four consecutive cycles without appreciable loss in catalytic activity and evaluated for in vitro antimicrobial activity against different Gram-positive and Gram-negative bacterial strains.The outcome of the screening study showed that compound 6d,6f and7 n exhibited excellent activity against E.coil.Whereas,compound 6f and 6h exhibited excellent activity against P.aeurginosa,and compound 6c,and 6e displayed again excellent activity against Staphylococcus aureus whereas compound 7o shows excellent activity against S.aureus and B.subtilis when compared with Ampicillin(standard control).The results indicated that maximum compounds are moderately effective against bacterial growth and their effectiveness is highest against standard drugs.     

Determination of Cadmium,Copper, Lead,and Zinc in Pilchard Sardines from the Bay of Boumerdés by Atomic Absorption Spectrometry

Fish are an important human food with increasing consumption in recent decades. Diet is the main route of exposure to heavy metals for human health. For this reason, many studies have been performed on the pollution by heavy metals in different species of fish. Our main objective was to evaluate the pollution by trace metals (Zn, Cu, Pb, and Cd) in sardine muscles collected from the bay of Boumerdés (Algeria). Flame atomic absorption spectroscopy (AAS) was used for the quantification of Cu and Zn. The quantification of Cd and Pb was achieved by graphite furnace (GF) AAS. The analysis revealed the presence of metal trace elements in the muscles of species at concentrations below the thresholds established by national and international regulations.     

Development of a novel quality by design–enabled stability-indicating HPLC method and its validation for the quantification of nirmatrelvir in bulk and pharmaceutical dosage forms

A systematic and novel quality by design–enabled, rapid, simple, and economic stability–indicating HPLC method for quantifying nirmatrelvir (NMT) was successfully developed and validated. An analytical target profile (ATP) was established, and critical analytical attributes (CAAs) were allocated to meet the ATP requirements. The method used chromatographic separation using a Purosphere column with a 4.6 mm inner diameter × 250 mm (2.5 μm). The analysis occurred at 50°C with a flow rate of 1.2 mL/min and detection at 220 nm. A 10 μL sample was injected, and the mobile phase consisted of two components: mobile phase A, containing 0.1% formic acid in water (20%), and mobile phase B, containing 0.1% formic acid in acetonitrile (80%). The diluent was prepared by mixing acetonitrile and water at a 90:10 v/v ratio. The retention time for the analyte was determined to be 2.78 min. Accuracy exceeded 99%, and the correlation coefficient was greater than 0.999. The validated HPLC method was characterized as precise, accurate, and robust. Significantly, NMT was found to be susceptible to alkaline, acidic, and peroxide conditions during forced degradation testing. The stability-indicating method developed effectively separated the degradation products formed during stress testing, underlining its effectiveness in stability testing and offering accuracy, reliability, and sensitivity in determining NMT.     

Stepwise Self-assembly of a Tripeptide from Molecular Dimers to Supramolecular β-sheets in Crystals and Amyloid-like Fibrils in the Solid State

The terminally protected tripeptide Boc–Ala(1)–Leu(2)–Ala(3)–OMe 1 forms antiparallel hydrogen-bonded dimers of two different conformers in the asymmetric unit and the individual dimers then self-associate to form supramolecular β-sheet structures in crystals and amyloid-like fibrils in the solid state.     

Asymmetric syntheses of α-amino acids via double chiral induction in alkylation of the ketimine derived from 2-hydroxypinan-3-one and menthyl glycinate

Asymmetric syntheses of (S)-α-amino acids in 28--98% optical yields via doublechiral induction in alkylations of ketimine 1 derived from (+)-2-hydroxypinan-3-one and (-)-men-thyl glycinate which is a chiral match pair have been studied. The factors controlling the diastereoselec-tivities in alkylation reactions of the ketimine, the properties of alkylating agents and various alkylationconditions are examined.