Synthesis and UV photoelectron investigation of the “peptide units” of α,β-dehydroamino acids

The He(I) and He(II) photoelectron (PE) spectra of the N-methylamides of N-acetyldehydroalanine (1) and N-acetyldehydrophenylalanine (2) are presented and discussed, within a comprehensive investigation of the electronic structure and conformation of, -dehydroamino acid derivatives. Three different procedures have been adopted for the synthesis of1. The PE results of the peptide units are in tune with theoretical predictions and they support the view that in the vapor phase the tendency towards a planar arrangement around the torsion angle is much more marked for N-acetyldehydroalanine-NHR systems than for (Z)-substituted ones.     

The Possibility of the Existence of a Positive Leslie Viscosity α2. Proposed Flow Behavior of Disk Like Nematic Liquid Crystals

The signs of the Leslie viscosities α₂ and α₃ for nematics consisting of disk like molecules are discussed. It is predicted on theoretical grounds that for disk like molecules α₃ is always positive while α₂ should be able to exhibit both positive and negative values. This is the first time that the possibility of a positive α₂ is discussed in the literature. Provided that the prediction is correct, the nature of the flow behavior of disk like molecules is discussed.     

Crystal structure of the meta hydroxyl phenyl analog of α(+)-prodine

The meta hydroxyphenyl analog of -prodine (1,3-dimethyl-(4-meta-hydroxy phenyl)-4-propionyloxypiperidine) free base crystallizes in the orthorhombic space group P2₁2₁2₁. The relative configuration of the compound shows the piperidine ring is in a chair conformation, the phenyl ring, the 3-methyl, and the N-methyl are equatorial, and the 4-propionyloxy group is axial. The molecular structure is similar to that of racemic -prodine and the potent opioid agonist, ketobemidone. There is a hydrogen bond between N(1) ... O(1) 2.79 Å.     

Crystal structure of the low-temperature modification of α-RbB3O5

The crystal structure of α-RbB₃O₅ was refined by the Rietveld method with due regard for anisotropic vibrations of rubidium atoms to R _p = 2.93, R _wp = 3.80, R _B = 2.53, R _F = 2.84, and s = 1.54. The compound is isostructural to CsB₃O₅: it is orthorhombic, sp. gr. P2₁2₁2₁, a = 8.209(1), b = 10.092(1), c = 5.382(1) Å, and V = 445.9 Å 3. The framework structure is formed by the boron-oxygen [B ₂ ^III B^IVO₅] ^? rings consisting of two [BO₃]-triangles and a [BO₄]-tetrahedron. The rings are linked to form systems of helical chains running along the twofold screw axes parallel 2₁ to the a-and b-axes and infinite channels parallel to the a-and c-axes, which accommodate Rb atoms. The data were collected on an ADP-2 diffractometer [CuK _α radiation, Ni-filter, 12.00° < 2θ < 110.00°, a step in 2θ equal to 0.02°, count time 8 s per step, and 711 reflections α₁ + α₂)]. All the calculations were performed using version 3.3 of the WYRIET program. The comparison of the structures of α-and β-RbB₃O₅ and CsB₃O₅ revealed that the type of deformations in the framework structures of alkali-metal borates due to the changes of the temperature or the substitution of cations is determined by the role played by metal atoms, and especially, by large and heavy ions.     

The Preparation and Properties of the α,ω-bis(4,4′-Cyanobiphenyloxy)Alkanes: Nematogenic Molecules with a Flexible Core

The first twelve members of the homologous series of α,ω-bis(4,4′-cyanobiphenyloxy) alkanes have been synthesised. The compounds are nematogenic, although the mesophases for the first and third members of the series are monotropic. Both the nematic-isotropic transition temperature and the entropy of transition exhibit a pronounced dependence on the length of the flexible core; this is analogous to that found for main chain thermotropic liquid crystal polymers.     

Influence of intermolecular hydrogen bonds on the luminescence properties of α-substituted cinnamonitriles

In view of the dramatic difference in the spectral-luminescence properties of α-(p-chlorobenzoyl)-4-diethylaminocinnamonitrile and α-ethoxycarbonyl-4-diethylaminocinnamonitrile in solutions and in the crystalline state, X-ray diffraction analysis has been applied to study crystals of these compounds. The intermolecular C-H...N and C-H...O hydrogen bonds are found to contribute to the quinoidization of molecules, which leads to a bathochromic shift in the absorption and fluorescence spectra. A spectral-luminescence study of the aforementioned compounds has revealed that the solvent temperature and polarity affect the position of absorption and luminescence peaks: a decrease in these parameters causes a hypsochromic shift.     

Peculiarities of the microstructure of a nanoscale modification of η-TiO2

Samples with nanoscale η-TiO₂ phase have been obtained by sulfate and modified sulfate methods and are characterized by a complex of techniques: X-ray diffraction, electron diffraction, small-angle X-ray scattering (SAXS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy, and X-ray energy-dispersive analysis. The effect of sample formation conditions on the size and shape of crystallites, content of the amorphous component in the samples, and their elemental composition has been established. A significant change (depending on the synthesis conditions) in the parameters of the diffraction reflection with d ～ 17–21 Å (intensity and interplanar spacing d, Å), pronounced for η-TiO₂, is revealed. This change is most likely related to the variation in the content of water molecules in the interlayer space of η-TiO₂ structure and/or the change in crystallite shape.     

Elastic, piezoelectric, and dielectric properties of α-GeO2 single crystals

The elastic, piezoelectric, and dielectric characteristics of α-GeO₂ crystals are studied at room temperature. The measurements were performed by the resonance method on crystals grown on α-quartz seeds from solutions in the recirculating mode. In germanium dioxide crystals, the piezoelectric moduli have high values. The characteristic features of variations of the electromechanical and elastic properties in crystals with the quartzlike structure observed earlier are confirmed.     

Supramolecular Complexes of Tetrapeptides Capable of Inducing the Human α-Lactalbumin β-Domain Conformational Transitions

Crystallography Reports - It is shown using small-angle X-ray scattering (SAXS) that the tetrapeptide, which is an inductor of fibrillogenesis for a fragment of α-human lactalbumin...     

Structure of a new crystal modification of the bacterial NAD-dependent formate dehydrogenase with a resolution of 2.1 Å

Formate dehydrogenase (FDG) from methylotrophic bacteria Pseudomonas sp. 101 catalyzes the reaction of oxidation of the formate ion to carbon dioxide, which is accompanied by the reduction of nicotinamid adenine dinucleotide (NAD⁺). The structures of the apo and holo (enzyme-NAD-azide triple complex) forms of the enzyme were determined earlier. In an attempt to prepare a complex of FDG with the product of the enzymatic reaction (NADH), a new crystal modification of FDG is obtained (space group P4₂2₁2, a = b = 93.3 Å, c = 103.05 Å). The FDG structure is solved by the molecular replacement method and refined to R = 20.7%. The asymmetric part of the unit cell contains one FDG molecule. In contrast to the previously studied FDG structures, the biologically active dimer is formed by the crystallographic rotation axis. A comparative structural analysis of the studied form with the apo and holo forms of the enzyme is performed. The influence of the molecular structure on the environment in the crystal is investigated.     

Order Parameters of α-Substituted Anthraquinone Dyes in a Nematic Liquid Crystalline Host

A study is presented of the optical order parameters and spectroscopic properties of α-substituted anthraquinone dyes in a nematic liquid crystalline mixture composed of cyanophenylcyclohexane derivatives. Then, the results of this study are discussed in terms of the effects of the dye structure on the order parameters. From the systematic variation in structure, the order parameters are found to be improved when a biphenylamino group is introduced into the α-position of the anthraquinone nucleus. In addition, preliminary data on the solubility and photostability of the dyes are reported.     

Temperature variation of effective geometry parameter, αg,refractive indices and the determination of orientational orders in three room temperature nematic liquid crystal mixtures

Orientational order parameter, S and effective geometry, α_g are calculated from the refractive index in three nematic room temperature liquid crystal mixtures, blOO3, blOO6, and blO38. The method due to Kuczynski et al. used to determine S from birefringence. The effective geometry parameter is α_g = n_o/n_e. (n_o and n_e are the ordinary and extraordinary refractive indices of the liquid crystal material.) The behavior of α_g as a function of temperature and order parameter has also been reported. The S value calculated from α_g is identically equal to the value obtained from Kuczynski et al method, i.e., from birefringence, δn. Finally, dn_e/dT and dn_o/dT are obtained and the crossover temperatures, T_CO the characteristic temperature, which is in fact the temperature of minimum in the ordinary index of refraction in three mixtures for different wavelengths are calculated from dn_o/dT.     

Synthesis,Crystal Structure,Antioxidant, and α-Glucosidase Inhibitory Activities of Methoxy-substituted Benzohydrazide Derivatives

Eight methoxy substituted at the benzylidene moiety benzohydrazide derivatives [R = 2-OCH₃ (1), 3-OCH₃ (2), 4-OCH₃ (3), 2,3-(OCH₃)₂ (4), 3,4-(OCH₃)₂ (5), 2,4,5-(OCH₃)₃ (6), 2,4,6-(OCH₃)₃ (7), and 3,4,5-(OCH₃)₃ (8)] were synthesized and characterized by ¹H NMR, FT-IR and UV-Vis spectroscopy. The crystal structure of 4 was determined by single crystal X-ray diffraction (sp. gr. Pbca, Z = 8). The molecule is slightly twisted with the dihedral angle between the two phenyl rings being 9.33(14)°. The methoxy group at the ortho position is twisted [C–O–C–C angle is–109.2(3)°] whereas the other at meta position is co-planar with the attached benzene ring. In the crystal packing, the molecules are linked into two-dimensional network parallel to the (001) plane by O–H···O, O–H···N, and N–H···O hydrogen bonds. Compounds 1–8 were evaluated for an antioxidant and α-glucosidase inhibitory activities and the results suggested that the ?OCH₃ substituent was ineffective for bioactivity enhancement.     

Crystal structure of 14, α-hydroxy,sandarocopimar-7, 15-diene-19-oic acid

C₂₀H₃₀O₃, M _r =318.46, Monoclinic,P2₁,a=12.232(2) ,b=11.491(2) ,c=14.160(3) , =115.2°(3),V=1800.3(7) ³,Z=4,D _x =1.18 Mg m^–3, (CuK)=1.5418 , =5.3 cm^–1,F(000)=696,T=298 K. FinalR=0.044 for 2645 reflections withI>2. The two crystallographically independent molecules in the unit cell adopt essentially similar conformations. The structure reveals atrans relationship between the hydrogen at C(9) and the methyl group at C(20). The stereochemistry at C(13) is S. Apart from van der Waals forces the structure is stabilized by hydrogen bonds.     

Synthesis and crystal structure of a palladium dimer containing ferrocenyldiphenylphosphine, trans-[Pd(μ-Cl)(CH3)(PPh2Fc)]2

The chloro-bridged dimeric complex, trans-[Pd(-Cl)(CH₃)(PPh₂Fc)]₂, was prepared by reaction of [Pd(Cl)(CH₃)(COD)] (COD = cis,cis-1,5-cyclooctadiene) with 1 equivalent of PPh₂Fc (PPh₂Fc = ferrocenyldiphenylphosphine) in acetone medium. The complex was characterized by ¹H and ³¹P NMR spectroscopy and a single crystal X-ray diffraction study revealed a trans geometry. The compound crystallizes in the triclinic space group P1, a = 11.476(2) Å, b = 14.059(3) Å, c = 14.658(3) Å and = 82.34(3)°, = 75.09(3)°, and = 68.48(3)° with Z = 2. Most important geometrical parameters include bond lengths Pd(1)–P(1) 2.2165(10), Pd(2)–P(2) 2.2290(10), Pd(1)–C(1) 2.157(3), Pd(2)–C(2) 2.079(4), Pd(1)–Cl(1) 2.4329(13), Pd(1)–Cl(2) 2.4106(11), Pd(2)–Cl(1) 2.4169(10), and Pd(2)–Cl(2) 2.4414(13) Å and bond angles P(1)–Pd(1)–C(1) 87.47(8), P(1)–Pd(1)–Cl(2) 176.88(4), C(1)–Pd(1)–Cl(1) 173.93(7), P(2)–Pd(2)–C(2) 89.03(11), P(2)–Pd(2)–Cl(1) 174.18(4), and C(2)–Pd(2)–Cl(2) 173.50(11)°. The effective and Tolman cone angles for P(1) were calculated as 170 and 167° and that for P(2) as 179 and 177°, respectively.     

First crystal structure of a pentacyclic biomarker: 4,4,6a,12b,14b-Pentamethyl-1,2,3,4,4a,5,6,6a,6b,7, 8,12b,13,14,14a,14b-hexadecahydropicen-3β-ol

The crystal structure of the title compound (1, C₂₇H₄₀O 1/8 H₂O) synthesized via photo-induced electron transfer promoted biomimetic cascade cyclization, is reported. Compound 1 crystallizes in the monoclinic space group C2/c, with cell parameters a = 53.982(4), b = 7.1889(6), c = 11.170(1) Å, = 96.86(1). The structure adopts a fully trans-fused ring junction with all methyl groups axially oriented. The geometrical aspects associated with 1 were discussed and compared with the previously determined X-ray data for structurally analogous compounds.     

The crystal and molecular structure of one of the possible conformers of “the smallest piece of ice”—A solid-state isolation of a hydrogen bonded water hexamer trapped within channels of a crystal of a dirhodium molecule

Reexamination of our study of [Rh₂(HNOCCH₃)₄(2H₂O)] 3H₂O (Ahsan, M.Q.; Bernal, I.; Bear, J.L. Inorg. Chem. 1986, 26 260) showed it to be interesting not just because the dirhodium molecule is an antineoplastic but because it contains a hexameric cluster of waters trapped in Rh–Rh lattice cavities. It may well provide an interesting model for the smallest piece of ice (Nauta, K.; Miller, R.E. Science 2000, 287, 293).     

What is the Shape of a Polymer Chain near the Theta Point?

We address the question of the conformation of a polymer near the θ point. To this end, we present an argument that the statistics of polymer rings at the θ point in two dimensions is exactly given by the statistics of the external perimeter (“hull”) of a percolation cluster. As a consequence, the fractal dimension d_f (θ) of a polymer chain at the θ point coincides with that of the hull of the percolating cluster, d_f (θ) - d_H . We also perform extensive simulations of the conventional θ point model—the interacting self-avoiding walk (ISAW)—and the smart kinetic walk (SKW). We demonstrate that the SKW predicts a higher order critical point, termed the θ′ point, which has the same critical behavior as the ISA W. We conclude that the SKW is a well-defined walk that gives the conformation of a polymer near the θ point in two dimensions. In particular, we report accurate calculations of all three tricritical point exponents that are needed to fully describe the θ point.