Showing papers in "Structural Chemistry in 2009"

Pectin, a versatile polysaccharide present in plant cell walls

Abstract: Pectin or pectic substances are collective names for a group of closely associated polysaccharides present in plant cell walls where they contribute to complex physiological processes like cell growth and cell differentiation and so determine the integrity and rigidity of plant tissue. They also play an important role in the defence mechanisms against plant pathogens and wounding. As constituents of plant cell walls and due to their anionic nature, pectic polysaccharides are considered to be involved in the regulation of ion transport, the porosity of the walls and in this way in the control of the permeability of the walls for enzymes. They also determine the water holding capacity. The amount and composition of pectic molecules in fruits and vegetables and other plant produce strongly determine quality parameters of fresh and processed food products. Pectin is also extracted from suitable agro-by-products like citrus peel and apple pomace and used in the food industry as natural ingredients for their gelling, thickening, and stabilizing properties. Some pectins gain more and more interest for their health modulating activities. Endogenous as well as exogenous enzymes play an important role in determining the pectic structures present in plant tissue, food products, or ingredients at a given time. In this paper functional and structural characteristics of pectin are described with special emphasis on the structural elements making up the pectin molecule, their interconnections and present models which envisage the accommodation of all structural elements in a macromolecule. Attention is also given to analytical methods to study the pectin structure including the use of enzymes as analytical tools.

Cell wall polysaccharides in cereals: chemical structures and functional properties

Abstract: β-Glucans and arabinoxylans are the two primary cell wall structural components in cereals, such as wheat, oat, barley and rye. The relative amounts of the two polysaccharides vary with species and growing environments. The cell walls of barley and oats are generally rich in β-glucan, whereas rye and wheat cell walls contain higher levels of arabinoxylans. Cereal β-glucan is a mix linked (1 → 3) (1 → 4)-β-d-glucan composed of two major building blocks: a trisaccharide and a tetrasaccharide unit: the combination of the two units is over 90%. The ratio of the two building blocks is used as a fingerprint for each β-glucans: it is 4.5, 3.3, 2.2 for wheat, barley and oat β-glucan, respectively. Of the two types of cell wall polysaccharides in cereals, β-glucan received greater attention due to its proved beneficial physiological effect as an excellent source of soluble dietary fibre for significantly attenuating blood glucose and insulin levels and its demonstrated ability to reduce low-density lipoprotein cholesterols (LDL) in serum. The ability of cereal β-glucan to attenuating blood glucose, insulin levels is linked to the viscosity produced by cereal β-glucans in a linear relationship. It is also demonstrated that the functional properties of cereal β-glucans are determined by their structural features and molecular weight: a higher trisaccharide to tetrasaccharide ratio favours gel formation and faster gelation process, and ultimately, gives stronger gels. Cereal β-glucan also demonstrated an unusual behaviour by forming gel faster and yielding stronger gels at lower molecular weight (above minimum gelation molecular weight). Such a structure–function relationship was established based on rheological, light scattering and computer modelling studies which cover both dilute and concentrated concentration regimes: high tri/tetra ratio gives high proportion of consecutive trisaccharide unit which favours the intermolecular association of β-glucan chains; on the other hand, low molecular weight chains have higher mobility that promotes intermolecular chain–chain interactions, hence, lead to faster gelation process and formation of stronger gels. Research also revealed that processing and storage conditions, such as temperature, pH, extrusion, baking and frozen before eating, have significant effects on the bioavailability of cereal β-glucans, hence, its ability to reduce blood glucose and cholesterol levels.

Molecular structure and reactivity of antituberculosis drug molecules isoniazid, pyrazinamide, and 2-methylheptylisonicotinate: a density functional approach

Abstract: Density functional theory calculations have been performed to determine structure and reactivity of two of the most commonly used antitubercular drug molecules, isoniazid and pyrazinamide. Having found good agreement with the available experimental data for these two compounds, we extended DFT calculation to predict the properties of 2-methylheptylisonicotinate which is a novel natural analogue of isoniazid. The chemical reactivity of these compounds was compared using density functional-based descriptors such as global softness, global electrophilicity, Fukui function, and philicity. The experimental better reactivity of 2-methylheptylisonicotinate with respect to the well-known antituberculosis drug molecule, isoniazid has been successfully established.

Intermetallic compounds of the NaCd2 family perceived as assemblies of nanoclusters

Abstract: In this work, we proceed with the concept of developing nanoclusters (fundamental configurations), which are the basic building blocks of certain crystal structures, allowed us to propose a universal algorithm for crystal self-assembly structure, and hence to elaborate a uniform approach to the analysis of the essentially hierarchic structure of natural objects under the nanoscale. It should be noted that we use the conception, where the configuration of points on the single sphere that minimize potential energy for broad class of potential functions (with the characteristic quadric function of potential versus distance between points) has been studied. The model is not connected both with the undefined isolated from structure nets (like diamond net) and polyhedra (like Friauf polyhedra), which are the basis of clusters, but is hierarchically constructed on local polyhedral structure’s searching according to universal algorithm. Crystal structure can explicitly store the information on the structure of the particles that formed it. These particles, geometrically distorted in some extent, can remain in parts of fundamental nanoclusters, coherently grow into them, and emerge as clusters-centaurs.

A computational study of the cooperativity in clusters of interhalogen derivatives

Abstract: The clusters, up to four monomers, of the interhalogen derivatives (FCl, FBr, and ClBr) have been studied by means of ab initio and DFT methods, up to MP2/aug-cc-pVTZ computational methods Two dispositions, linear and cyclic, of the clusters have been studied Cooperative effects in the geometry, energy, and electron density have been observed in the linear and cyclic dispositions of these clusters The Natural Energy Decomposition Analysis shows that the main source of the interaction corresponds to the polarization term

Molecular associations in acacia gums

Abstract: The tendency of polysaccharides to associate in aqueous solution has long been recognised. Molecular associations can profoundly affect their performance in a given application due to its influence on the molecular weight, shape and size. This will ultimately determine how the molecules will interact with each other, with other molecules and with water. There are several factors, such as hydrogen bonding, hydrophobic association, ion mediated association, electrostatic interaction, concentration dependence and the presence of proteinaceous components, which affect this behaviour. Our objective is to highlight the role of the proteinaceous component, present in acacia gum, to promote associations when the gum is subjected to various processing treatments such as maturation, spray drying and irradiation. The results demonstrate the ability of the proteinaceous component to promote hydrophobic associations which influence the size and proportion of the arabinogalactan high molecular weight component (AGP). Heat treatment in solid state (maturation) increases the hydrophobic character of the gum and hence its emulsification performance. Spray drying also involves aggregation through hydrophobic association but changes the surface properties of peptide moieties to become more hydrophilic compared to the association promoted by the maturation treatment in the solid state. Irradiation induced cross-linking, in the presence of unsaturated gas, was used to introduce C–C bonds into the carbohydrate moiety and thus confirms the hydrophobic association prompted by the heat used in the maturation and spray drying. This association can be reversed by treatments, such as filtration or high pressure homogenisation. The results reported here reconcile the contradiction about structure of gum arabic proposed by the wattle-blossom and twisted hairy rope models and shows that the AGP fraction is basically an aggregated fraction made up of AG units stabilized by low molecular weight highly proteinaceous components.

Paradoxes and paradigms: why is quinoline less basic than pyridine or isoquinoline? A classical organic chemical perspective

Abstract: An explanation for the reported lower basicity of quinoline as compared with pyridine or isoquinoline has been provided. The competing roles of steric hindrance and solvation effects are discussed along with resonance stabilization of neutral molecules and related ions.

Geometries and stabilities of various configurations of benzene dimer: details of novel V-shaped structure revealed

Abstract: Benzene dimer configurations namely T-shaped, parallel-displaced, sandwich, and V-shaped that were proposed by experimental studies are investigated using second- and fourth-order Moller–Plesset perturbation theory. The MP2 method with aug-cc-pVDZ and aug-cc-pVTZ basis sets unequivocally shows that the parallel-displaced configuration is considerably more stable than T-shaped structure. On the other hand, the MP4(SDTQ)/aug-cc-pVDZ level predicts that the T-shaped and parallel-displaced configurations are nearly isoenergetic, which is parallel to the previous results of estimated CCSD(T)/CBS level reported recently. The lowest energy T-shaped configuration is stabilized by 0.17 kcal/mol over the parallel-displaced configuration at the MP4(SDTQ)/aug-cc-pVDZ level. Although the structures of all the four different types of configurations are found to be stable at both MP2 and full MP4 methods, the V-shaped configuration is the least stable among them. The calculated interaction energy of −2.3 kcal/mol for the lowest energy T-shaped structure at the MP4(SDTQ)/aug-cc-pVDZ level is in good agreement with the experimental value of −2.4 ± 0.4 kcal/mol. We conclude that the MP4(SDTQ) with a reasonably good basis set can be used for systems involving π–π interactions to obtain qualitative and quantitative results.

Therapeutic use of hyaluronan

Abstract: Hyaluronan used today as a therapeutic agent in human and animal medicine must be in a highly purified form, free of immunologically active protein molecules, from endotoxin and from inflammatory molecules originating from the tissues or bacterial cultures—the source of hyaluronan. All hyaluronans, whether in liquid or in covalently crosslinked gel form, must have certain elastoviscous properties to be usable for current therapeutic applications. Elastoviscous properties are especially important in the ophthalmic viscosurgical use. The metabolism of hyaluronan is very fast in most tissues, except in the vitreus. The ability of injected hyaluronan with a short half-life time in the joint to accomplish long-lasting analgesia represents a challenge for the design of the various products. The history of the therapeutic use of hyaluronan and its present status is described, with emphasis on its use in ophthalmic surgery and for a long-lasting analgesia in arthritic joints in humans and animals. The use of hyaluronan gels is described in viscoaugmentation as injected into the dermal layer of the skin. Hyaluronan gels were also used as viscosupplements injected into sphincter muscles to improve their function in aging or disease. The use of hyaluronan and its gel for drug delivery was suggested several decades ago, and is also mentioned.

Effects of different GIAO and CSGT models and basis sets on 2-aryl-1,3,4-oxadiazole derivatives

Abstract: The direct molecular structure implementations of the gage-including atomic orbital (GIAO), individual gages for atoms in molecules (IGAIM) and continuous set of gage transformations (CSGT) methods for calculating nuclear magnetic shielding tensors at both the Hartree-Fock (HF) and density functional (B3LYP) levels of theory with 6-31G(d), 6-311G(d), 6-31++G(d,p), 6-311++G(d,p), and 6-311++G(df,pd) basis sets are presented. Dependence on the 1H and 13C NMR chemical shifts on the choice of method and basis set have been investigated. Also, these chemical shifts of 2-aryl-1,3,4-oxadiazoles 5a–g have been performed related to dihedral angles (C4–C3–C2–O) of two conformers. The optimized molecular geometries and 1H and 13C chemical shift values of 2-aryl-1,3,4-oxadiazoles 5a–g in the ground state have been obtained. The linear correlation coefficients of 13C NMR chemical shifts for these molecules were given. The new nuclear magnetic shielding tensors of tetramethylsilane (TMS) were calculated. The data of 2-aryl-1,3,4-oxadiazole derivatives display significant molecular structure and NMR analysis. Also, these provide the basis for future design of efficient materials having the 1,3,4-oxadiazole core.

An examination of the vaporization enthalpies and vapor pressures of pyrazine, pyrimidine, pyridazine, and 1,3,5-triazine

Abstract: The vaporization enthalpies and liquid vapor pressures from T = 298.15 K to T = 400 K of 1,3,5-triazine, pyrazine, pyrimidine, and pyridazine using pyridines and pyrazines as standards have been measured by correlation-gas chromatography. The vaporization enthalpies of 1,3,5-triazine (38.8 ± 1.9 kJ mol−1) and pyrazine (40.5 ± 1.7 kJ mol−1) obtained by these correlations are in good agreement with current literature values. The value obtained for pyrimidine (41.0 ± 1.9 kJ mol−1) can be compared with a literature value of 50.0 kJ mol−1. Combined with the condensed phase enthalpy of formation in the literature, this results in a gas-phase enthalpy of formation, Δf H m (g, 298.15 K), of 187.6 ± 2.2 kJ mol−1 for pyrimidine, compared to a value of 195.1 ± 2.1 calculated for pyrazine. Vapor pressures also obtained by correlation are used to predict boiling temperatures (BT). Good agreement with experimental BT (±4.2 K) including results for pyrimidine is observed for most compounds with the exception of the pyridazines. The results suggest that compounds containing one or two nitrogen atoms in the ring are suitable standards for correlating various heterocyclic compounds provided the nitrogen atoms are isolated from each other by carbon. Pyridazines do not appear to be evaluated correctly using pyridines and pyrazines as standards.

Uncommon hydrogen bonds between a non-classical ethyl cation and π hydrocarbons: a preliminary study

Abstract: This study undertakes a theoretical investigation into uncommon hydrogen bonds between the ethyl cation (C2H5 +) and π hydrocarbons. Firstly, it considers the hyperconjugation effect of the ethyl cation, in which the non-localized hydrogen (H+) is taken to be a pseudoatom bound to the carbons of the methyl groups. The goal of the research is to use this electronic phenomenon to gain a better understanding of the (H+···π) and (H+···p-π) hydrogen bonds, which are considered uncommon because they are formed through the interaction of the H+ of the ethyl cation with the π bonds of the acetylene (C2H2) and ethene (C2H4), as well as with the pseudo-π bond of the cyclopropane (C3H6). In view of this, B3LYP/6-311++G(d,p) calculations were used to determine the geometries of the C2H5 +···C2H2, C2H5 +···C2H4, and C2H5 +···C3H6 hydrogen-bonded complexes. Deformations of the bond lengths and bond angles of these systems were analyzed geometrically. Examination of the stretch frequencies and absorption intensities of the (H+···π) and (H+···p-π) hydrogen bonds has revealed red-shifts in π and p-π bonds. After structural modeling and vibrational characterization, analysis of the charge transfer following the ChelpG approach and subsequently quantification of the hydrogen bond energies (basis sets superpostition error and zero point vibrational energies being considered) were used to predict the strength of the (H+···π) and (H+···p-π) hydrogen bonds. In addition, the molecular topography was estimated using the quantum theory of atoms in molecules (QTAIM). QTAIM was chosen because of a desire to understand the (H+···π) and (H+···p-π) hydrogen bonds chemically on the basis of the quantity of charge density and interpretation of Laplacian fields.

Three new coordination complexes of cobalt(III), manganese(II), and copper(II) with N,N,O-donor hydrazone ligands: syntheses and structural characterizations

Abstract: Three new coordination complexes, 2{[Co(L1)2]ClO4} · 0.5CH3OH (1), [Mn(L2)2] (2), and [Cu(HL2)(L2)]ClO4 · 2H2O (3) have been synthesized from two tridentate N,N,O-donor hydrazone ligands HL1, 2-acetylpyridine-salicyloylhydrazone, and HL2, 2-benzoylpyridine-salicyloylhydrazone, respectively and thoroughly characterized by elemental analysis, FT-IR, UV–Vis, electrochemical, and room temperature magnetic susceptibility measurements. Structures of the complexes have been unequivocally established by single crystal X-ray diffraction technique. Structural analysis reveals that 1 consists of two chemically similar but crystallographically independent cationic [Co(L1)2]+ units and 2 consists of a neutral [Mn(L2)2] molecule while 3 consists of a cationic [Cu(HL2)(L2)]+ unit. Metal ions display distorted octahedral geometry in 1 and 2 while in 3 it shows a distorted square pyramidal geometry. Ligand conformations around the metal ions are stabilized by the presence of intra-ligand hydrogen bonding in all the complexes. Structure of 3 reveals that a perchlorate ion linked to the complex by hydrogen bonding via a water molecule.

Density functional theory study on the –SO3H functionalized acidic ionic liquids

Abstract: In this work, the structures of the –SO3H functionalized acidic ionic liquid 1-(3-sulfonic acid) propyl-3-methylimidazolium hydrogen sulfate ([C3SO3Hmim]HSO4), including its precursor compound (zwitterion), cation, and cation–anion ion-pairs, were optimized systematically by the DFT theory at B3LYP/6-311++G** level, and their most stable geometries were obtained. The calculation results indicated that a great tendency to form strong intramolecular hydrogen bonds was present in the zwitterion, and this tendency was weakened in the cation that was the protonation product of zwitterion. The intramolecular hydrogen bonds and intermolecular hydrogen bonds coexisted in the ionic liquid, and they played an important role in the stability of the systems. The strongest interaction in the ionic liquid was found between the anion and the functional group. The transition state research and the intrinsic reaction coordinate analysis of the hydrogen transfer reaction showed that, when the cation and the anion interacted near the functional group by double O–H···O hydrogen bonds, the ionic liquid was inclined to exist in a form of the zwitterion and H2SO4.

Small heterocyclics as hydrogen bond acceptors and donors: the case of the C2H3XS···NH3 complexes (X = H, F and CH3)

Abstract: B3LYP/6-311++G(d,p) calculations were employed in order to examine the molecular parameters of the C2H3XS···NH3 heterocyclic hydrogen-bonded complexes with X = H, F and CH3. Intermolecular criteria were taken into account when studying the formation of these hydrogen-bonded complexes, such as geometry analysis, charge density quantification and interpretation of the harmonic vibrational spectrum, in which case the appearance of red-shift and blue-shift effects was discussed. It was assumed from the outset that many hydrogen bond types may exist in these systems, and these were investigated using the results of topological integrations from the quantum theory of atoms in molecules (QTAIM) and intermolecular charge transfer calculations using the ChelpG scheme. The proton donor/acceptor behavior of C2H3XS was interpreted in terms of hydrogen bond energies, whose values were corrected using the basis sets superposition error (BSSE) and zero point energy (ZPE).

Thermal behavior of trehalose dihydrate (Th) and β-anhydrous trehalose (Tβ) by in-situ laboratory parallel-beam X-ray powder diffraction

Abstract: Thermal behaviors of trehalose dihydrate (T h) and β-anhydrous trehalose (T β) have been investigated by in-situ laboratory parallel-beam X-ray powder diffraction. Data indicate that both phases show essentially the same volume expansion but expansion of the anhydrous form is markedly anisotropic due to the features of the hydrogen-bond network. Under the present experimental conditions, dehydration starts at 66 °C and within the 75 < T < 114 °C the presence of the T α anhydrous polymorphic form has been detected.

Interplay of thermochemistry and Structural Chemistry , the journal (volume 19, 2008) and the discipline

Abstract: In the current review paper the content of the journal Structural Chemistry for the calendar year 2008 is related to thermochemistry. Papers are summarized and a thermochemical slant added. Often questions are asked and research topics suggested.

A B3LYP and QTAIM study of a new proton donor for dihydrogen bonds: the case of the C2H5 +···nBeH2 complexes (n = 1 or 2)

Abstract: B3LYP/6-311++G(d,p) calculations and molecular integrations from the quantum theory of atoms in molecules (QTAIM) were performed for the purposes of studying a new class of dihydrogen-bonded hyperconjugation complexes formed by C2H5 +···n(BeH2), when n = 1 (bimolecular) or n = 2 (trimolecular). Whether bimolecular or trimolecular, when the hyperconjugation on the ethyl cation (C2H5 +) is taken into account, this enables the earth alkaline hydride, BeH2, to interact efficiently with the nonlocalized hydrogen (H+) of the C2H 5 . In addition to computation of QTAIM topological parameters, analysis of the infrared harmonic spectrum at the B3LYP/6-311++G(d,p) level of theory revealed the existence of red-shifts on BeH2, and this effect is explained by means of the atomic charges derived from the ChelpG approach.

Hydrogen binding property of Co- and Ni-based organometallic compounds

Abstract: The binding property of hydrogen on organometallic compounds consisting of Co, and Ni transition metal atoms bound to C m H m rings (m = 4, 5) is studied through density functional theory calculation. CoC m H m and NiC m H m complexes can store up to 3.49 wt% hydrogen with an average binding energy of about 1.3 eV. The adsorption characteristics of hydrogen to organometallic compounds are investigated by analyzing vibrational spectra of CoC4H4(H2) n and NiC4H4(H2) n (n = 0, 1, 2). The kinetic stability of these hydrogen-covered organometallic complexes is assured by analyzing the energy gap between the highest occupied molecular orbitals and the lowest unoccupied molecular orbitals. It is also discussed the application of 18-electron rule in predicting maximum number of hydrogen molecules that could be adsorbed by these organometallic compounds.

Interplay of thermochemistry and Structural Chemistry , the journal (volume 17, 2006) and the discipline

Abstract: In the current study we relate the contents of the journal of “Structural Chemistry” for the calendar year 2006 to thermochemistry. Each paper is briefly summarized and supplemented by a comment which explicitly interrelates the content of the study to chemical energetics.

Polyelectrolyte properties of a plant and animal polysaccharide

Abstract: The principal properties of polyelectrolytes in aqueous solution are reviewed; experimental data obtained about polysaccharides in our laboratory identify the main electrostatic interactions which occur between macromolecular chains, between ionic sites on isolated chains or between counterions and polyelectrolytes. Two examples are described. Hyaluronan, a linear alternated copolymer type with one anionic site per repeat unit, is very soluble in water, with a persistence length around 8 nm and a rheological behavior directly related to the molecular weight and polymer concentration in the presence of external monovalent electrolyte. The second example is that of pectin for which the influence of the charge density is studied. It exhibits an important ionic selectivity among divalent counterions in dilute solution with a dimer formation followed by aggregation and gelation.

Novel type of mixed O–H···N/O–H···π hydrogen bonds: monohydrate of pyridine

Abstract: Investigation of characteristics of hydrogen bonding between pyridine and water by MP2/aug-cc-pvdz method reveals that these two molecules may form three types of hydrogen bonds depending on nature of proton withdrawal site of pyridine. Change of orientation of water with respect to plane of aromatic ring leads to transformation of the O–H···N bond to O–H···π bond via wide region of the potential energy surface where both lone pair of the nitrogen atom and π-system make significant contribution into hydrogen bonding. Hydrogen bond in this intermediate region may be considered as mixed O–H···N/O–H···π bond representing new type of H bonds.

Quantum Chemical Analysis of the Energetics of the anti- and gauche- Conformers of Ethanol

Abstract: Ethanol displays two stable conformers, the classic anti (or trans) form and a gauche conformation in which the hydroxyl hydrogen points toward one of the methyl hydrogens. Surprisingly, the two forms have nearly equal energies, and in the vapor phase the gauche form predominates because of its twofold degeneracy. An analysis of the energetics of these conformers based on natural bond orbital analysis helps to explain the apparently anomalous near degeneracy of these conformers.

Synthesis, characterization and crystal structures of the tetrachlorocuprate and tetrabromocadmate salts of the antimalarial mefloquine

Abstract: Two new compounds, bis (DL-erythro-α-2-piperydylium-2,8-bis(trifluoromethyl)-4-quinolinemethanol) tetrachlorocuprate(II) tetrahydrate [LH+]2[CuCl4]2−∙4H2O 1 [L = mefloquine] and bis(DL-erythro-α-2-piperydylium-2,8-bis(trifluoromethyl)-4-quinolinemethanol) tetrabromocadmate (II) bis(methanol) [LH+]2[CdBr4]2−·2CH3OH 2, have been synthesized and characterized by elemental analysis, 1H-NMR and IR spectroscopy. Single-crystal X-ray diffraction analysis of 1 showed the structure to be ionic with formula unit comprising two protonated monocationic mefloquine molecules of opposite chirality, a tetrachlorocuprate (II) anion and a complement of four water molecules, disordered over several sites. The crystals are orthorhombic, space group Pnma, a = 9.6975(1) A, b = 29.5385(3) A, c = 15.9423(1) A, V = 4566.67(7) A3, Z = 4. The formula unit of compound 2 comprises two protonated monocationic mefloquine molecules, a tetrabromocadmate(II) anion and two molecules of methanol. The crystals are orthorhombic, space group Fdd2, a = 17.2185(5) A, b = 55.456(2) A, c = 9.5140(3) A, V = 9084.6(5) A3, Z = 8. The mefloquine molecule is protonated at the piperidinyl N atom and extensive hydrogen bonding occurs in both crystal structures. The conformation of the mefloquine cation in 1 and 2 is very similar to that recently observed in other salts of the drug, confirming its relevance in the context of antimalarial action.

Natural bond orbital analysis of some para -substituted N -nitrosoacetanilide biological molecules

Abstract: Theoretical study of several para-substituted N-nitrosoacetanilide biological molecules has been performed using density functional B3LYP method with 6-31G(d,p) basis set. Geometries obtained from DFT calculation were used to perform natural bond orbital analysis. The p characters of two nitrogen natural hybrid orbital (NHO) σ N3–N2 bond orbitals increase with increasing σ p values of the para substituent group on the benzene, which results in a lengthening of the N3–N2 bond. The p characters of oxygen NHO σ O1–N2 and nitrogen NHO σ O1–N2 bond orbitals decrease with increasing σ p values of the para substituent group on the benzene, which results in a shortening of the N2=O1 bond. It is also noted that decreased occupancy of the localized σ N3–N2 orbital in the idealized Lewis structure, or increased occupancy of $$ \sigma_{\rm N3-N2}^{\ast}$$ of the non-Lewis orbital, and their subsequent impact on molecular stability and geometry (bond lengths) are also related with the resulting p character of the corresponding nitrogen NHO of σ N3–N2 bond orbital.

Synthesis, characterization and spectra studies on Zn(II) and Cu(II) complexes with thiocarbamide ligand containing Schiff base group

Abstract: The ligand of 4-phenyl-1-(1-phenylethylidene)thiosemicarbazide (HL) and its metal complexes of ZnL2 (1) and CuL2 (2) have been synthesized. Elemental analysis, IR and X-ray single crystal diffraction characterizations for 1 and 2 have been carried out. In the complexes 1 and 2, the central metallic ions Zn2+ and Cu2+, coordinate with two deprotonated ligands, L−, respectively and adopt distorted tetrahedral geometries. The thermal analyses result shows that the two complexes undergo two similar decomposition processes because of their similar geometric configurations. For the two complexes, UV–Vis spectra have been measured and DFT calculations at B3LYP/LANL2DZ level of theory prove that the electronic spectra of 1 are corresponding with intraligand electronic transitions, and the electronic spectra of 2 are attributed to intraligand electronic transitions as well as d–d electronic transitions. Fluorescence spectra measurements indicate that the complex of 1 emits stronger fluorescence than the ligand of HL, and the complex 2 does not emit fluorescence at all. Electrochemical analyses reveal that both the oxidation peak currents and the reduction peak currents of 1 and 2 are stronger than those of the ligand, respectively.

The role of hyaluronan in the structure and function of the biomatrix of connective tissues

Abstract: This review focuses on the only polysaccharide compartment of the biomatrix of connective tissues: hyaluronan. This pure polysaccharide is present in the largest amount and highest concentration in three connective tissues of the adult vertebrate body: the vitreus of the eye, the synovial fluid of the joint, and the dermis of the rooster comb. The molecular properties, weight average molecular weight, polydispersity, and elastoviscous properties of hyaluronan, as well as its distribution in these biomatrices, are presented. The differences and similarities in the biological function of this polysaccharide in the biomatrix are discussed. The effect of aging on the hyaluronan content of the vitreus, synovial fluid, and the dermis of human skin is also presented.

Hetero-heptanuclear (Fe–Na) complexes of salicylaldehydes: crystal and molecular structure of [Fe2(3-OCH3-salo)8 · Νa5] · 3OH · 8H2Ο

Abstract: The reaction of Fe(III) with the substituted salicylaldehydes [X-saloH, where X = 3-OCH3 (L 1 ), 5-CH3 (L 2 ), 5-Cl (L 3 ), 5-NO2 (L 4 )] led to the formation of four new iron(III) hetero-heptanuclear complexes (Fe–Na) under the general formula [Fe2(X-salo)8Νa5] · 3OH · zH2Ο. The two different coordination modes of the ligand, as well as the geometry around the metal ions were deduced by X-ray structure analysis of compound 1, [Fe2(3-OCH3-salo)8Νa5] · 3OH · 8H2Ο. The complexes have also been characterized by physicochemical and spectroscopic (IR, UV–Vis, Mossbauer) methods.

1D and 2D solid-state metallosupramolecular arrays of freebase 5,10,15,20-tetra(4-pyridyl)porphyrin, peripherally linked by zinc and manganese ions

Abstract: Freebase 5,10,15,20-tetra(4-pyridyl)porphyrin (TPyP) is able to assemble into crystalline rigid 1D and 2D metallosupramolecular arrrays via exocyclic coordination with tetracoordinate and hexacoordinate metal ions, respectively. In this contribution we report on two coordination polymers of μ 4-bridging freebase TPyP with zinc and manganese halide moieties. The structure of [(ZnBr2)2TPyP] · 6 TCE (2) (TCE = 1,1,2,2-tetrachloroethane) consists of 1D polymeric chains. The topology of [(MnCl2)TPyP] · 6 TCE (3) is the (4,4) square grid type. The crystal structure of the known compound [(HgI2)2TPyP] · 4 TCE (1) was redetermined by X-ray crystallography.

Conformational flexibility of pyrimidine rings of nucleic acid bases in polar environment: PCM study

Abstract: The results of calculations of molecular structures of nucleic acid bases in polar environment using Polarized Continuum Model of solvent combined with the MP2/cc-pvdz level of ab initio theory demonstrate considerable polarization of thymine, cytosine, and guanine. This phenomenon can be related to considerable contribution of zwitter-ionic resonant forms into total structure of the studied species. It leads to significant increase (about 30%) of frequencies of the out-of-plane pyrimidine ring vibrations and is related to considerable decrease of conformational flexibility of heterocycle due to smaller out-of-plane deformability of pyrimidine ring in zwitter-ionic resonant forms. In the case of adenine, the presence of a polar environment results in an increase of conformational flexibility of pyrimidine ring.