Enthalpy and entropy of transfer of hydrogen ion from water to mixed solvents

Attempts have been made to determine the enthalpy and entropy of transfer of H⁺ ion from water to mixed solvents using the calorimetric data of earlier experiments. The results are in qualitative agreement with the facts that ΔH _t ⁰ (H⁺) passes through an exothermic maximum andTΔS _t ⁰ passes through a minimum at about 20 to 30 wt% of organic solvent indicating the initial structure formation and the ultimate breakdown of the solvent structure with the addition of organic solvent.     

Studies in the metathesis of beryllium chloride and sodium silicate solutions

Summary From the conduotometrio and electrometric studies of the metathesis between beryllium chloride and sodium silicate, it has been concluded that two silicates of beryllium are formed along with the formation of beryllium hydroxide and silicic acid. It has been further concluded that the hydrolysis of beryllium chloride is not so great as that of chlorides of iron and chromium.

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Zusammenfassung Aus Leitf?higkeitsmessungen und elektrometrischen Untersuchungen an Reaktionsprodukten aus Berylliumchlorid und Natriumsilikat wird auf Entstehung von zwei Berylliumsilikaten geschlossen, gleichzeitig auf Bildung von Berylliumhydroxyd und Kiesels?ure. Die Hydrolyse von Berylliumchlorid ist nicht so stark wie die von Eisen- und Chromchlorid.

     

Diastereoselective formation of glycoluril dimers: isomerization mechanism and implications for cucurbit[n]uril synthesis

Cucurbit[6]uril (CB[6]) is a macrocyclic compound, prepared in one pot from glycoluril and formaldehyde, whose molecular recognition properties have made it the object of intense study. Studies of the mechanism of CB[n] formation, which might provide insights that allow the tailor-made synthesis of CB[n] homologues and derivatives, have been hampered by the complex structure of CB[n]. By reducing the complexity of the reaction to the formation of S-shaped (12S-18S) and C-shaped (12C-18C) methylene bridged glycoluril dimers, we have been able to probe the fundamental steps of the mechanism of CB[n] synthesis to a level that has not been possible previously. For example, we present strong evidence that the mechanism of CB[n] synthesis proceeds via the intermediacy of both S-shaped and C-shaped dimers. The first experimental determination of the relative free energies of the S-shaped and C-shaped dimers indicates a thermodynamic preference (1.55-3.25 kcal mol(-)(1)) for the C-shaped diastereomer. This thermodynamic preference is not because of self-association, solvation, or template effects. Furthermore, labeling experiments have allowed us to elucidate the mechanism of this acid-catalyzed equilibrium between the S-shaped and C-shaped diastereomers. The equilibration is an intramolecular process that proceeds with high diastereoselectivity and retention of configuration. On the basis of the broad implications of these results for CB[n] synthesis, we suggest new synthetic strategies that may allow for the improved preparation of CB[n] (n > 8) and CB[n] derivatives from functionalized glycolurils.     

Chemoselective reduction of aldehydes with zinc borohydride in tetrahydrofuran

A variety of structurally different aldehydes undergo chemoselectire reduction over ketones with zinc borohydride in tetrahydrofuran at −10°C to the corresponding alcohols.     

Facile catalyzed acylation of alcohols, phenols, amines and thiols based on ZrOCl2·8H2O and acetyl chloride in solution and in solvent-free conditions

Acylation of heteroatoms (O, N and S) with acetyl chloride based on the use of a catalytic amount of the moisture stable, inexpensive ZrOCl₂·8H₂O, proceeds efficiently producing the corresponding acylated products in excellent yields.     

On the estimation of higher order resistance coefficients from electroosmotic transport data

Summary First and second order resistance coefficients between membrane matrix and permeant for the permeation of methanol, acetone and methyl ethyl ketone through pyrex and quartz membranes have been evaluated from the experimental data on electroosmotic effects. Analysis of the data shows that-the following non-linear relationP =R ₁₁ J _v +R ₁₂ I +R ₁₂₂ I ²[1] exists between pressure difference,P, and the fluxes and a linear relation =R ₂₁ J _v +R ₂₂ I [2] expresses the dependence of potential difference,, on the fluxes. The first order cross resistance coefficients have been found to obey theOnsager's reciprocity relation. An attempt has been made to explain the occurrence of higher order resistance coefficients in terms of electrokinetic character of the membrane, permeant interface.

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Zusammenfassung Die Widerstandskoeffizienten 1. und 2. Ordnung zwischen einer Membranmatrix und dem permeierenden Stoff wurden für die Permeation von Methanol, Aceton und Methyläthylketon durch Pyrex- und Quarz-Membranen auf Grund elektroosmotischer Effekte bestimmt. Die Auswertung gibt die nichtlineare BeziehungP =R ₁₁ J _v +R ₁₂ I +R ₁₂₂ I ² [1] zwischen der Druckdifferenz und dem Fließen; die lineare Beziehung =R ₂₁ J _v +R ₂₂ I [2] beschreibt die Abhängigkeit der Potentialdifferenz vom Fließen. Die gekreuzten Widerstandskoeffizienten 1. Ordnung gehorchen dem ReziprozitätsgesetzOnsager. Das Vorkommen von Widerstandskoeffizienten höherer Ordnung wird versuchsweise auf den elektrokinetischen Charakter der Membran-Grenzfläche zurückgeführt.

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With 2 figures and 2 tables     

Synthesis, X-ray structure and properties of a new nitrite-bound copper(II) complex with 2-(3,5- dimethylpyrazol-1-ylmethyl)pyridine in a CuN4(O) coordination

Synthesis and characterization of a nitrite-bound copper(II) compound [CuL⁴)₂(ONO)]ClO₄ have been achieved (L⁴ = 2-(3,5-dimethylpyrazol-1-ylmethyl)pyridine]. The bidentate ligand L⁴ provides a pyridine and a pyrazole donor site; however, they are separated by a methylene spacer. The complex has been structurally characterized and it belongs to only a handful of complexes having nitrito-bound mononuclear copper(II) centre. The metal atom has a distorted square pyramidal geometry with the copper atom displaced from the equatorial plane by 0.25 Å. In MeCN solution the green complex exhibits a broad ligand-field transition at 655 nm with a shoulder at 675 nm and in dichloromethane-toluene glass (80 K) it exhibits an EPR spectral feature characteristic of the unpaired electron in the d_x²−y² orbital. Variable-temperature (80–300 K) magnetic susceptibility measurements in the solid state as well as room temperature measurement in MeCN solution reveal mononuclear magnetically dilute copper(II) centre. When examined by cyclic voltammetry (MeCN solution) it displays electrochemically irreversible Cu^II---Cu^I response [cathodic peak potential, E_pc (V vs saturated calomel electrode (SCE)): −0.32]. An oxidative response is observed at 1.14 V, probably due to bound-nitrite oxidation and is partially removed to generate a solvated complex at the electrode surface. The latter species gives rise to reversible Cu^II---Cu^I redox response [ ].     

Isolation of sesquiterpenoids from sponge Dysidea avara and chemical modification of avarol as potential antitumor agents

The marine sponge Dysidea avara contained avarol (1) and avarone (2). Avarol on acylation yielded 2',5'-O-dibenzoylavarol (3); 2,5'-O-(4-chlorobenzoyl)avarol (4); 2,5'-O-dicinnamoylavarol (5); 2,5'-O-(4-bromobenzoyl)avarol(6); 2',5'-O-dioctanoylavarol (7); 2',5'-O-(4-fluorobenzoyl)avarol (8) and diacetylavarol (9). The structural elucidation of all the compounds 1-9 have been done by spectral analysis. The cytotoxicity of these compounds were also determined and evaluated. Compounds 6 and 9 showed selective cytotoxicity against Hepa (human hepatoma) and KB cell lines respectively.     

Sensitivity of polar solvation dynamics to the secondary structures of aqueous proteins and the role of surface exposure of the probe

The structure and dynamics of water around a protein is expected to be sensitive to the details of the adjacent secondary structure of the protein. In this article, we explore this sensitivity by calculating both the orientational dynamics of the surface water molecules and the equilibrium solvation time correlation function of the polar amino acid residues in each of the three helical segments of the protein HP-36, using atomistic molecular dynamics simulations. The solvation dynamics of polar amino acid residues in helix-2 is found to be faster than that of the other two helices (the average time constant is smaller by a factor of 2), although the interfacial water molecules around helix-2 exhibit much slower orientational dynamics than that around the other two helices. A careful analysis shows that the origin of such a counterintuitive behavior lies in the dependence of the solvation time correlation function on the surface exposure of the probe-the more exposed is the probe, the faster the solvation dynamics. We discuss that these results are useful in explaining recent solvation dynamics experiments.     

Conformational studies of 3,4-dideoxy furanoid sugar amino acid containing analogs of the receptor binding inhibitor of vasoactive intestinal peptide

Conformational analysis of vasoactive intestinal peptide (VIP) receptor binding inhibitor Leu¹-Met²-Tyr³-Pro⁴-Thr⁵-Tyr⁶-Leu⁷-Lys⁸1 by various NMR techniques and constrained molecular dynamics (MD) simulation studies revealed that the molecule had a turn structure involving its Tyr³-Pro⁴-Thr⁵-Tyr⁶ moiety with intramolecular hydrogen bond between Tyr⁶NH→Tyr³CO. In order to mimic the structure of 1, peptidomimetic analogs 2-4 were synthesized using conformationally constrained scaffolds of 3,4-dideoxy furanoid sugar amino acids (2S,5R)-ddSaa1 5 and its enantiomer (2R,5S)-ddSaa2 6. All these analogs displayed well defined three-dimensional structures akin to that found in 1. Peptides 2 and 3, which differed only in the sugar amino acid stereochemistry, show propensity of structures with identical intramolecular hydrogen bonds between ThrNH→MetCO. A similar structure with a hydrogen bond between TyrNH→MetCO was observed in 4.