Complexation of natural and methylated β-cyclodextrins with long-chain Carboxylic acids in aqueous solutions. Calorimetric studies at 298 K

The formation of complexes between β-cyclodextrin or methyl-β-cyclodextrin and monocarboxylic acids from C₅ to C₁₂ has been studied calorimetrically at 298 K in aqueous buffer phosphate, pH 11.3. When a complex forms, calorimetry enables the calculation of both the enthalpy and the association constant, from which the Gibbs free energy and the entropy of the process can be obtained. The forces involved in the association process are then discussed in the light of the signs and values of the thermodynamic parameters obtained. For β-cyclodextrin, 1:1 inclusion complexes form, characterized by an irregular variation in the values of enthalpies, entropies and association constants at increasing alkyl chain length. A model is proposed to explain this unusual behaviour for acids longer than ten carbon atoms. The association involving methyl-β-cyclodextrin shows, instead, a regular variation of the thermodynamic parameters up to the C₁₂ term. An elongation of the cavity effect is discussed: the cavity behaves as it were deeper than that of the natural cyclodextrins. Association is characterized prevailingly by hydrophobic interactions; for the longer terms, the high and almost invariant entropic contribution determines the large association constants, notwithstanding the positive, unfavorable enthalpic contribution. This is an indication that the relaxation of water molecules from the hydrophobic hydration shells of the external methyl groups is the contribution that mainly determines the association process.     

PHOTOOXIDATION OF LANTHANIDE ION-LYSOZYME COMPLEXES. A NEW APPROACH TO THE EVALUATION OF INTRAMOLECULAR DISTANCES IN PROTEINS

Abstract— Rare-earth metal ions give 1:1 complexes with hen's egg-white lysozyme. Spectroscopic and enzymic activity measurements suggest that the binding site consists of the side chains of glutamic-35 and aspartic-52. The spatial conformation of these complexes is practically identical to that of native lysozyme, especially as concerns the environment of the tryptophyl side chains. Irradiation of La³⁺-lysozyme by visible light, in the presence of proflavine as photosensitizer, causes the oxidative modification of all the tryptophyl and methionyl residues at almost the same rate as in uncomplexed protein. On the other hand, when lanthanide ions with nonvanishing magnetic moments were coordinated with lysozyme, at least some tryptophans and methionines were protected from photooxidative attack. The distance of the protected residues from the coordination site increased with increasing magnetic moment of the bound metal ion, which suggests that inhibition of the photoprocess was mainly due to perturbation of the lifetime of the electronically excited intermediate species. On the basis of the atomic coordinates of lysozyme in the crystal state, it is thus possible to define a "quenching radius" for the various lanthanide ions; these radii could in turn be used to evaluate intramolecular distances in proteins of unknown tertiary structure, by identifying the amino acid residues that are protected or photooxidized upon irradiation of complexes between the given protein and several different lanthanide ions. Our studieson lysozyme allow us to define five radii of protection, ranging from 6·7 Å for Sm³⁺ to over 17 Å for Dy³⁺, Ho³⁺, Er³⁺ and Tb³⁺. Therefore, this technique opens the possibility of mapping appreciably large regions of a protein molecule.     

Kinetics and equilibria of the interactions of hydroxamic acids with gallium(III) and indium(III)

The thermodynamics and kinetics of the binding of Ga(III) and In(III) to two hydroxamic acids, C6H5-C(O)N(OH)H (BHA) and C6H5-C(O)N(OH)C6H5 (PBHA), have been investigated in acidic media. Spectrophotometric titrations in the UV region reveal that, with excess metal, only the chelate ML forms, whereas the concentration of the protonated species, MHL, is negligible. The thermodynamic parameters indicate that the driving force for formation of ML from MOH2+ and HL is mainly enthalpic, with entropic contributions favoring InL2+ and disfavoring GaL2+ formation. The kinetic (stopped-flow) experiments are interpreted on the basis of two parallel reaction paths both involving reaction of the undissociated ligand (HL): (a) M + HL <==> MHL <==> ML + H where MHL is in a steady state and (b) MOH + HL <==> ML + H2O. Whereas gallium binding to BHA and PBHA proceeds mainly through path b, indium binding to PBHA proceeds through both a and b paths. The rates of both the a and b steps are ligand dependent. Two alternative mechanisms are proposed. The first is based on the electronic characteristics of the ligands and is of the Ia type. The second, of the Id type, assumes that a considerable fraction of the ligand is unreactive owing to intramolecular hydrogen bonding (possibly including a water molecule) which blocks the reaction site. The reasons for preferring the former mechanism are discussed.     

Asymmetric aminolysis of aromatic epoxides: a facile catalytic enantioselective synthesis of anti-beta-amino alcohols

[reaction: see text] The first asymmetric aminolysis of trans-aromatic epoxides with anilines is described. The process affords enantioenriched anti-beta-amino alcohols in up to 99% ee. The complete regio- and diastereoselectivity observed uses commercially available [Cr(Salen)Cl] as a Lewis acid catalyst and in combination with a very simple experimental procedure renders the present reaction a facile and practical tool for the synthesis of chiral nonracemic anti-beta-amino alcohols.     

S‐Trifluoromethylation of Thiols by Hypervalent Iodine Reagents: A Joint Experimental and Computational Study

The radical trifluoromethylation of thiophenol in condensed phase applying reagent 1 (3,3‐dimethyl‐1‐(trifluoromethyl)‐1λ³,2‐benziodoxol) has been examined by both theoretical and experimental methodologies. On the basis of ab initio molecular dynamics and metadynamics we show that radical reaction mechanisms favourably compete with polar ones involving the S‐centred nucleophile thiophenol, their free energies of activation, ΔF^≠, lying between 9 and 15 kcal mol^?1. We further show that the origin of the proton activating the reagent is important. Hammett plot analysis reveals intramolecular protonation of 1 , thus generating negative charge on the sulfur atom in the rate‐determining step. The formation of a CF₃ radical can be thermally induced by internal dissociative electron transfer, its activation energy, ΔF^≠, amounting to as little as 10.8 and 2.8 kcal mol^?1 for reagent 1 and its protonated form 2 , respectively. The reduction of the iodine atom by thiophenol occurs either subsequently or in a concerted fashion.     

Aromatic garlands,as new foldamers,to mimic protein secondary structure

Proteins modulate the majority of all biological functions and are composed of highly organized secondary structural elements such as helices, turns, and sheets. Many of these functions are affected by a small number of key structural element, protein–protein interactions. Their mimicry by peptide and non-peptide scaffolds has become a major focus of contemporary research. This paper examines oligomeric system as new foldamers, which either reproduce the local topography of the helix, or project appropriately functionality in a similar manner to residues of an alpha-helix.     

Proton‐conducting electrolytes based on silylated and sulfonated polyetheretherketone: Synthesis and characterization

A derivative of polyetheretherketone (PEEK) having sulfonic acid groups and silicon‐containing substituents covalently bound to the aromatic backbone has been prepared as proton‐exchange membrane material. The polymer 4 (PhSiSPEEK) has been synthesized via (i) sulfonation of PEEK up to 0.9 degree of sulfonation (DS, the number of sulfonic groups per repeat unit), (ii) conversion of sulfonated PEEK 1 (SPEEK09) into sulfonyl chlorinated derivative 2 (PEEKSO₂Cl), (iii) lithiation of 2 and subsequent addition of PhSiCl₃, followed by hydrolysis. The chemical structure of the synthesized polymers has been investigated by ¹H NMR and ¹³C NMR and ATR/FTIR spectroscopy and their thermal stability has been evaluated by thermogravimetric analysis. The presence of inorganic moieties increases the thermal stability of 4 with respect to the sulfonated and not silylated product. Despite its very high DS, PhSiSPEEK is insoluble in water but does not possess the plastic properties needed to be used as an electrolyte membrane. Blend membranes made of SPEEK05 (DS = 0.5) and containing 10 and 25 wt % of 4 (DS = 0.9, degree of silylation DSi = 0.1) have been prepared and characterized by water uptake measurements and electrochemical impedance spectroscopy. The combination of the two functionalized polymers having different properties allows to obtain proton‐conducting electrolytes that are potential candidates for fuel cells applications. © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 2178–2186, 2010     

The cavity elongation effect. Calorimetric studies of the complexes of long-chain carboxylic acids with methyl-α-cyclodextrin in aqueous solutions

The formation of complexes between methyl-α-cyclodextrin (MαCD) and monocarboxylic acids from C₅ to C₁₂ or cycloalkanols has been studied calorimetrically at 298 K in aqueous phosphate buffer, pH 11.3. The forces involved in the association process are discussed according to the signs and values of the thermodynamic parameters obtained: association enthalpy, constant, Gibbs energy and entropy. Methyl-α-cyclodextrin forms 1:1 inclusion complexes with monocarboxylic acids, characterized by a monotonous increase in the values of enthalpies and association constants at increasing alkyl chain length. Association is characterized by negative enthalpies and by positive entropies: that determines large association constants. That behaviour is compared to the unusual trend in the values of the association constants shown by the parent α-cyclodextrin interacting␣with the same monocarboxylic acids. The model proposed to rationalize the present data provides a cavity elongation effect. Namely, because of the presence of the methyl groups on the outside, the cavity behaves as it were deeper than that of the parent cyclodextrin. The association with cycloalkanols—cyclohexanol, cycloheptanol, cyclooctanol and 1-cylohexyl-ethanol—is characterized by lower entropies, as determined by the␣enhanced negative contribution originating from the␣tighter fit of the guest into the cavity.     

Time‐Resolved In Situ Synchrotron X‐ray Study and Large‐Scale Production of Magnetite Nanoparticles in Supercritical Water

A general solution : In situ synchrotron X‐ray scattering in a high‐pressure pulsed injection reactor (see picture) shows that magnetite nucleation and growth are temporally separated. Gram‐scale crystalline, pure phase, superparamagnetic magnetite nanoparticles were synthesized without surfactants in supercritical water in less than one hour using a laboratory‐scale continuous‐flow reactor.

     

Solute-solvent and solvent-solvent interactions in the preferential solvation of 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide in 24 binary solvent mixtures

The molar transition energy (E(T)) polarity values for the dye 4-[4-(dimethylamino)styryl]-1-methylpyridinium iodide were collected in binary mixtures comprising a hydrogen-bond accepting (HBA) solvent (acetone, acetonitrile, dimethyl sulfoxide (DMSO), and N,N-dimethylformamide (DMF)) and a hydrogen-bond donating (HBD) solvent (water, methanol, ethanol, propan-2-ol, and butan-1-ol). Data referring to mixtures of water with alcohols were also analyzed. These data were used in the study of the preferential solvation of the probe, in terms of both solute-solvent and solvent-solvent interactions. These latter interactions are of importance in explaining the synergistic behavior observed for many mixed solvent systems. All data were successfully fitted to a model based on solvent-exchange equilibria. The E(T) values of the dye dissolved in the solvents show that the position of the solvatochromic absorption band of the dye is dependent on the medium polarity. The solvation of the dye in HBA solvents occurs with a very important contribution from ion-dipole interactions. In HBD solvents, the hydrogen bonding between the dimethylamino group in the dye and the OH group in the solvent plays an important role in the solvation of the dye. The interaction of the hydroxylic solvent with the other component in the mixture can lead to the formation of hydrogen-bonded complexes, which solvate the dye using a lower polar moiety, i.e. alkyl groups in the solvents. The dye has a hydrophobic nature and a dimethylamino group with a minor capability for hydrogen bonding with the medium in comparison with the phenolate group present in Reichardt's pyridiniophenolate. Thus, the probe is able to detect solvent-solvent interactions, which are implicit to the observed synergistic behavior.