Modeling procyanidin self-association processes and understanding their micellar organization: a study by diffusion NMR and molecular mechanics

The colloidal behavior of eight synthetic procyanidins (three monomers, four dimers, and a trimer) has been investigated in water or in a winelike medium using DOSY NMR spectroscopy and molecular dynamics simulations. Different behavior was observed for monomers and oligomers. Monomers self-associate with a high affinity constant (37-53 M(-1)) to form micelles at low cmc (critical micelle concentration) values (1-5 g.L(-1)). These micelles undergo a time-dependent coalescence process to form hazes and precipitates. As for dimers and the trimer, self-association also occurs but with a lower affinity (approximately 6 M(-1)) and at higher cmc values (10-20 g.L(-1)) to form small micelles (<5 nm) that remain stable throughout the experiment. The presence of 10% ethanol does not significantly affect the self-association constant for monomers and oligomers but increases their cmc values by approximately 50% and decreases the micelle size by a factor 2. However, the presence of 20 mM NaCl appears to negate the effect of ethanol. This study helps to clarify the role of procyanidin monomers versus oligomers in wine turbidity and demonstrates that procyanidin oligomers are fully available to interact with saliva proteins.     

Electrospray tandem mass spectrometry of alendronate analogues: fingerprints for characterization of new potential prodrugs

1-hydroxymethylene-1,1-bisphosphonic acids (HMBPs) are important drugs for the treatment of a variety of bone diseases. Since these compounds have no chromophore, their detection is challenging and mass spectrometry (MS) appears to be an appropriate sensitive tool. Our work deals with the analysis by electrospray ionization tandem mass spectrometry (ESI-MSn) of the well-known nitrogen-containing HMBP alendronate and of three analogues, considered as potential prodrugs. These four molecules share a common structure with different protecting groups on the phosphonic acid and on the amine functions. We describe the dissociation mechanisms of nitrogen-containing HMBPs in positive ion mode and we compare, in negative ion mode, our results with literature data. In both modes, the dissociations are essentially losses of ROH, and of phosphorus-containing species (HPO2, ROP(OH)2 and ROPO(OH)2), where R=H, C6H5, or CH3OC6H5. These fingerprints will be of great value for differentiating alendronate from its potential prodrugs in complex biological mixtures.     

Dynamic kinetic resolution of racemic beta-haloalcohols: direct access to enantioenriched epoxides

The direct chemo-enzymatic DKR of racemic beta-haloalcohols is reported, yielding the corresponding optically active epoxides in a single step. The mutant haloalcohol dehalogenase HheC Cys153Ser Trp249Phe is used for the asymmetric ring closure, whereas racemization of the remaining enantiomer of the haloalcohol is achieved using the new iridacycle 3, one of the most effective racemization catalysts to date for beta-haloalcohols.     

Deracemization of a dynamic combinatorial library induced by (-)-cytidine and (-)-2-thiocytidine

A dynamic combinatorial library composed of racemic hydrazone-based dipeptides becomes deracemized on binding to the chiral analytes (-)-cytidine and (-)-2-thiocytidine through the amplification of two receptors, (SS)-dimer and (RRRR)-tetramer. The deracemization phenomenon was investigated by laser polarimetry, mass-tagged pseudo-enantiomers in conjunction with electrospray ionization mass spectrometry, HPLC/UV-MS, UPLC/UV-MS, rapid-resolution LC-MS, collision-induced dissociation MS/MS, and numerical simulations. These data were consistent with a phenomenon where (SS)-dimer and (RRRR)-tetramer selectively bind the chiral analyte in preference to their enantiomeric counterparts, which ultimately causes them to be amplified and the library to become deracemized.     

Synthesis and properties of supramolecular ionic networks

The synthesis of a supramolecular ionic network, its physical properties, and the use of this network property to form macroscopic porphyrin fibers are described. These ionic networks are compared to ionic liquids. Current ionic liquid compositions have a charge and molar ratio of 1:1 where an anionic species is matched with a cationic species; however, alteration of this molar ratio while maintaining the charge ratio of 1:1 by using multivalent cationic/anionic molecular pairs affords new ionically cross-linked networks with interesting properties.     

Reversibility effects on the stereoselectivity of Pt(II)-mediated cascade poly-ene cyclizations

Cyclization of 1,5-dienes bearing nucleophilic traps with electrophilic trisphosphine pincer ligated Pt(II) complexes results in the formation of a polycyclic Pt-alkyl via a Pt(eta2-alkene) intermediate. With electron-rich triphosphine ligands, an equilibrium between the Pt(eta2-alkene) and Pt-alkyl was observed. The position of the equilibrium was sensitive to ligand basicity, conjugate acid strength, solvent polarity, and ring size. In cases where the ligand was electron poor and did not promote retrocyclization, the kinetic products adhering to the Stork-Eschenmoser postulate were observed (E-alkenes give trans-ring junctions). When retrocyclization was rapid, alternative thermodynamic products resulting from multistep rearrangements were observed (cis-[6,5]-bicycles). Under both kinetic and thermodynamic conditions, remote methyl substituents led to highly diastereoselective reactions. In the case of trienol substrates, long-range asymmetric induction from a C-ring substituent was considerably attenuated and only modest diastereoselectivity was observed (approximately 2:1). The data suggest that for a tricyclization, the long-range stereocontrol results from diastereo-selecting interactions that develop during the organization of the nascent rings. In contrast, the bicyclization diastereoselectivities result from reversible cascade cyclization.     

Structure and magnetic properties of oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) with Fe2O square planar layers representing an antiferromagnetic checkerboard spin lattice

The oxychalcogenides A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se), which contain Fe2O square planar layers of the anti-CuO2 type, were predicted using a modular assembly of layered secondary building units and subsequently synthesized. The physical properties of these compounds were characterized using magnetic susceptibility, electrical resistivity, specific heat, (57)Fe Mossbauer, and powder neutron diffraction measurements and also by estimating their exchange interactions on the basis of first-principles density functional theory electronic structure calculations. These compounds are magnetic semiconductors that undergo a long-range antiferromagnetic ordering below 83.6-106.2 K, and their magnetic properties are well-described by a two-dimensional Ising model. The dominant antiferromagnetic spin exchange interaction between S = 2 Fe(2+) ions occurs through corner-sharing Fe-O-Fe bridges. Moreover, the calculated spin exchange interactions show that the A2F2Fe2OQ2 (A = Sr, Ba; Q = S, Se) compounds represent a rare example of a frustrated antiferromagnetic checkerboard lattice.     

Ab initio study of ladder-type polymers: Polythiophene and polypyrrole

An ab initio study of four polymers, polythiophene, polypyrrole, ladder-type polythiophene, and ladder-type polypyrrole is presented. Upon an analysis of the variation of the band gap when comparing the non-ladder and the ladder-type polymers, a discrepancy is found between the thiophene and the pyrrole polymer families. For polythiophene, the ladder-type polymer has a larger band gap than the non-ladder polymer whereas the opposite is found for the pyrrole polymers. This difference is explained in terms of the competition between the bond length alternation and the effect of the charge density distribution in the carbon backbone.     

Stereocontrol by a pair of epimeric sugar-derived ligands of the coordination sphere of copper(II) complexes

A pair of novel C3-epimeric sugar-derived ligands (glycoligands) with a neutral N4O donor set was synthesized. Copper(II) complexes of both ligands were obtained and characterized by X-ray crystallography. Cyclic voltammetry, electron paramagnetic resonance, and UV-vis spectroscopies showed similar electronic properties. Mirror-image CD spectra were obtained for the Cu(II) d-d band, indicating an enantiomeric character of the coordination sphere, which has been rationalized structurally. This example shows the possible predetermination of stereochemistry for complexes by ligands based on a glycoscaffold.     

Status of trivalent copper and charge-transfer excitons in high-TC cuprates

A chemical bonding approach based on tight-binding cluster and band calculations, taking into account on-site Coulomb repulsion (Hubbard U parameter) to differentiate doubly and singly occupied states, was applied to high- T C superconducting cuprates and related compounds. This work provides rational insight and explanations for issues such as (i) the actual oxidation number Cu (I+) for formally trivalent copper in oxides such as La 2Li 1/2Cu 1/2O 4, (ii) the dominant oxygen character of the doping holes in (CuO 2) ( n- ) planes, (iii) the Mott-Hubbard character of the insulator-to-metal transition triggered by hole doping, leading to an oxygen-to-copper charge transfer of avalanche type, (iv) the occurrence of an excitonic phase with anisotropic Frenkel-type excitons, (v) the role of Coulomb interactions between excitons and between doping holes and their exciton surroundings, and (vi) the on-time pairing of doping holes by means of an "excitonic glue".