Long-range molecular resonances in a cold Rydberg gas

We present evidence for molecular resonances in a cold dense gas of rubidium Rydberg atoms. Single UV photon excitation from the 5s ground state to np Rydberg states (n=50-90) reveals resonances at energies corresponding to excited atom pairs (n-1)d+ns. We attribute these normally forbidden transitions to avoided crossings between the long-range molecular potentials of two Rydberg atoms. These strong van der Waals interactions result in avoided crossings at extremely long range, e.g., approximately 58 000 times the Bohr radius (a(0)) for n=70.     

Absorbing phase transitions of branching-annihilating random walks

The phase transitions to absorbing states of the branching-annihilating reaction-diffusion processes mA-->(m+k)A, nA-->(n-l)A are studied systematically in one space dimension within a new family of models. Four universality classes of nontrivial critical behavior are found. This provides, in particular, the first evidence of universal scaling laws for pair and triplet processes.     

Solid State Reactions of Hemin with Basic Substances: Formation of bis and Mixed Complexes

The solid state reactions of hemin with KCN, Na₂SxH₂O, arginine, imidazole, 1-methylimidazole, 2-methylimidazole, benzimidazole, and mixtures of these basic reagents were monitored using IR, Mössbauer, and XRD techniques. All these basic substances react at the peripheral propionic acid group of hemin-forming salts. Binary mixtures of KCN, arginine, imidazole, 1-methylimidazole, 2-methylimidazole, and benzimidazole were found to form complexes with mixed ligands at the iron site of hemin. According to the structural information obtained for these mixed complexes, mechanisms for their formation are proposed. The solid state synthesis and the properties of the obtained products reveal the specifities of the involved ligands.     

Concentration of sodium ion as the determining factor for the association of dansyl amino acids with the teicoplanin molecule in reversed-phase liquid chromatography

The mechanism of the binding of D,L dansyl amino acids to teicoplanin was investigated. Na+ was used as an indicator of the interactions between the solutes and teicoplanin. The number (n) of sodium ions, Na+, excluded from the solute-teicoplanin interface when analyte transfer occurred was determined. A thermodynamic study and enthalpy-entropy compensation were performed to further explore the interaction mechanism. From these results, it was shown that teicoplanin was balanced between 2 conformational states characterized by distinct enantioselective properties. This approach indicates that liquid chromatography (LC) is a useful tool to extract physicochemical and molecular information from retention data. Thus, LC can be used as a complementary technique with the conventional techniques of molecular interaction analysis.     

Reductive cyclizations of hydroxysulfinyl ketones: enantioselective access to tetrahydropyran and tetrahydrofuran derivatives

The stereocontrolled formation of cis-2,5-disubstituted tetrahydrofurans and cis-2,6-disubstituted tetrahydropyrans is achieved from enantiopure ketosulfinyl esters by reduction, Weinreb's amide, and ketone formation, followed by the reductive cyclization (Et3SiH/TMSOTf) of the resulting hydroxysulfinyl ketones. The sulfoxide-bearing heterocycles were transformed into two natural products, (-)-centrolobine (1) and both enantiomers of cis-(6-methyltetrahydropyran-2-yl)acetic acid (2).     

Ring opening of nonactivated 2-(1-aminoalkyl) aziridines: unusual regio- and stereoselective C-2 and C-3 cleavage

We have studied the ring opening of nonactivated amino aziridines 1 by water under acidic conditions. Depending on the acid used, amino aziridines are cleaved at C-3 or C-2 with high regioselectivity, and total stereoselectivity, affording chiral 2,3-diaminoalkan-1-ols 3 or 1,3-diaminoalkan-2-ols 4 in high yield.     

Interaction of a laminin derived peptide with phosphatidyl choline/phosphatidyl glycerol

The physicochemical interactions between an active peptide sequence derived from laminin and phospholipids have been studied. The main aim is to determine the suitability of this peptide fragment to be linked to liposome's with the purpose to develop targeting devices. Results indicate that this peptide is able to insert in lipid monolayers and also to form monomolecular layers at an air/water interface. Besides, miscibility studies carried out through compression isotherms of mixed monolayers [dipalmitoyl phosphatidylcholine (DPPC)/peptide], indicate a strong interaction at 60-80% DPPC molar composition. Studies carried out with lipid bilayers indicate that the interaction is restricted to the external face of the vesicles. Moreover, the presence of this peptide in the incubation media promotes a low level of carboxyfluorescein (CF) leakage and no fusion of vesicles. These results indicate that the association of this sequence to vesicles do not produce damage of the bilayer and can be used as potential targeting vector.     

HPLC separation of diastereomers of Ln(III)-ethylenepropylene-triamine-pentaacetate complexes. Direct assessment of their water exchange rate

The diastereomers of two Ln(III)-EPTPA derivatives have been separated by reversed-phase HPLC, and the water exchange rate on their Gd(III) complexes has been directly determined by 17O NMR (H5EPTPA = ethylenepropylene-triamine-pentaacetic acid).     

A supramolecular cup-and-ball C60-porphyrin conjugate system

In addition to the ammonium-crown ether recognition, pi-stacking interactions between the C60 sphere and the porphyrin moiety have been evidenced in a supramolecular complex obtained from a porphyrin-crown ether conjugate and a fullerene derivative bearing an ammonium unit.