Chiral N-isobutyryl-cysteine protected gold nanoparticles: preparation, size selection, and optical activity in the UV-vis and infrared

We have prepared gold nanoparticles covered with N-isobutyryl-l-cysteine and N-isobutyryl-d-cysteine, respectively. These particles with a mean particle size smaller than 2 nm are highly soluble in water and are amenable to chiroptical techniques such as vibrational circular dichroism (VCD) and circular dichroism (CD) spectroscopy. Density functional theory shows that the VCD spectra are sensitive toward the conformation of the adsorbed thiol. Based on the comparison between the experimental VCD spectrum and the calculated VCD spectra for different conformers, a preferential conformation of the thiol adsorbed on the gold particles can be proposed. In this conformation the carboxylate group interacts with the gold particle in addition to the sulfur. The particles could furthermore be separated according to their charge and size into well-defined compounds. The optical absorption spectra revealed a well-quantized electronic structure and a systematic red-shift of the absorption onset with increasing gold core size, which was manifested in a color change with particle size. Some compounds showed basically identical absorption spectra as analogous gold particles protected with l-glutathione. This shows that these particles have identical core sizes (10-12, 15 and 18 gold atoms, respectively) and indicates that the number and arrangement of the adsorbed thiol are the same, independent of the two thiols, which have largely different sizes. Some separated compounds show strong optical activity with opposite sign when covered with the d- and l-enantiomer, respectively, of N-isobutyryl-cysteine. The origin of the optical activity in the metal-based transitions is discussed. The observations are consistent with a mechanism based on a chiral footprint on the metal core imparted by the adsorbed thiol.     

Interfacial properties of amiodarone: the stabilizing effect of phosphate anions

Amiodarone, a drug used in heart therapy, is poorly soluble in water at room temperature, but forms transparent phases much more concentrated than the critical micellar concentration (CMC), when crystals are heated (above 60 degrees C) in presence of water and cooled down to room temperature. These pseudosolutions were supposed to be made of a complex system of micelles. In order to better understand the effects of pH and ion species on the supramolecular organization of amiodarone, interfacial pressure measurements were performed at the air/water interface on a Langmuir trough. Monolayers spread from chloroformic solutions over non bufferered subphases were insoluble at basic pH (NaOH, pH 10) but soluble at acidic pH (HCl, pH 4). However, a higher ionic strength obtained by adding NaCl (0.15 N) or NaH(2)PO(4) (0.15 N) to the subphase stopped the amiodarone solubilization. On an acidic phosphate subphase (NaH(2)PO(4), pH 4.4, 0.15 N), abnormally high surface pressures (>1 mN/m) were measured for high molecular areas (80-200 ?(2)/molecule) suggesting a supramolecular organization of the surface film. Insoluble monolayers were also obtained when the amiodarone supramolecular pseudosolution was spread on neutral (NaH(2)PO(4), pH 6.25, 0.15 N) or acidic (NaH(2)PO(4), pH 4.4, 0.15 N) subphases. However, a great instability on basic subphase (phosphate buffer pH 8.8) indicated the breakage of the supramolecular structure during spreading. These results are discussed taking into account the amiodarone state of ionization and the electrostatic interactions with counterions. Combining the use of phosphate counterions and that of acidic pH opens new perspectives in the optimization of amiodarone intravenous formulations.     

Electrochemical transduction on self-assembled monolayers: are covalent links essential?

Electrochemical transduction without covalent links between redox and complexant units in a complexing self-assembled monolayer has been established. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable.     

A water soluble Cu(I)-NHC for CuAAC ligation of unprotected peptides under open air conditions

A reducing agent-free version of CuAAC able to operate under open air conditions is reported. A readily-synthesizable, hydrophilic and highly stable Cu(I)-NHC allows the clean ligations of unprotected peptides comprising sensitive side chains, at millimolar concentrations.     

Structure and Stability of VO2+ in Aqueous Solution: A Car—Parrinello and Static ab initio Study.

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Sc0.43(2)Rb2Mo15S19, a partially Sc‐filled variant of Rb2Mo15S19

The structure of scandium dirubidium pentadecamolybdenum nonadecasulfide, Sc_{0.43 (2)}Rb₂Mo₁₅S₁₉, constitutes a partially Sc‐filled variant of Rb₂Mo₁₅S₁₉ [Picard, Saillard, Gougeon, Noel & Potel (2000), J. Solid State Chem. 155 , 417–426]. In the two compounds, which both crystallize in the Rc space group, the structural motif is characterized by a mixture of Mo₆Sⁱ₈S^a₆ and Mo₉Sⁱ₁₁S^a₆ cluster units (`i' is inner and `a' is apical) in a 1:1 ratio. The two components are interconnected through interunit Mo—S bonds. The cluster units are centred at Wyckoff positions 6b and 6a (point‐group symmetries and 32, respectively). The Rb⁺ cations occupy large voids between the different cluster units. The Rb and the two inner S atoms lie on sites with 3. symmetry (Wyckoff site 12c), and the Mo and S atoms of the median plane of the Mo₉S₁₁S₆ cluster unit lie on sites with .2 symmetry (Wyckoff site 18e). A unique feature of the structure is a partially filled octahedral Sc site with symmetry. Extended Hückel tight‐binding calculations provide an understanding of the variation in the Mo—Mo distances within the Mo clusters induced by the increase in the cationic charge transfer due to the insertion of Sc.