Dynamics of hyaluronan aqueous solutions as assessed by fast field cycling NMR relaxometry

Fast field cycling (FFC) NMR relaxometry has been used to study the conformational properties of aqueous solutions of hyaluronan (HYA) at three concentrations in the range 10 to 25 mg mL^–1. Results revealed that, irrespective of the solution concentration, three different hydration layers surround hyaluronan. The inner layer consists of water molecules strongly retained in the proximity of the HYA surface. Because of their strong interactions with HYA, water molecules in this inner hydration layer are subject to very slow dynamics and have the largest correlation times. The other two hydration layers are made of water molecules which are located progressively further from the HYA surface. As a result, decreasing correlation times caused by faster molecular motion were measured. The NMRD profiles obtained by FFC-NMR relaxometry also showed peaks attributable to ¹H–¹⁴N quadrupole interactions. Changes in intensity and position of the quadrupolar peaks in the NMRD profiles suggested that with increasing concentration the amido group is progressively involved in the formation of weak and transient intramolecular water bridging adjacent hyaluronan chains. In this work, FFC-NMR was used for the first time to obtain deeper insight into HYA–water interactions and proved itself a powerful and promising tool in hyaluronan chemistry.     

Tetramethyl(perfluoroalkyl)cyclopentadienyl rhodium(I) complexes with ethylene or diene ligands. Crystal structure of [(η-C5Me4C6F13)Rh(CO)2]

Tetramethyl(perfluoroalkyl)cyclopentadienyl rhodium(I) complexes with ethylene or diene (norbornadiene, cycloocta-1,5-diene, 2,3-dimethylbuta-1,3-diene, cyclohexa-1,3-diene) ligands were obtained by reduction of tetramethyl(perfluoroalkyl)rhodium(III) dichloro dimers by zinc in THF or by propan-2-ol/sodium carbonate in the presence of the ligands. Reduction in the presence of cycloocta-1,3-diene gave a different product, an η³-cyclooctenyl complex, which was not reduced further. During the reduction in the presence of ethylene, a new tetramethyl(perfluoroalkyl)-η⁴-cyclopentadiene complex was observed by NMR. This compound, formed by hydrogen transfer from the metal to the ligand, is probably in an equilibrium with the parent hydridocyclopentadienyl complex. Crystal and molecular structure of dicarbonyltetramethyl(perfluorohexyl)cyclopentadienylrhodium(I) complex was determined by X-ray diffraction. The structure shows a moderate ring slippage of the rhodium atom which was not observed in the only other known structure of a complex with the same ligand, the rhodium(III) dichloro dimer.     

Copper-catalyzed enantioselective synthesis of trans-1-alkyl-2-substituted cyclopropanes via tandem conjugate addition-intramolecular enolate trapping

Cu-TolBINAP-catalyzed conjugate addition of Grignard reagents to 4-chloro-α,β-unsaturated esters, thioesters, and ketones leads to 4-chloro-3-alkyl-substituted thioesters and ketones in up to 84% yield and up to 96% ee upon protonation of the corresponding enolates at low temperature. Tandem conjugate addition-enolate trapping, however, yields trans-1-alkyl-2-substituted cyclopropanes in up to 92% yield and up to 98% ee. The versatility of this reaction is illustrated by the formation of key intermediates for the formal syntheses of cascarillic acid and grenadamide.     

Aza-Michael reaction of 12-N-carboxamide of (–)-cytisine under high pressure conditions

The first example of aza-Michael reaction of 12-N-carboxamide of quinolizidine alkaloid (–)-cytisine with α,β-unsaturated ketones, dimethyl acetylenedicarboxylate and β-nitrostyrene under high pressure condition has been described. It has been shown that the [4+2]-cycloaddition takes place in the case with N-phenylmaleimide.     

Spectrochemical determination of tungsten and boron in mineral matrix

The spectrochemical behavior of nonvolatile tungsten and boron was tested by adding different buffers and using 12 A ac arc excitation. The oxidization and sulfidization processes were found to be most effective for facilitating the volatilization. The optimal admixture was the copper oxide buffer for both the detection limit and the precision of the method. The reference values of the background or a palladium line blackening were used.     

Application of emission spectrometer with laser sampler to microanalysis of pigments from Hubert Robert’s canvas painting

Laser-induced breakdown spectroscopy has been applied to layer-by-layer pigment material microanalysis from the different sections of Hubert Robert’s (1733–1808) painting “Landscape of a Pool with an Obelisk and Ruins of an Aqueduct”. This painting consists of two sections and, therefore, requires thorough examination of the pigments from both sections in order to identify their authenticity. The data obtained on the elemental composition of the paint layers including the ground layer alongside with art examination have formed the basis for the identification, attribution and restoration of both investigated sections of the painting.     

Environmental analysis in the Czech Republic: organisation of proficiency testing and its use in laboratory assessment

The history, origin, and development of a system for monitoring and assessing water and other environmental laboratories in the Czech Republic is described. The system started in 1991 and has matured to its present complexity with similarities to the accreditation systems found in other countries. Differences from internationally recognized procedures are being corrected step by step. During the first year of its existence ASLAB, as part of its brief, organised proficiency testing (PT) programs for fifty laboratories. Today the total number of regularly participating laboratories exceeds 700 from the Czech Republic, the Slovak Republic, and Germany. This paper describes the ASLAB PT system, discusses some experiences with its use, and describes the use of PT results in assessment of the competence of laboratories. Received: 12 October 2000 Accepted: 7 January 2001     

A stereoselective total synthesis of the HCl salts of mycestericins F,G and ent-F

The total synthesis of the HCl salts of two natural sphingolipid-related amino acid derivatives, mycestericins F 4 and G 5 together with unnatural ent-4·HCl, starting from the four crucial scaffolds 6, 8, 9, 11 and utilizing the Wittig reaction to build the C₂₀ backbone, has been achieved. The selection of selective functional group interconversions accompanied with suitable protection–deprotection protocols in the coupling products 20 and 34 gave the desired structures.