Systematic Dissection of an Aminopyrrolic Cage Receptor for β‐Glucopyranosides Reveals the Essentials for Effective Recognition

A set of structures designed for the recognition of glucosides has been obtained by systematically destructuring a tripodal aminopyrrolic cage receptor that selectively recognizes octyl‐β‐D ‐glucopyranoside (OctβGlc). NMR spectroscopy and isothermal titration calorimetry binding measurements showed that cleavage of one pillar of the cage was beneficial to the binding properties of the receptor, as long as two residual amino groups of the cleaved pillar were present. Removal of these two residual amino groups produced a dramatic loss of affinity for OctβGlc of the resulting monocyclic analogue of the parent cage receptor. A significant improvement in the binding ability was achieved by replacing one pillar with two aminopyrrolic hydrogen‐bonding arms, despite the loss of a preorganized structure. In contrast to the cage receptor, recognition of OctβGlc was observed, even in a competitive medium (30 % DMF in chloroform). Structural studies in solution, carried out through NMR spectroscopy and molecular modeling calculations, led to the elucidation of the 3D binding modes of the side‐armed monocyclic receptors; this highlighted the key role of the amino groups and demonstrated the occurrence of a rotaxane‐like complex, which featured the octyl chain of the glucoside threaded through the macrocyclic ring.     

Synthesis,NMR spectra and chromatographic properties of five trimethylarsonioribosides

Five trimethylarsonioribosides were prepared from naturally occurring and synthetic dimethylarsinylribosides (arsenosugars) by reducing them with 2,3-dimercaptopropanol and quaternizing the resultant arsine with methyl iodide. The trimethylarsonioribosides prepared in this manner were the four novel compounds methyl 5-deoxy-5-trimethylarsonio-β-D -riboside (as the iodide), (2′R)-2′, 3′-dihydroxypropyl 5-deoxy-5-trimethylarsonio-β-D -riboside (as the formate), 3′-[(2″, 3″ -dihydroxypropyl)hydroxyphosphinyloxy] - 2′ -hydroxypropyl 5-deoxy-5-trimethylarsonio-β-D -riboside and 3-(5′-deoxy-5′-trimethylarsonio-β-D -ribosyloxy)-(2S)-2-hydro xypropanesulfonate, and the known (2′S)-2′-hydroxy-3′-(sulfooxy)propyl 5-deoxy-5-trimethylarsonio-β-D -riboside. They were synthesized to serve as standards for chromatographic analyses of arsenic compounds in marine samples and for investigations into the biotransformation of arsenic in marine organisms. NMR spectral and chromatographic data for the five trimethylarsonioribosides are presented and compared with those of their dimethylarsinyl analogues.     

Arsenic speciation in freshwater organisms from the river Danube in Hungary

Total arsenic and arsenic species were determined in a range of freshwater samples (sediment, water, algae, plants, sponge, mussels, frog and fish species), collected in June 2004 from the river Danube in Hungary. Total arsenic concentrations were measured by ICPMS and arsenic species were measured in aqueous extracts of the samples by ion-exchange HPLC-ICPMS. In order to separately determine the efficiency of the extraction method and the column recovery, total arsenic concentrations in the extracts were obtained in three ways: (i) ICPMS determination after acid digestion; (ii) flow injection analysis performed directly on the extract; (iii) the sum of arsenic species eluting from the HPLC column. Extraction efficiencies were low (range 10-64%, mean 36%), but column recovery was acceptable (generally >80%) except for the fish samples, where substantial, currently unexplained, losses were observed. The dominating arsenic species in the extracts of freshwater algae were arsenosugars, whereas arsenate [As(V)] was present only as a minor constituent. On the other hand, plant extracts contained only inorganic arsenic, except for two samples which contained trace amounts of dimethylarsinate (DMA) and the tetramethylarsonium cation (TETRA). The oxo-arsenosugar-phosphate (ca. 35% of extractable arsenic) and the oxo-arsenosugar-glycerol (ca. 20%) as well as their thio-analogues (1-10%) were found in the mussel extracts, while arsenobetaine (AB) was present as a minor species only. In general, fish extracts contained only traces of arsenobetaine, and the oxo-arsenosugar-phosphate was the major arsenic compound. In addition, samples of white bream contained thio-arsenosugar-phosphate; this is the first report of a thio-arsenical in a fish sample. The frog presented an interesting arsenic speciation pattern because in addition to the major species, arsenite [As(III)] (30%) and the tetramethylarsonium cation (35%), all three intermediate methylation products, methylarsonate (MA), dimethylarsinate and trimethylarsine oxide (TMAO), and arsenate were also present. Collectively, the data indicate that arsenobetaine, the major arsenical in marine animals, is virtually absent in the freshwater animals investigated, and this represents the major difference in arsenic speciation between the two groups of organisms.     

Human metabolism of arsenolipids present in cod liver

We report results from the first investigation of the human metabolism of arsenic-containing lipids (arsenolipids), significant arsenic constituents of some seafood products. Two male volunteers ingested canned cod liver and the arsenic metabolites in their urine were monitored by high-performance liquid chromatography inductively coupled plasma mass spectrometry over a 66-h period. Volunteer A consumed 85 g (wet mass) of cod liver containing a total of approximately 120 μg arsenic, 77% of which was present as arsenolipids, and volunteer B consumed 85 g (wet mass) of cod liver, 25% of which was present as arsenolipids, together with 20 g of cod liver oil, containing a total of about 180 μg arsenic. The structures of the arsenolipids are currently unknown, whereas the majority of the non-lipid arsenic in the cod liver was identified as arsenobetaine, which was excreted unchanged. The arsenolipids were rapidly metabolised to water-soluble compounds and excreted in the urine; peak arsenic concentrations were recorded between 7 and 15 h (volunteer A) and between 6.5 and 15 h (volunteer B), and by the end of the experiment about 90% of the ingested arsenic had been accounted for in the urine for both volunteers. The major arsenolipid metabolite was dimethylarsinate (DMA), constituting 73% (volunteer A) or 41% (volunteer B) of the total urinary arsenic, and most of the remaining arsenolipid-derived arsenic, constituting about 10% (volunteer A) and 5% (volunteer B), comprised four novel arsenic-containing fatty acids, namely oxo-dimethylarsenopropanoic acid, thio-dimethylarsenopropanoic acid, oxo-dimethylarsenobutanoic acid, and thio-dimethylarsenobutanoic acid. Unchanged arsenobetaine (15% for volunteer A and 51% for volunteer B) made up the remaining urinary arsenic together with trace quantities of other, mostly unknown, arsenicals. In a second experiment (volunteer A only), performed with pure cod liver oil, which contains only arsenolipids, DMA and the same four arsenic fatty acids were excreted in the urine. The study shows that arsenolipids in cod liver are bioavailable, and that they are quickly biotransformed to several water-soluble arsenicals, the structures of which suggest that the native arsenolipids contain a dimethylarsine oxide moiety.     

Cadmium ions and cadmium sulphide particles in γ-titanium dihydrogen phosphate. Synthesis, thermal behaviour and X-ray characterization

Layered compounds with CdS particles supported on the ion-exchanger, γ-titanium dihydrogen phosphate, were prepared by the stepwise reaction of the ion-exchanger and cadmium solution, followed by reaction with H₂S gas. The CdS content on the ion-exchanger is dependent on the timeframe of the H₂S gas flow. The materials obtained were layered, as shown by the X-ray measurements that exhibit both precursor and CdS diffraction peaks. The thermal treatment of the material obtained gives evidence of its stability (≤ 320°C) before the CdS decomposition, which occurs in a single step.      

Influence of steric and electronic properties of the defect site, lanthanide ionic radii, and solution conditions on the composition of lanthanide(III) alpha1-P2W17O6110- polyoxometalates

This study identifies the principles that govern the formation and stability of Ln complexes of the (alpha(1)-P(2)W(17)O(61))(10-) isomer. The conditional stability constants for the stepwise formation equilibria, K(1cond) and K(2cond), determined by (31)P NMR spectroscopy, show that the high log K(1cond)/log K(2cond) ratio predicts the stabilization of the 1:1 Ln/ (alpha(1)-P(2)W(17)O(61))(10-) species. The value of log K(1cond) increases as the Ln series is traversed, consistent with the high charge/size requirement of the basic alpha(1) defect site. The conditional stability constants, K(2), are very low and are highly dependent on the countercations in the buffer. The source of the instability is understood from the crystal structures of the early-mid lanthanide analogues, where the close contact of the (alpha(1)-P(2)W(17)O(61))(10-) units result in severe steric encumbrance. The electronic properties of the alpha(1) defect along with the lanthanide ionic radii and countercation composition are important parameters that need to be considered for a rational synthesis of lanthanide polyoxometalates.     

Negative magnetoresistance in Ba2CoS3

A small negative magnetoresistance and metallic-like behaviour has been detected for the first time in a one-dimensional sulfide containing Co2+.