Synthesis of the O-linked hexasaccharide containing β-D-Galf-(1→2)-β-D-Galf in Trypanosoma cruzi mucins

The hexasaccharide β-D-Galp-(1→2)-[β-D-Galp-(1→3)]-β-D-Galp-(1→6)-[β-D-Galf(1→2)-β-D-Galf(1→4)]-D-GlcNAc (1) is the largest carbohydrate structure released as alditol by reductive β-elimination from mucins of some strains of T. cruzi. The terminal β-D-Galp units are sites of sialylation by trans-sialidase which transfers sialic acid from the host to the parasite. Hexasaccharide 1 was synthesized by a [3 + 3]-convergent strategy based on a nitrile assisted glycosylation, using the trichloroacetimidate method. The β-D-Galf-(1→2)-β-D-Galf-D-GlcNAc synthon was sequentially constructed from the reducing end to the non-reducing end employing benzyl α-D-galactofuranoside as starting material for the internal Galf unit. The choice of this novel precursor, obtained in one-reaction step from galactose, allowed the introduction of an orthogonal and participating levulinoyl group at O-2. Thus, the diastereoselective construction of the Galf-β(1→4)-GlcNAc linkage by the trichloroacetimidate method of glycosylation was achieved. The (1)H NMR spectrum of alditol 2 was identical to the product released by β-elimination from the parasite mucin.     

A combined method of neutron activation analysis and radiometric measurements for <Superscript>234</Superscript>U and <Superscript>238</Superscript>U determination in soil samples of low uranium concentration

A method that combines the use of non-destructive neutron activation analysis and high-resolution α spectrometry has been developed for determination of the activities of ²³⁴U and ²³⁸U in geological samples of low uranium content. The ²³⁸U content is determined by k₀-based neutron activation analysis, whereas the ²³⁴U/²³⁸U relationship is measured by α spectrometry after isolation and electrodeposition of the uranium extracted from a lixiviation with 6 M HCl. The main advantage of the method is the simplicity of the chemical operations, including the fact that the steps destined to assure similar chemical state for the tracer and the uranium species present in the sample are not necessary. The method was applied to soil samples from sites of the North Peru Coast. Uranium concentration range 3–40 mg/kg and the isotopic composition correspond to natural uranium, with about 10% uncertainty.     

Extending the formula to calculate the spectral radius of an operator

In a Banach space, Gelfand's formula is used to find the spectral radius of a continuous linear operator. In this paper, we show another way to find the spectral radius of a bounded linear operator in a complete topological linear space. We also show that Gelfand's formula holds in a more general setting if we generalize the definition of the norm for a bounded linear operator.

     

Absence of phosphate hydrolysis in the nucleotide substitution reaction on cis-[Co(H2O)2(cyclen)] at physiological pH: Importance of hydrogen-bonding and conjugate base-catalysis

The use of classical Werner-type cis-[Co(Cl)₂(tetraamine)]⁺ (tetraamine = cyclen or tren) complexes for their complexation study of biologically relevant ligands has been pursued. These chlorocomplexes are found to be in the chloroaqua/chlorohydroxo forms under the physiological conditions used, their chloride substitution reactivity being dominated by conjugate base pathways, specially when tetraamine = cyclen. Further studies with nucleotides indicate that the substitution processes on cis-[Co(H₂O)₂(tetraamine)]³⁺, up to neutral pH, correspond to a simple reaction producing final stable phosphato bound mononucleotide complexes. These complexes are found to be an equilibrium mixture between monodentate O-phosphato and chelate O-phosphato-N-nucleotide forms. No evidence has been found for hydrolytic cleavage of the phosphato-nucleoside bond, as found in other systems with activated phosphates or higher pH values. A full kinetic profile of the process is proposed for the systems in the 2–7 pH range which is the same for chloride, nucleoside and nucleotide substitutions. The results are indicative of an important degree of outer-sphere hydrogen bonding between the cobaltocomplex and the entering biologically relevant ligands, as expected for these processes.     

Palladium(II) coordination and cyclometallated complexes derived from 3- and 5-aryl-substituted pyrazoles

Palladium(II) coordination complexes of nine 3- or 5-arylpyrazoles (phenyl, 2-bromophenyl, or 3-methoxyphenyl), as well as of 3,5-diphenylpyrazole, are reported. A cis-trans mixture of [PdL₂Cl₂] isomers is found in the case of 3-aryl-1-methylpyrazoles, the cis-isomers being transformed into trans by heating. Only trans isomers are isolated with the other ligands. Cyclopalladation of 3-ary]-1-methylpyrazoles can be performed with palladium(II) acetate, and the resultant μ-acetate bridged dimers can be transformed into μ-chloro bridged dimers or acetylacetonate monomers. The structures of the complexes have been characterized by ¹H- and ¹³C-NMR spectroscopy.     

2‐[3‐Furyl­(hydroxy)­methyl]‐2,3‐di­methyl­cyclo­hexanone

In the mol­ecule of the title compound, C₁₃H₁₈O₃, there is a syn relationship between the two vicinal methyl groups. The six‐membered ring adopts a chair conformation, with one equatorial and two axial groups, and the furyl group is almost parallel to the ketone group. Intermolecular hydrogen bonds [O—H?O=C 2.814 (3) Å] form chains along [100].     

Synthesis and conformational studies on hexa-O-alkyl p-unsubstituted calix[6]arenes

[structure: see text] In this article we describe the selective O-benzylation of para-unsubstituted calix[6]arene 1 in rings 1 and 4 (2a-c) and the subsequent alkylation of phenol groups with alpha-haloesters (methyl esters 3a, 3c, and 3e; tert-butyl esters 3b, 3d, and 3f) to determine the effect of these groups on their conformational behavior. 2D NMR studies at 188 K reveal that compounds 2a-c, 3b, 3d, and 3f are less flexible, showing a 1,2,3-alternate conformation. The same conformation has been observed in the solid state.