Thermodynamics of the conversion of aqueous glucose to fructose

The thermodynamics of the conversion of aqueous glucose to fructose has been investigated using both heat conduction microcalorimetry and high pressure liquid chromatography (HPLC). The reaction was carried out in both aqueous Tris/HCl buffer and in aqueous phosphate buffer in the pH range 7–8 using the enzyme glucose isomerase and the cofactors CoCl₂ and MgSO₄. The temperature range over which this reaction was investigated was 298.15–358.15 K. We have found that the enthalpy of reaction is independent of pH over the range investigated. A combined analysis of both the HPLC and microcalorimetric data leads to the following results at 298 15 K:ΔG° = 349 ± 53 J mol^-1, ΔH° = 2.78 ± 0.20 kJ mol^-1, and ΔC _p ^° = 76 ± 30 J mol^-1 K^-1. The stated uncertainties are based upon an analysis of both the random and systematic errors inherent in the measurements. Comparisons are made with literature data. The percent conversion of glucose to fructose has been calculated for the temperature range 300–373.15 K.     

Photolysis of p-benzoquinone in N2 matrices at 12K: a novel decomposition pathway

Photolysis of _p-benzoquinone in N₂ matrices at 12K has been found to give acetylene and CO; 1,4-naphthoquinone and 9,10-anthraquinone, however, are inert under similar conditions.     

Enantioselective aziridination using copper complexes of biaryl Schiff bases

Racemic 2,2'-diamino-6,6'-dimethylbiphenyl is resolved using simulated moving bed chromatography, and the absolute configuration of the enantiomers is confirmed via the X-ray crystal structure of a derivative. The diamine is condensed with a range of aldehydes to give bidentate aldimine proligands L. Molecular structures of the complexes formed between L and Cu(I) fall into two classes; bimetallic double helices ([Cu(2)L(2)](2+)) and monometallic ([CuL](+)). The latter are strikingly more efficient in the aziridination of alkenes than are the former in terms of rate, turnover, and enantioselection. In particular, the imine ligand formed from the diamine and 2,6-dichlorobenzaldehyde gives, in combination with Cu(I) or Cu(II), up to 99% ee in the aziridination of 6-acyl-2,2-dimethylchromene and 88-98% ee for a range of cinnamate esters. Styrenic and other alkenes are converted with lower selectivities (5-54%). The catalytic system shows a linear response in product ee to catalyst ee, and the product ee does not vary significantly during the reaction. UV spectrophotometric investigations indicate that conversion of Cu(I) to Cu(II) is not essential for catalysis but that Cu(II) is probably also a competent system.     

Effect of modified virtual orbitals on local Coulomb correlation holes in FCN

Local Coulomb correlation hole distribution functions may be used to assess the extent to which electron correlation effects are present in large scale SCF + CI wave functions. From a set of modified virtual orbitals, ordered according to their interaction with the SCF configuration, we have constructed a limited SCF + CI wave function with improved convergence characteristics with respect to that formed from the canonical virtual orbital set. These wave functions, of the same size yet with different energies, have been used to examine the range and depth of local Coulomb correlation holes in FCN. In all cases, the depth of the local Coulomb hole is no more than 10% or so of that of the corresponding Fermi hole, and the range Fermi correlation is generally less than that of Fermi correlation. This is particularly marked in the high density regions around the nuclei. The significance of our results is discussed in relation to a recent proposal for the incorporation of Coulomb correlation into the local exchange method.     

Compton profiles of solvated electrons

The importance of applying a variety of experimental techniques to unravel the nature of solvated electrons is emphasized. Compton profiles are evaluated for these species from a range of models. Some comparisons are made with positron annihilation studies.     

Synthesis of NSC-341,964: An unexpected study on the stability of 2-aryl-4H-3,1-benzoxazin-4-ones

Resynthesis of NSC 341,964, which had been assigned structure 1 (1-[[3-(7-chloro-4-oxo-4H-3,1-benzoxazin-2-yl)phenyl]methyl]pyridinium chloride) was approached via 7-chloro-2-(3-methylphenyl)-4H-3,1-benzoxazin-4-one ( 5 ) obtained from 3-methylbenzoyl chloride ( 2 ) and 2-amino-4-chlorobenzoic acid ( 3 ) followed by dehydration in acetic anhydride. Radical bromination provided 6 which with pyridine afforded the bromide analog 7 of 1 . Ion exchange, however, gave ring-opened benzoic acid 8 rather than 1 . The original sample of NSC 341,964 also proved to be ring-opened material. However, 7 upon standing exhibited slow hydrolysis to 8 so that the structure of the original NSC 341,964 remains uncertain. A more direct route to compound 8 is also described.     

The roles of template complexation and ligand binding conditions on recognition in bupivacaine molecularly imprinted polymers

A model for the molecular basis for ligand recognition in bupivacaine imprinted methacrylic acid-ethylene glycol dimethacrylate co-polymers has been developed based upon a series of (1)H-NMR studies in conjunction with HPLC and radioligand binding analyses. (1)H-NMR studies indicated that functional monomer-template complexes survive the polymerisation process, at least up until the stage of gelation. Polymers were synthesised and characterised by surface area analysis (BET), FT-IR and SEM. A combination of zonal and frontal chromatographic studies in aqueous and non-polar media indicate that selectivity arises from a combination of hydrophobic and electrostatic interactions. However, in the concentration regime employed for LC-based studies, ligand recognition in aqueous media was shown to be predominantly non-specific and hydrophobic in character. Radioligand binding studies, in lower ligand binding concentration regimes, permitted closer examination of the higher affinity binding sites. It was shown that the presence of a polar modifier in a non-polar solvent, or an organic modifier in water, produced enhanced selectivity. Variable temperature studies showed that the temperature of binding influences selectivity as well as the apparent number of sites available and that this effect is different in organic and aqueous environments. This indicates that the system studied is more complex in character than is generally appreciated. A comparison of the techniques employed here indicates that although chromatographic studies provide a valuable first-round screen for polymer-ligand selectivities, the level of detail obtainable using radioligand binding studies (lower concentrations and true equilibrium binding) makes them superior for detailed evaluations of molecularly imprinted polymers.     

Formation and structure of a novel enediyne-RNA base covalent adduct

The structure of an unusual covalent adduct formed by thiol-activated neocarzinostatin chromophore (NCS-chrom) and a RNA-DNA hybrid having an overhang of four unpaired residues at the 3'-end of the RNA strand has been elucidated by MS and NMR spectroscopic analyses. Unlike previously characterized adducts formed by NCS-chrom on the sugar residue of the DNA target, this adduct has been found to be on one of the uracil bases in the RNA overhang. Covalent linkage is between C-6 of the post-activated NCS-chrom and C-5 of the uracil. A novel mechanism involving adduction of the NCS-chrom C-6 radical, generated by 2-mercaptoethanol activation, to C-5 of the uracil at the U9 position of the RNA 11-mer, oxidation by dioxygen, reduction by the thiol, and subsequent dehydration is proposed for adduct formation.     

Actin-binding marine macrolides: total synthesis and biological importance

Marine organisms produce a fascinating range of structurally diverse secondary metabolites, which often possess unusual and sometimes unexpected biological activities. This structural diversity makes these marine natural products excellent molecular probes for the investigation of biochemical pathways. Recently, a number of novel and stereochemically complex macrolides, having a large macrolactone (22- to 44-membered) ring, that interact with the actin cycloskeleton have been isolated from different marine sources. Actin, like tubulin, is a major component of the cytoskeleton and has important cellular functions. Although the details of these interactions are still under investigation, these marine macrolides are becoming increasingly important as novel molecular probes to help elucidate the cellular functions of actin. Owing to their potent antitumor activities, these compounds, for example the aplyronines, also have potential for preclinical development in cancer chemotherapy. Their appealing molecular structures, with an abundance of stereochemistry, and biological significance, coupled with the extremely limited availability from the marine sources, have stimulated enormous interest in the synthesis of these compounds. This review summarizes the biological properties of these unusual marine natural products and features the recently completed total syntheses of swinholide A, scytophycin C, aplyronine A, mycalolide A--all of these being potent cytotoxic agents that target actin--and a diastereoisomer of ulapualide A. Rather than detailing each individual step of these multistep total syntheses, the different synthetic strategies, key reactions, and methods adopted for controlling the stereochemistry are compared.     

The pzc of mercury in the presence of humic acids and their complexes with aluminium

Variation of the lifetime of a mercury drop with potential was used to determine the pzc of mercury in the presence of soil humic acids and their aluminium complexes. In all cases there was an overall net shift in the pzc in the cathodic direction. Variation in the extent of the shift with pH and concentration indicated greater adsorption of negatively charged species. The shift was smaller in the presence of aluminium humates, probably due to a decrease in the negative charge of the humic acid molecules after complexing. The use of electroanalytical techniques for metal speciation studies in soils and natural water, if humic materials are known to be present, might therefore be limited.