Cations M2(O2CtBu)4+, where M = Mo and W, and MoW(O2CtBu)4+. Theoretical, spectroscopic, and structural investigations

With the aid of density function theory, the molecular and electronic structures of the molecules Mo2(O2CMe)4, MoW(O2CMe)4, and W2(O2CMe)4 and their single-electron oxidized radical cations have been determined; this includes calculated observables such as v(MM) and the delta --> delta* electronic transition energies. The calculated properties are compared with those for the corresponding pivalates, M2(O2CtBu)4 (M = Mo or W) and MoW(O2CtBu)4 and their radical cations prepared in situ by oxidation with Cp2FePF6. The EPR spectra of the radical cations are also reported. The EPR spectrum of the MoW(O2CtBu)4+ cation reveals that the unpaired electron is in a polarized MM delta orbital having 70% Mo and 30% W character. The MM stretching frequencies show good correlation with the MM bond lengths obtained from single-crystal X-ray diffraction studies of MoW(O2CtBu)4, W2(O2CtBu)4, and W2(O2CtBu)4+PF6- compounds, along with previously reported structures. These data provide benchmark parameters for valence trapped dicarboxylate bridged radical cations of the type [(tBuCO2)3M2]2(micro-O2C-X-CO2)+ (X = conjugated spacer).     

Comprehensive metabolic profiling of mono- and polyglutamated folates and their precursors in plant and animal tissue using liquid chromatography/negative ion electrospray ionisation tandem mass spectrometry

This work reports the use of reversed-phase ion-pair chromatography coupled to electrospray ionisation mass spectrometry for the simultaneous profiling of folate-based metabolites including natural folates, their polyglutamatyl derivatives and their biosynthetic precursors in plant and animal tissue. A simple sample preparation method, using 0.1% citric acid and ascorbic acid in ice-cold methanol, was used to extract and stabilise the folates, and three internal standards were used. Chromatography was on a C18 column using slow gradient elution with a mobile phase consisting of methanol/water with 5 mM dimethylhexylamine. Mass spectrometric detection was performed by multiple reaction monitoring in seven separate time windows in negative ion mode over the 25 min run time. Full, quantitative analysis was obtained for 16 folates and a 'semi-quantitative' analysis was possible for all other folates with up to eight conjugated glutamate residues by reference to structurally related calibration standards. The precision, accuracy and recovery of the method were generally within the accepted guidelines for a quantitative bioanalytical method and the method was linear over the range 0.2 to 10 ng of individual folate per sample. The method was applied to profile mono- and polyglutamated tetrahydrofolates (including subcellular analysis) in a range of plant species, including Arabidopsis, spinach, Brassica and wheat; the technique was also successfully applied to the profiling of folates in mouse tissue.     

A 119mSn Mössbauer study of the photochemical degradation of organotin stabilised PVC

Although organotin compounds are widely used as thermal and photochemical stabilisers for PVC, their mode of action and their fate on degradation of the polymer are not yet fully understood. ^119mSn Mössbauer spectroscopy has been used to study chemical changes undergone by two commonly used stabilisers, dibutyltinbis(iso-octylthioglycollate) and dibutyltinbis(iso-octylmaleate), during irradiation of stabilised PVC in artificial sunlight. After 1000 hours' exposure, the maleate stabiliser is chemically unchanged, whereas the thioglycollate is rapidly converted to the monochloroester, Bu₂SnCl(SCH₂CO₂C₈H₁₇). Prolonged exposure of the thioglycollate stabilised polymer leads to the formation of stannic oxychloride.     

On the electron delocalization in the radical cations formed by oxidation of MM quadruple bonds linked by oxalate and perfluoroterephthalate bridges

Electron paramagnetic resonance, electronic absorption, and resonance Raman spectroscopy reveal that in the oxalate-bridged compounds, [[(tBuCO2)3M2]2(mu-O2CCO2)]+[PF6]-, the unpaired electron is delocalized over four metal centers (M = Mo or W) as a result of M2 delta to bridge pi conjugation, but in the related cationic perfluoroterephthalate-bridged species, the tungsten complex is delocalized and the molybdenum analogue valence trapped.     

Electronically coupled MM quadruply-bonded complexes (M = Mo or W) employing functionalized terephthalate bridges: toward molecular rheostats and switches

Toluene solutions of M2(O2C(t)Bu)4 (M = Mo, W; 2 equiv) react with a range of functionalized terephthalic acids, HO2CArCO2H (Ar = C6H4, C6F4, C6Cl4, C6H2-2,5-Cl2, C6H2-2,5-(OH)2, C6H3-2-F), to give [(tBuCO2)3M2]2[mu-O2CArCO2]. These compounds show intense ML(bridge)CT absorptions in the visible region of the electronic spectrum, and the terephthalate bridge serves to electronically couple the two M2 units via interactions between the M2 delta and bridge pi orbitals. Electronic structure calculations reveal how the degree of electronic coupling is controlled by the dihedral angles between the terephthalate C6 ring and the two CO2 units and the degree of interaction between the M4 delta MOs and the LUMO of the bridge. Both of these factors are controlled by the aryl substituents, and collectively these determine the thermochromism displayed by these complexes in solution together with the physical properties of the oxidized radical cations as determined by electrochemical studies (CV, DPV), UV-vis-NIR and EPR spectroscopic methods.     

Open-cage fullerene derivatives with 15-membered-ring orifices

The addition reaction of the N-MEM-ketolactam derivative of [60]fullerene with phenyl, p-Br-phenyl, and p-MeO-phenyl hydrazines proceeds regioselectively, affording three open-cage fullerene derivatives bearing a 15-membered-ring orifice on the fullerene cage. Both experimental data and theoretical calculations were utilized for the structure determination of the new [60]fullerene adducts.     

Routine liquid charomatography/fast atom bombardment mass spectrometry of peptides and enzymatically digested proteins

The development of a high-performance liquid chromatography (HPLC)/fast atom bombardment (FAB) interface and the subsequent commercial availability of such systems has facilitated the routine application of the technique to problems in pharmaceutical research and development. Although many products are amenable to FAB analysis and hence LC/FAB, the greatest benefit of the interface has been in the field of peptide and protein analysis. It has been found that, even with post-column matrix addition, chromatographic resolution is maintained and, by plotting mass chromatograms, the resolution may be greater than that achieved by the less specific UV detector. As only 1% matrix is required in the final eluent, the system is stable for extended periods and has been used for 3 h LC/FAB experiments or used continuously for multiple analyses over 8 h periods. In addition to the acquisition of relative molecular mass information, the constant background can be completely subtracted to yield structurally significant fragment ions which may allow sequencing of components from the single LC/FAB experiment. Applications of LC/FAB to date include the characterization of Iys(78)-plasminogen by the on-line analysis of complex mixtures of peptides resulting from the various enzymatic digests.     

Half-sandwich arene ruthenium(II)-enzyme complex

The 1.6 [Angstrom] X-ray crystal structure of [(eta(6)-p-cymene)Ru(lysozyme)Cl(2)], the first of a half-sandwich complex of a protein, shows selective ruthenation of Nepsilon of the imidazole ring of His15.     

In vivo determination of amino acid bioavailability in humans and model animals

Because the digestion of many dietary proteins is incomplete, and because there is a continuous (but variable) entry into the intestinal lumen of endogenous protein and amino acid nitrogen that is also subject to digestion, the fluxes of nitrogen, amino acids, and protein in the gut exhibit a rather complicated pattern. Methods to distinguish and quantitate the endogenous and dietary components of nitrogen and amino acids in ileal chyme or feces include the use of a protein-free diet, the enzyme-hydrolyzed protein method, different levels of protein intake, multiple regression methods, and stable-isotope labelling of endogenous or exogenous amino acids. Assessment of bioavailability can be made, with varying degrees of difficulty, in man directly but, for routine evaluation of foods, the use of model animals is attractive for several reasons, the main ones being cost and time. Various animals and birds have been proposed as models for man but, in determining their suitability as a model, their physiological, enzymological, and microbiological differences must be considered. Fecal or ileal digestibility measurements, as well as apparent and true nitrogen and amino acid digestibility measurements, have very different nutritional significance and can, thus, be used for different objectives. Measurements at the ileal level are critical for determining amino acid losses of both dietary and endogenous origin, whereas measurements at the fecal level are critical in assessing whole-body nitrogen losses. A complementary and still unresolved aspect is to take into account the recycling of intestinal nitrogen and bacterial amino acids to the body.     

Revised structure of palau’amine

The structure of palau’amine, a bioactive hexacyclic pyrrole-imidazole alkaloid, from the sponge Stylotella aurantium was revised by detailed analysis of its 2D ROESY and 1D NOESY data.