DFT investigation of structural, electronic, and catalytic properties of diiron complexes related to the [2Fe](H) subcluster of Fe-only hydrogenases

Hydrogenases catalyze the reversible oxidation of dihydrogen to protons and electrons. The structures of two Fe-only hydrogenases have been recently reported [Peters, J. W.; Lanzilotta, W. N.; Lemon, B. J.; Seefeldt, L. C. Science 1998, 282, 1853-1858. Nicolet, Y.; Piras, C.; Legrand, P.; Hatchikian, E. C.; Fontecilla-Camps, J. C. Structure 1999, 7, 13-23], showing that the likely site of dihydrogen activation is the so-called [2Fe](H) cluster, where each Fe ion is coordinated by CO and CN(-) ligands and the two metals are bridged by a chelating S-X(3)-S ligand. Moreover, the presence of a water molecule coordinated to the distal Fe2 center suggested that the Fe2 atom could be a suitable site for binding and activation of H(2). In this contribution, we report a density functional theory investigation of the structural and electronic properties of complexes derived from the [(CO)(CH(3)S)(CN)Fe(II)(mu-PDT)Fe(II)(CO)(2)(CN)](-1) species, which is related to the [2Fe](H) cluster observed in Fe-only hydrogenases. Our results show that the structure of the [2Fe](H) cluster observed in the enzyme does not correspond to a stable form of the isolated cluster, in the absence of the protein. As a consequence, the reactivity of [(CO)(CH(3)S)(CN)Fe(II)(mu-PDT)Fe(II)(CO)(2)(CN)](-1) derivatives in solution may be expected to be quite different from that of the active site of Fe-only hydrogenases. In fact, the most favorable path for H(2) activation involves the two metal atoms and one of the bridging S atoms and is associated with a very low activation energy (5.3 kcal mol(-1)). The relevance of these observations for the catalytic properties of Fe-only hydrogenases is discussed in light of available experimental and theoretical data.     

Density functional theory investigation of the active site of Fe-hydrogenases. systematic study of the effects of redox state and ligands hardness on structural and electronic properties of complexes related to the [2Fe](H) subcluster

Density functional theory has been used to investigate complexes related to the [2Fe](H) subcluster of [Fe]-hydrogenases. In particular, the effects on structural and electronic properties of redox state and ligands with different sigma-donor pi-acceptor character, which replace the cysteine residue coordinated to the [2Fe](H) subcluster in the enzyme, have been investigated. Results show that the structural and electronic properties of fully reduced Fe(I)Fe(I) complexes are strongly affected by the nature of the ligand L, and in particular, a progressive rotation of the Fe(d)(CO)(2)(CN) group, with a CO ligand moving from a terminal to a semibridged position, is observed going from the softest to the hardest ligand. For the partially oxidized Fe(I)Fe(II) complexes, two isomers of similar stability, characterized either by a CO ligand in a terminal or bridged position, have been observed. The switching between the two forms is associated with a spin and charge transfer between the two iron atoms, a feature that could be relevant in the catalytic mechanism of dihydrogen activation. The structure of the fully oxidized Fe(II)Fe(II) models is extremely dependent on the nature of the L ligand; one CO group coordinated to Fe(d) switches from terminal to bridging position going from complexes characterized by neutral to anionic L ligands.     

Liquid chromatography/tandem mass spectrometry for the identification and determination of trichothecenes in maize

A reliable, sensitive and selective liquid chromatography/tandem mass spectrometry (LC/MS/MS) method has been developed to determine four trichothecene mycotoxins (nivalenol, deoxynivalenol, fusarenon X and 3-acetyldeoxynivalenol) in maize. Sample preparation was performed by extracting the analytes with a mixture of acetonitrile and water, followed by a solid-phase extraction with Carbograph-4 cartridges as the purification step. For the LC/MS/MS analysis two interfacing systems, Turbo IonSpray (TISP) and atmospheric pressure chemical ionization (APCI), were compared in both negative and positive ion modes. LC and MS parameters were optimized to achieve better results and sensitivity. The effect of mobile phase modifiers such as ammonium acetate and formic acid on the ionization yield was also evaluated. The best results were obtained using the electrospray ionization (ESI) interface in negative ion mode and the multiple reaction monitoring mode (MRM) for the quantitation. The detection limits ranged between 10 ng/g for fusarenon X and 1.5 ng/g for deoxynivalenol. A linear working range was achieved with a standard deviation between 3 and 10% and recovery rates from the maize samples above 81%. The procedure was applied to the analysis of a set of maize samples collected from farms located in different areas of northern and central Italy. The investigated samples turned out to be contaminated primarily with deoxynivalenol and, to a minor extent, with its derivatives.     

Photometrische Titration der Carboxylendgruppen in Poly?thylenterephthalat

Zusammenfassung Es wird eine schnelle Methode zur Bestimmung von Carboxylendgruppen in Polyäthylenterephthalat beschrieben; sie beruht auf der Messung der Farbänderung von Bromphenolblauindicator während der Titration einer Lösung des Polymeren in o-Kresol/Chloroform mit alkoholischer Lauge.Resultate von Bestimmungen an einer organischen Säure und an Polymerproben werden gegeben. Die Resultate von Bestimmungen an Polymeren wurden mit denen nach der von Pohl beschriebenen visuellen, nach der von Maurice beschriebenen potentiometrischen Methode und nach der von Ward beschriebenen Infrarotmethode verglichen und gleiche Resultate erhalten. Die photometrische Methode wird aus verschiedenen Gründen bevorzugt.

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Photometric titration of carboxyl end groups in polyethylene terephthalate

The method is based on the continuous measurement of the changes in absorbance of bromophenol blue indicator during titration of a solution of the polymer in o-cresol/chloroform with standard alcoholic potassium hydroxide solution.Results are given of tests with an organic acid and polymers amples. The results obtained with the polymer samples are compared with those obtained by the visual titration method of Pohl, the potentiometric titration method of Maurice and the infrared method of Ward. The four methods yield the same results. For several reasons the photometric method is preferred.

     

Metabolic profiling of urinary organic acids by single and multicolumn capillary gas chromatography

High-resolution gas chromatography (HRGC) and gas chromatography/mass spectrometry (GC/MS) are the techniques of choice to determine the retention indices of more than 200 organic acids as their trimethylsilyl (TMS) or oxime-trimethylsilyl derivatives. Several types of apolar and semipolar fused-silica capillary columns (OV-1, SE-52, and OV-1701), used to analyze and separate organic acids isolated from urine samples, are evaluated.     

The structures of [C4H4]+˙ ions by photodissociation in an ion cyclotron resonance spectrometer

The photodissociation of [C₄H₄]⁺˙ fragment ions at the ion cyclotron resonance time-scale competes with relaxation of the internal energy by infrared emission. As a result the fraction of photodissociating ions increases with light intensity. The experiments indicate that [C₄H₄]⁺˙ from benzene and 1,5-hexadiyne consists of a mixture of 60% vinyl acetylene ions, 10% butatriene ions and 30% cyclic ions. This confirms previous conclusions from studies of the ion-molecule reactions of [C₄H₄]⁺˙ with benzene.     

PHOTOSENSITIZING PROPERTIES OF BACTERIOCHLOROPHYLLIN a and BACTERIOCHLORIN a, TWO DERIVATIVES OF BACTERIOCHLOROPHYLL a

Abstract The photosensitizing properties of two water soluble derivatives of bacteriochlorophyll a , bacteriochlorophyllin a and bacteriochlorin a (lacking the central Mg-ion) were investigated and compared to those of hematoporphyrin derivatives. At physiological pH the oxygen consumption rate of histidine, tryptophan, dithiothreitol and guanosine upon illumination was 3 to 4 times higher when bacteriochlorin a was used as photosensitizer than when hematoporphyrin derivatives were used. Especially bacteriochlorin a proved to be an effective sensitizer for the killing of L929 cells. Because bacteriochlorin a has an absorption maximum at 765 nm in phosphate buffered saline (allowing a light penetration in tissue about ten times larger than at 630 nm) and a high molar absorption coefficient (32 000 M cm⁻¹) it has promising possibilities for the application in photodynamic therapy.     

The chemical ionization mass spectra of 2-tert-butyl-substituted 1,3-cycloalkanediol diacetates and dimethyl ethers

The chemical ionization (CI) mass spectra of the 2-tert-butyl-substituted 1,3-cyclopentane- and 1,3-cyclohexanediol diacetates and dimethyl ethers have been determined using isobutane and methane as reagent gases. From the differences in the spectra of these compounds, it clearly follows that steric and conformational effects are expresssed in the CI mass spectra. The relative impact of these effects, however, is strongly dependent on diol derivatization and 2-alkyl substitution.     

The signal vs. concentration relationships in fluorimetry

Formulae for the fluorescence signal are derived which take into account some factors hitherto neglected. From these formulae conditions are deduced, for which a linear relationship exists between the concentration of the fluorescent components and the intensity of the fluorescence. In some cases of practical interest the fulfilment of these conditions cause too low a sensitivity. For such cases a method of optimization is outlined, by means of which the conditions are found which lead to the least deviation from linearity at the required sensitivity.