Phosphoester Hydrolysis: The Incoming Substrate Turns the Bridging Hydroxido Nucleophile into a Terminal One

Identifying the active nucleophile in hydrolysis reactions catalyzed by binuclear hydrolases is a recurrent problem and a matter of intense debate. We report on the phosphate ester hydrolysis by a Fe^IIIFe^II complex of a binucleating ligand. This complex presents activities in the range of those observed for similar biomimetic compounds in the literature. The specific electronic properties of the Fe^IIIFe^II complex allowed us to use ¹H NMR and Mössbauer spectroscopies to investigate the nature of the various species present in the solution in the pH range of 5–10. Both techniques showed that the hydrolysis activity is associated to a μ‐hydroxido Fe^IIIFe^II species. Further ¹H NMR experiments show that binding of anions or the substrate changes this bonding mode suggesting that a terminal hydroxide is the likely nucleophile in these hydrolysis reactions. This view is further supported by the structure determination of the hydrolysis product.     

Benefits of mesopores in sulfated zirconia catalyst

Activities of two catalysts obtained from hydroxide and alkoxide precursors were correlated to their different structural and textural properties. Activity is mainly determined by the presence of active tetragonal phase, which is more stable at high temperatures in the catalyst of alkoxide origin than in one from hydroxide. An appropriate structure must be accompanied with beneficial pore structure, as is the case of mesopores of the catalyst from alkoxide. In the case of very small pores of the catalyst from hydroxide precursor, activity is negligible due to pores plugged by coke.     

Dihedral semigroups,their defining relations and an application to describing knot semigroups of rational links

It is known that a knot (link) is rational if and only if its \(\pi \)-orbifold group is dihedral. A semigroup version of this result has been formulated as a conjecture. Working towards proving the conjecture, we describe certain semigroups associated with twist links, clarify how these semigroups are related to dihedral groups and find defining relations of these semigroups.     

35-Cl nuclear quadrupole resonance spectra in diazepam and its 1:1 complex with chloral hydrate

The 35-Cl NQR spectra of diazepam and some of its related compounds are reported; they yield information about the hydrogen bonded complex that diazepam forms with chloral hydrate. The results reflect large changes in electron distribution at the CCl₃ group while the diazepam chlorine at the 7 position remains almost unaffected.     

Evidence for two light-induced metastable states in Cl3[Ru(NH3)5NO]H2O

Differential Scanning Calorimetry measurements on irradiated Cl₃[Ru(NH₃)₅NO]H₂O reveal the existence of two light-induced long-lived metastable states SI, SII. Irradiation with light in the spectral range 400–500 nm leads to the excitation of SI. For the first time we report experimental evidence for the state SII in this compound, which can be excited by transferring SI into SII with irradiation of light in the spectral range 1000–1200 nm. The excitation and transfer of the metastable states is described and the exponential decays are evaluated according to Arrhenius' law yielding activation energies of E_A(SI)=0.73(3) eV, E_A(SII)=0.66(3) eV and frequency factors of Z(SI)=1 × 10¹² s⁻¹, Z(SII) = 5 × 10¹² s⁻¹.     

Selective inhibition of Trypanosoma cruzi GAPDH by "bi-substrate" analogues

A new series of "bi-substrate" analogues have been synthesized as potential inhibitors of the glyceraldehyde-3-phosphate dehydrogenase and one lead compound has been identified that inhibits the enzyme from Trypanosoma cruzi with good affinity and very high (50-fold) specificity.     

Postsynthesis vapor-phase functionalization of MCM-48 with hexamethyldisilazane and 3-aminopropyldimethylethoxylsilane for bioseparation applications

MCM-48 was surface modified via vapor-phase reactions with hexamethyldisilazane (CH(3)-MCM-48) and 3-aminopropyldimethylethoxysilane (NH(2)-MCM-48). (29)Si NMR confirmed that the resulting materials contained covalently attached trimethylsilane and 3-aminopropyldimethylsilane moieties, both important functionalities for bioseparation applications. The surface coverage was approximately 1.8 and 0.9 groups per nm(2), respectively. The X-ray diffraction patterns and the narrow pore size distributions obtained from the gas sorption isotherms showed that the modified materials retained the characteristic pore structure of the underlying MCM-48 material. CH(3)-MCM-48 exhibited significantly improved hydrolytic stability over the unmodified MCM-48 under the aqueous conditions tested, whereas NH(2)-MCM-48 appeared to be less stable than the unmodified MCM-48. The decrease in stability is most likely due to the nature of the attachment of the 3-aminopropyldimethylsilane moiety, where the conversion of surface silanol groups is limited by H bonding with the amino end, leading to a 50% lower surface concentration and resulting in an increased likelihood of nucleophilic attack on the silica surface, enhancing the rate of hydrolysis. Hexamethyldisilazane thus appears to be a superior functional group for modifying the MCM-48 surface.     

Essential Oil Composition of Four <Emphasis Type="Italic">Achillea</Emphasis> Species from the Balkans and Its Chemotaxonomic Significance

The chemical composition of the essential oils of Achillea clavennae L., Achillea holosericea Sibth. & Sm., Achillea lingulata W. & K., and Achillea millefolium L. from the Balkans was determined by GC and GC/MS analyses. The main components were 1,8-cineole in A. holosericea, camphor in A. clavennae, β-pinene in A. millefolium, and τ-cadinol in A. lingulata. A detailed chemotaxonomic discussion is presented. __________ Published in Khimiya Prirodnykh Soedinenii, No. 6, pp. 555–558, November–December, 2005.     

New trends in the chemistry of iron(III) citrate complexes: correlations between X-ray structures and solution species probed by electrospray mass spectrometry and kinetics of iron uptake from citrate by iron chelators

Despite the crucial role of "iron(III) citrate systems" in the iron metabolism of living organisms (bacteria as well as plants or mammals), the coordination chemistry of ferric citrate remains poorly defined. Variations in the experimental conditions used for the preparation of so-called ferric citrates (iron salt, Fe:cit molar ratio, base, pH, temperature, solvent) lead to several different species, which are in equilibrium in solution. To date, six different anionic complexes have been structurally characterized in the solid state, by ourselves or others. In the work described herein, we have established the experimental conditions leading to each of them. Five were obtained from aqueous solution. With the exception of a nonanuclear species (of which fragments have been detected), all were identified in aqueous solution on the basis of electrospray ionization mass spectrometry. In addition, the spectra revealed a new trinuclear species, which could not be crystallized. Kinetic studies of iron uptake from citrate species by iron chelators confirmed the results indicated by the ESI-MS studies. These studies also allowed the relative molar fraction of mononuclear versus polynuclear complexes to be determined, which depends on the Fe:cit molar ratio.