Excitonic recombination in superstoichiometric nanocrystalline TiO2 grown by cluster precursors at room temperature

Unprecedented room temperature excitonic emissions are achieved from TiO(2) nanocrystals synthesized at 300 K by supersonic cluster beams. Transmission electron microscopy studies show the crystalline nature of the nanoparticles (NPs) with a diameter ranging from 5 to 30 nm. All the samples show mixed rutile and anatase phases as confirmed by Raman spectroscopy. XPS core level analyses evidence an O/Ti ratio of the as-grown nanoparticles of 2.30 ± 0.04. Two room temperature cathodoluminescence excitonic peaks observed at 3.16 and 3.25 eV are ascribed to the coexistence of rutile and anatase crystallographic phases respectively. Subsequent thermal treatments at 450 °C cause the complete quenching of the UV excitonic emissions and result in a more conventional broad visible band centered at 2.5 eV. HRTEM and XPS studies reveal that, after annealing, the NPs remain single crystals in nature with an O/Ti ratio of 2.20 ± 0.04. These results suggest a correlation between the emission properties and the oxygen concentration of our NPs. The achieved ability to tune the optical properties of TiO(2) nanoparticles is very promising for sensing and energy applications.     

Numeration and enumeration

In this paper, numeration systems defined by recurrent sequences are considered. We present a class of recurrences yielding numeration systems for which the words corresponding to greedy expressions for natural numbers are easily described. Those sequences, in turn, enumerate classes of words with forbidden substrings.     

Room temperature ionic liquids structure and its effect on the mononuclear rearrangement of heterocycles: an approach using thermodynamic parameters

The kinetics of the rearrangement of the Z-phenylhydrazone of 3-benzoyl-5-phenyl-1,2,4-oxadiazole (1) into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole (2) induced by amines have been studied in five room-temperature ionic liquids (RTILs) at different temperatures. The kinetic data collected show that both cationic and anionic parts of RTILs significantly influence the reactivity of the title reaction. The calculated activation parameters allow us to advance hypotheses about the weak interactions operating in RTIL solutions.     

A polarizable continuum approach for the study of heterogeneous dielectric environments

We present a computational method, exploiting some features of the polarizable continuum model (PCM) to describe heterogeneous media; it belongs to the family of electrostatic embedding mixed methods, such as the more common quantum-mechanical (QM)/molecular mechanics approaches, with the electrostatic long range effects accounted for by a polarized continuum instead of atomic point charges. Provided effective dielectric constants are determined for the various parts of the system, the method is much faster than its atomistic counterpart, and allows for high-level QM calculations on the fragment of interest, using all the highly efficient computational tools developed for homogeneous PCM. Two case studies (the calculation of the pKa of solvent exposed acidic residues in a model protein, and the calculation of the electron spin resonance spectrum of a typical spin probe partially embedded in a membrane) are analyzed in some detail, to illustrate the application of the method to complex systems.     

Coordinated and reversible reduction of enzymes involved in terminal oxidative metabolism in skeletal muscle mitochondria from a riboflavin-responsive, multiple acyl-CoA dehydrogenase deficiency patient

In this case report we studied alterations in mitochondrial proteins in a patient suffering from recurrent profound muscle weakness, associated with ethylmalonic-adipic aciduria, who had benefited from high dose of riboflavin treatment. Morphological and biochemical alterations included muscle lipid accumulation, low muscle carnitine content, reduction in fatty acid beta-oxidation and reduced activity of complexes I and II of the respiratory chain. Riboflavin therapy partially or totally reversed these symptoms and increased the level of muscle flavin adenine dinucleotide, suggesting that aberrant flavin cofactor metabolism accounted for the disease. Proteomic investigation of muscle mitochondria revealed decrease or absence of several flavoenzymes, enzymes related to flavin cofactor-dependent mitochondrial pathways and mitochondrial or mitochondria-associated calcium-binding proteins. All these deficiencies were completely rescued after riboflavin treatment. This study indicates for the first time a profound involvement of riboflavin/flavin cofactors in modulating the level of a number of functionally coordinated polypeptides involved in fatty acyl-CoA and amino acid metabolism, extending the number of enzymatic pathways altered in riboflavin-responsive multiple acyl-CoA dehydrogenase deficiency.     

NO* release from MbFe(II)NO and HbFe(II)NO after oxidation by peroxynitrite

In this work, we showed that the reaction of peroxynitrite with MbFe(II)NO, in analogy to the corresponding reaction with HbFe(II)NO (Herold, S. Inorg. Chem. 2004, 43, 3783-3785), proceeds in two steps via the formation of MbFe(III)NO, from which NO* dissociates to produce iron(III)myoglobin (Mb = myoglobin; Hb = hemoglobin). The second-order rate constants for the first steps are on the order of 10(4) and 10(3) M(-1) s(-1), for the reaction of peroxynitrite with MbFe(II)NO and HbFe(II)NO, respectively. For both proteins, we found that the values of the second-order rate constants increase with decreasing pH, an observation that suggests that HOONO is the species responsible for oxidation of the iron center. Nevertheless, it cannot be excluded that the pH-dependence arises from different conformations taken up by the proteins at different pH values. In the presence of 1.2 mM CO2, the values of the second-order rate constants are larger, on the order of 10(5) and 10(4) M(-1) s(-1), for the reaction of peroxynitrite with MbFe(II)NO and HbFe(II)NO, respectively. The pH-dependence of the values for the reaction with MbFe(II)NO suggests that ONOOCO2- or the radicals produced from its decay (CO3*-/NO2*) are responsible for the oxidation of MbFe(II)NO to MbFe(III)NO. In the presence of large amounts of nitrite (in the tens and hundreds of millimoles range), we observed a slight acceleration of the rate of oxidation of HbFe(II)NO by peroxynitrite. A catalytic rate constant of 40 +/- 2 M(-1) s(-1) was determined at pH 7.0. Preliminary studies of the reaction between nitrite and HbFe(II)NO showed that this compound also can oxidize the iron center, albeit at a significantly slower rate. At pH 7.0, we obtained an approximate second-order rate constant of 3 x 10(-3) M(-1) s(-1).     

Isothiazolo[5,4‐d]isoxazole S,S‐dioxides and pyrazolo [3,4‐d]‐isothiazole S,S‐dioxides through cycloaddition reaction on 3‐benzylaminoisothiazole S,S‐dioxides

By reacting 4,5‐unsubstituted isothiazole dioxides with diazoalkanes and nitrile oxides bicyclic pyrazolo[3,4‐d]isothiazole and isothiazolo[5,4‐d]isoxazole S,S‐dioxides were obtained in good yield through a regioselective cycloaddition reaction. Through cycloaddition reaction of 3‐benzylamino‐4‐bromo‐isothiazole S,S‐dioxide labile cycloadducts were formed that underwent in situ dehydrobromination affording the corresponding aromatized compounds.     

Tuning anisotropy barriers in a family of tetrairon(III) single-molecule magnets with an S = 5 ground state

Tetrairon(III) Single-Molecule Magnets (SMMs) with a propeller-like structure exhibit tuneable magnetic anisotropy barriers in both height and shape. The clusters [Fe4(L1)2(dpm)6] (1), [Fe4(L2)2(dpm)6] (2), [Fe4(L3)2(dpm)6].Et2O (3.Et2O), and [Fe4(OEt)3(L4)(dpm)6] (4) have been prepared by reaction of [Fe4(OMe)6(dpm)6] (5) with tripodal ligands R-C(CH2OH)3 (H3L1, R = Me; H3L2, R = CH2Br; H3L3, R = Ph; H3L4, R = tBu; Hdpm = dipivaloylmethane). The iron(III) ions exhibit a centered-triangular topology and are linked by six alkoxo bridges, which propagate antiferromagnetic interactions resulting in an S = 5 ground spin state. Single crystals of 4 reproducibly contain at least two geometric isomers. From high-frequency EPR studies, the axial zero-field splitting parameter (D) is invariably negative, as found in 5 (D = -0.21 cm(-1)) and amounts to -0.445 cm(-1) in 1, -0.432 cm(-1) in 2, -0.42 cm(-1) in 3.Et2O, and -0.27 cm(-1) in 4 (dominant isomer). The anisotropy barrier Ueff determined by AC magnetic susceptibility measurements is Ueff/kB = 17.0 K in 1, 16.6 K in 2, 15.6 K in 3.Et2O, 5.95 K in 4, and 3.5 K in 5. Both |D| and U(eff) are found to increase with increasing helical pitch of the Fe(O2Fe)3 core. The fourth-order longitudinal anisotropy parameter B4(0), which affects the shape of the anisotropy barrier, concomitantly changes from positive in 1 ("compressed parabola") to negative in 5 ("stretched parabola"). With the aid of spin Hamiltonian calculations the observed trends have been attributed to fine modulation of single-ion anisotropies induced by a change of helical pitch.