Ab-initio simulation of elastic constants for some ceramic materials

Athermal elasticity for some ceramic materials (α-Al₂O₃, SiC (α and β phases), TiO₂ (rutile and anatase), hexagonal AlN and TiB₂, cubic BN and CaF₂, and monoclinic ZrO₂) have been investigated via density functional theory. Energy-volume equation-of-state computations to obtain the zero pressure equilibrium volume and bulk modulus as well as computations of the full elastic constant tensor of these ceramics at the experimental zero pressure volume have been performed. The present results for the single crystal elasticity are in good agreement with experiments both for the aggregate properties (bulk and shear modulus) and the elastic anisotropy. In contrast, a considerable discrepancy for the zero pressure bulk modulus of some ceramics evaluated from the energy-volume fit to the computational zero pressure volume has been observed.     

Reactivity of Singlet Oxygen Toward Proteins: The Effect of Structure in Basic Pancreatic Trypsin Inhibitor and in Ribonuclease A

The reactions of singlet oxygen, ¹O₂, with large peptides have been described previously. It was found that even in these systems, which in their native form are generally not supposed to possess a stable structure in solution, the polypeptide does impede the access of ¹O₂ to the amino acids that react readily with ¹O₂. Here we describe the ¹0₂ reaction with two proteins of well-defined structure. The quenching of ¹O₂ by bovine pancreatic trypsin inhibitor (BPTI) and by ribonuclease A (RNase A) was compared to that of a solution at the same concentration as those of its constituent amino acids that react readily with ¹O₂. The proteins were studied in their native form, when partly denatured by splitting their S-S bonds and when fully denatured. It was found that while in the native form the quenching rate constant was seven times lower in BPTI (2.2 vs 15.2 times 10⁷WM^-1 s^-1) and three times lower in RNase A (11.0 vs 32 times 10⁷M^-l s^-1) than in a mixture of its constituent amino acid residues, it increased upon denaturation reaching in the fully denatured state the value of the corresponding amino acid mixture. More striking is the effect of the protein structure when comparing the fraction of the encounters between ¹O₂ and protein, which cause damage to the protein, as reflected in the decrease of its biological activity. This decrease is assumed to be due to the chemical (oxidative) reactions of ¹O₂ in the protein. In the exceptionally stable BPTI the fraction of such encounters was 0.05 and in RNase A it was 0.2, whereas for the amino acid tryptophan in solution, 0.7 of the collisions with ¹O₂ led to a chemical reaction.     

QUENCHING OF TRYPTOPHAN PHOSPHORESCENCE IN ALCOHOL DEHYDROGENASE FROM HORSE LIVER and ITS TEMPERATURE DEPENDENCE

Abstract— The phosphorescence of alcohol dehydrogenase from horse liver (LADH) can be observed at room temperature. The quenching of this long-lived light emission, which comes from a tryptophan residue well buried within the interior of the enzyme structure, was measured. The rate constants for the quenching by the small oxygen molecule and by the I ^-1ion were found to be 1.4 → 10⁸ M ^-1 s^-1 and 10⁸ M ^-1 s^-1, respectively, at room temperature. The temperature dependence of the quenching yields an activation energy of about 14 kcal/mol. This activation energy and the meaning of the accompanying large pre-exponential factor in the Arrhenius equation, A = 10¹⁸ M ^-l s^-1, are discussed in terms of a model in which the quencher threads its way through the protein network.     

Mass spectral fragmentation pathways in phthalate esters. A tandem mass spectrometric collision-induced dissociation study

A collision-induced dissociation study of a series of phthalate esters was carried out using a tandem BB mass spectrometer. Fragmentation pathways of the phthalates were determined in the electron impact mode. Two major daughter ions are formed, one by a McLafferty rearrangement and hydrogen transfer and the other by loss of an alkoxy radical Another major daughter ion, at m/z 149—which is the base peak in the electron impact mass spectra of most phthalate esters—is being formed through four alternate pathways.     

REACTIVITY OF SINGLET OXYGEN TOWARD LARGE PEPTIDES

Abstract— The reactions of singlet oxygen, ¹O₂, with amino acids and their derivatives have been studied previously. It was found that only five amino acid residues interact readily with ¹O₂. Here we describe its reactions with the large peptides melittin, neuropeptide Y (NPY) and insulin in their native and in their denatured forms. The singlet oxygen quenching by a polypeptide was compared with that of a solution at the same concentration as those of its constituent amino acids, which are known to react efficiently with ¹O₂. It was found that the quenching rate by such a mixture exceeded that of the polypeptides in their native form. The ratio of the rate constants for NPY to that of the corresponding amino acid mixture in solution was 0.75. For melittin in its monomeric form it was 0.83 and for a tetramer of melittin (at high ionic strength) it was 0.70. For native insulin the ratio of the rate constants was 0.55. For oxidized insulin with its -S-S- bridges opened the figure became 0.80. However, the quenching by all the polypeptides in their fully denatured form (in the presence of 6 M urea) equalled that of the corresponding amino acid mixtures. Although polypeptides are generally supposed not to possess a stable secondary structure in solution the effects are explained by shielding of some of the reactive amino acid residues in the chain by temporary folding or incipient secondary structures of the native polypeptide.
It is shown that the kinetics for a homogeneous solution of quenchers applies also to measurements in a polypeptide solution where the quenchers are localized along the polypeptide backbone and thus form clusters in solution.     

Forensic identification of explosive oxidizers by electrospray ionization mass spectrometry

The mass spectrometry of a group of inorganic oxidizers was studied using the electrospray ionization technique. It was found that a series of cluster ions were predominant in both positive- and negative-ion mode, allowing for the characterization of the investigated oxidizers. The identity of the recorded cluster ions was further confirmed by using some isotopically labeled compounds and tandem mass spectrometry with collision-induced dissociation. The use of electrospray ionization mass spectrometry for positive identification of major oxidizer components in explosive formulations was demonstrated by three samples of forensic interest.     

PHOTOCHEMISTRY OF REDUCED LUMIFLAVIN IN ITS ACIDIC FORM

Abstract— The photooxidation of reduced lumiflavin in its acidic form, LfH 3 ⁺, takes place in two consecutive steps. Upon illumination of LfH 3 ⁺ in its absorption band at 313 nm the semiquinone, LfH 3 ⁺, is formed. Two LfH 2 ⁺ ions are consumed for every LfH 2 ⁺ formed. Illumination of the semiquinone in its absorption band at 495 nm causes further oxidation so that the oxidized LfH⁺ ion is formed. In this reaction one LfH 3 ⁺ ion is photolyzed for every LfH⁺ formed. In addition, a hydrogen atom is released in the photooxidation of LfH 2 ⁺. Mechanisms for the two photoreactions are proposed.     

Quotient categories,stability conditions,and birational geometry

This article deals with the quotient category of the category of coherent sheaves on an irreducible smooth projective variety by the full subcategory of sheaves supported in codimension greater than c. We prove that this category has homological dimension c. As an application, we describe the space of stability conditions on its derived category in the case c  \(=\) 1. Moreover, we describe all exact equivalences between these quotient categories in this particular case, which is closely related to classification problems in birational geometry.