Long-Term Changes in a Heat Exchanger Steel

The steel ASTM A213 P22 is used for superheater outlet header in power plants. During duty cycles lasting over tens of years the temperature is of the order of 545°C and the pressure 125 atm. The microscopic changes in these steels are hard to analyze under working conditions, but they are believed to be responsible for the appearance of creeps in such devices. Investigation of the microscopic modifications will help to predict future failures due to creeps, increasing the reliability and saving a lot of money.We report the investigation of such changes in ASTM A213 P22 steel in use for 30 years as a Superheater header at Haifa power station. The analysis has been carried out using Mössbauer Spectroscopy, X-ray diffraction and optical microscopy. As a result it seems that the morphology and phase change of the carbide phases M₃C/M₇C₃ and M₂₃C₆ are the most prominent changes leading to the material failures.     

Thermal decomposition of pentacoordinate uranium(VI) complexes

The thermal decomposition of some mixed uranyl complexes with Schiff bases and DMSO, EtOH or (Ph)₃PO as neutral ligand, were investigated and the corresponding activation energy, E*_a, and enthalpy of dissociation, ΔH_d, values were calculated.

The results obtained indicate that for the same neutral ligand, the thermal stability of the uranyl complexes is influenced by the Schiff base used; for the same Schiff base, the presence of (Ph)₃PO results in a greater thermal stability of the mixed complexes than when the other two neutral ligands are present.     

The stereocontrolled synthesis of orthogonally protected (R)-α-methyltryptophan

An expedient and highly stereocontrolled route to (R)-α-methyltryptophan and its orthogonally protected analogs has been developed via a four-step conversion from L-alanine in good overall yields. The stereochemistry of the products is confirmed by X-ray diffraction analysis, NMR spectroscopy and optical rotations.     

Effects of variable selection on CoMFA coefficient contour maps in a set of triazines inhibiting DHFR

Summary An example of a CoMFA study is described with the aim to discuss one of the major problems of this 3D QSAR method: lack of variable selection. It is shown that the use of nonrelevant energy parameters might produce CoMFA contour maps which poorly reflect the actual nature of the binding site and are in part statistical artefacts. The data set employed in our analysis comparises triazine inhibitors of dihydrofolate reductase (DHFR), isolated from chicken liver, which have already been the object of a QSAR study by other authors. Since three-dimensional structures of triazine-DHFR complexes are known, it was possible not only to reduce ambiguities in the superimposition of the ligands, but also to compare the resulting CoMFA contour maps with the enzyme active site.Supplementary material available: The Cartesian coordinates and the atomic charges of the PM3-optimized structures used in the CoMFA study are available as MOL2 files upon request.To whose memory this paper is dedicated.     

RESOLUTION OF THE INDIVIDUAL TRYPTOPHANYL CONTRIBUTIONS TO THE NEAR-ULTRAVIOLET DICHROIC ACTIVITY OF APOMYOGLOBIN

Abstract –The individual tryptophanyl contributions to the near-ultraviolet dichroic activity of apomyoglobin in its native conformation have been resolved. This was accomplished by comparing the spectra of two classes of apomyoglobin with different aromatic residue contents and observing the effect of a specific modification of indole residues. The circular dichroism (CD) spectra of apomyoglobins containing two tryptophanyl residues, i.e. Trp A-5 and A-12, show the presence of a positive peak centered at 292 nm, attributable to indolic chromophore, which is missing in the CD spectrum of tuna apomyoglobin possessing only Trp A-12. Moreover, the specific modification of Trp A-5 by 2-hydroxy-5-nitrobenzyl bromide is shown by the lack of the 292 nm peak and the appearance of a positive band at longer wavelength. The pH dependence of the position of this band suggests that it arises from the 2-hydroxy-5-nitrobenzyl moiety. The results suggest that Trp A-12 does not substantially contribute to the optical activity in the near ultraviolet.     

Sugli zeri delle funzioni zeta di Dedekind

Riassunto Scopo di questo lavoro è dare una formula asintotica per il numero degli zeri di ReF _K(λ+it) e di ImF _K(λ+it), dove eζ _K(8) è la funzione zeta di Dedekind associata al campo numericoK, con 0<t<T e λ numero reale fissato tale che 1−1/n<λ<1 doven è il grado diK.

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Summary The aim of this paper is to give an asymptotic formula for the number of zeros of ReF _K(λ+it) and ImF _K(λ+it), where andζ _K(8) is the Dedekind zeta function for a number fieldK, with 0<t<T and λ fixed real number such that 1−1/n<λ<1, wheren is the degree ofK.

     

Convenient diastereospecific synthesis of a rociverine precursor and its resolution by lipase-catalyzed transesterification

(±)-1-Cyclohexyl-c-2-hydroxymethyl-r-1-cyclohexanol 3, a precursor of the antimuscarinic drug Rociverine 1, was obtained diastereospecifically in very high yield, from the Grignard reaction between C₆H₁₁MgCl and an appropriately protected 2-(hydroxymethyl)cyclohexanone. The preparation of enantiomerically enriched cis diol (+)-(1R,2S)-3 and the corresponding 2-acetoxymethyl derivative (+)-(1S,2R)-12 was achieved by lipase PPL-catalyzed transesterification of racemic diol (±)-3.     

Thermal behaviour of Uranium(VI) complexes. Uranyl acetate dihydrate-thiourea system: Synthesis and decomposition

The solid-state syntheses of uranyl acetate adducts with thiourea were attempted directly in a DSC apparatus according to the scheme: UO₂Ac₂·2 H₂O_(s)+n thiourea_(s) → → UO₂Ac₂·thiourea_n(s)+2 H₂O_(v) (n=1–6), and the correspondingδH values were evaluated. At all stoichiometric ratios the same UO₂Ac₂·thÌourea product was obtained, to which a pentagonal bipyramidal structure was attributed. Thermoanalytical (TG and DTG) and infrared spectra measurements were also performed and agree with the DSC results.