The Exciton Model and the Circular Dichroism of Polypeptides

Summary. Exciton coupling of the 190nm ^* transition is an important factor in the CD spectrum of peptides and proteins. The CD spectrum of the -helix is dominated by the exciton effect. The spectrum is sensitive to the direction of the ^* transition dipole moment, especially for short helices. Exciton theory is much less successful in accounting for the CD spectrum of the poly(proline)II (PPII) conformation, an important conformer in collagen and in unordered peptides. Mixing of the ^* transition with high-energy transitions in the peptide backbone and in side chains must be considered to explain the strong negative CD band near 200nm of PPII.     

Thermal stability and phase transformations in a commercial vanadia-molybdena alkali-promoted catalyst

Temperature-programmed decomposition, thermal analysis, thermogravimetry and high-temperature X-ray analysis have been used to investigate a commercial catalyst for the selective oxidation of benzene into maleic anhydride. It was shown that phase transformations took place in the vanadia-molybdena alkali-promoted catalyst. The transformations were caused either by high-temperature treatment or by catalytic work in an industrial plant. As a result, the-bronze phase was formed. In the high-temperature region a reversible transformation of the bronze phase was observed. This was interpreted as the phase transformation.

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Zusammenfassung Temperaturprogrammierte Zersetzung, Thermoanalyse, Thermogravimetrie und Hochtemperatur-Röntgenanalyse wurden zur Untersuchung eines kommerziellen Katalysators für die selektive Oxydation von Benzol zu Maleinsäure herangezogen. Es wird gezeigt, daß in dem einen Alkalipromotor enthaltenden Vanadinoxid-Molybdänoxid-Katalysator Phasenübergänge vor sich gehen. Die Phasenübergänge, als deren Ergebnis eine-Bronzephase gebildet wird, wurden entweder durch Hochtemperaturbehandlung oder durch sich in einer industriellen Anlage während des Betriebes abspielenden Vorgänge verursacht. Im Bereich hoher Temperaturen wurde eine reversible Umwandlung der Bronzephase beobachtet, die als Phasenübergang interpretiert wurde.

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- , , . , - , , . , , -. .

     

A mechanistic study of cycloaddition of carbon dioxide to propyne over supported Rh4 and Fex Rhy carbonyl cluster derived catalysts

CO₂ undergoes cycloaddition to propyne on a cationic rhodium site with the assistance of O^2–/OH^– of support or iron oxide over Rh₄ and Fex Rhy carbonyl clusterderived catalysts.¹³CO₁₂ labeling and FTIR studies suggest that monodentate carbonate formed on the catalyst surface is most likely a key species to offer an active CO₂ to be converted into a carbonyl intermediate prior to the product 4,6-dimethyl-2-pyrone.

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CO₂ O^2–/OH Rh₄ Fex Rhy. ¹³CO₂ , CO₂ , 4,6--2-.

     

Thermal decomposition of iron(II) sulphate heptahydrate in air in the presence of (basic) beryllium carbonate

Iron(II) sulphate heptahydrate undergoes decomposition in the presence of basic beryllium carbonate without any interaction with the carbonate. The components of the mixture decompose individually. Iron(II) sulphate decomposes with the formation of tetrahydrate, monohydrate, anhydrous salt, oxysulphate and ferric sulphate as intermediate phases. The basic beryllium carbonate decomposes to the oxide with BeO·BeCO₃ as the intermediate compound.

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Zusammenfassung In Gegenwart von basischem Berylliumcarbonat zersetzt sich Eisen(II)-sulfat-Heptahydrat, ohne mit dem Carbonat in Wechselwirkung zu treten. Die Komponenten des Gemisches zersetzen sich individuell. Die Zersetzung des Eisen(II)-sulfats verläuft über die Zwischenphasen Tetrahydrat, Monohydrat, wasserfreies Salz, Oxysulfat und Eisen(III)-sulfat. Das basische Berylliumcarbonat wird über die intermediäre Verbindung BeO·BeCO₃ in das Oxid überführt.

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, . . , , , . .₃.

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The authors are grateful to Dr. B. R. Sant for his keen interest and useful suggestions throughout this work. They also express their sincere thanks to Prof. P. K. Jena, Director, for his kind permission to publish the results. One of us (MSRS) thanks the Director for a Senior Fellowship.     

A desk computer program for the calculation of rate constants

A desk computer program has been written in BASIC programming language to optimize the values of rate constants determined from kinetic measurements of time and concentration data. The program is capable of the simultaneous refinement of a maximum of 5 rate constants. A FORTRAN version using the Gear method is also available.

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, BASIC , . 5 . FORTRAN .

     

Conversion of dehydrolinalyl acetate in catalytic hydrogenation on metals

Low selectivity of dehydrolinalyl acetate (DeHLA) hydrogenation into linalyl acetate and ester group detachment are predicted in accordance with quantum-chemical data obtained for its homologue dimethyl ethynyl carbinol acetate. Experimental data are confirmed by theoretical calculations. DeHLA hydrogenation produces a variety of products.

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, - . . .

     

Fluctuations and dynamics of chemical reactions near neutral steady-state

The effect of fluctuations on the behavior of a simple kinetic model of catalytic reactions allowing a neutral steady-state, is analyzed.

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, , .

     

Structure of Pt/ZnAl2O4 catalysts

The effect of the support preparation technique (solgel and coprecipitation) on the final Pt/ZnAl₂O₄ catalyst is presented. The structural properties of the solids obtained are correlated to the selectivity and activity for isobutane dehydrogenation in H₂ and He reaction media. If a highly dispersed catalyst is suitable, the support has to be prepared by the sol-gel method.

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( — ) Pt/ZnAl₂O₄. H₂ He. , — .

     

Catalytic oxidation of octene-1 in the presence of palladium(II) salts and heteropolyacids

In the presence of the catalytic system (PdSO₄+HPA), where HPA=H₉PMo₆V₆O₄₀, octene-1 is found to be selectively (95%) oxidized to octanone-2. The optimum concentration ratio for catalyst stability is [HPA]:[PdSO₄]=30–40. The substitution of PdCl₂ for PdSO₄ leads to the formation of all three isomeric octanones without decrease of the overall selectivity. Effective rate constants and the apparent activation energy have been determined in the temperature range from 60 to 80 °C.

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(PdSO₄+), =H₉PMo₆V₆O₄₀, -1 ( 95%) -2. []:[PdSO₄]=30–40. PdSO₄ PdCl₂ . 60–80 °C.

     

Hydrogenation of CO2

The hydrogenation of CO₂ has been studied on three different series of catalysts: Fe/MgO, Ni/Ti, Cr/Zn doped with potassium. Reaction was carried out in a conventional flow microreactor system at 15 bar at 250–350°C. Three independent competing reactions: Reverse gas-water shift (RGWS), Fischer-Tropsch (FT) and methanol synthesis were observed.

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CO₂ : Fe/MgO, Ni/Ti, Cr/Zn . 15 250–350°C. : RGWS, F.T. .

     

Kinetics and mechanism of OsO4 catalyzed oxidation of chalcones by Ce4 in aqueous acetic sulfuric acid media

Kinetics of OsO₄ catalyzed oxidation of chalcones by Ce⁴⁺ has been studied in aqueous acetic-sulfuric acid medium in the temperature range 313 to 338 K. The order in [oxidant] is zero while the order with respect to [substrate] and [catalyst] are each fractional. The rate of the reaction decreased with increase in percentage of acetic acid while [H⁺] had practically no effect on the rate The rates of various substituted chalcones (with substituents in the phenyl. ring attached to the moiety) are in the orderp-CH₃>m-CH₃>H>p-Cl>m-Cl>m-NO₂>p-NO₂. A mechanism in which formation of a cyclic ester between chalcone and OsO₄ in a fast step followed by its decomposition in a rate-determining step is envisaged.

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Ce⁴⁺, OsO₄, - 313–338 . —, —. , [H⁺] . ( , : -CH₃>-CH₃>H>-Cl>-NO₂>-NO₂. , OsO₄ , , .

     

Reaction between the OH group of hydroxyproline and diisocyanate catalyzed by an organotin compound

We have studied the kinetics of the reaction of L-hydroxyproline with OCH(CH₂)₆NCO, catalyzed by organotin, avoiding side reactions by protecting the other active hydrogens of the aminoacid, and comparing it with the MeOH/n-BuNCO system.

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L-- -OCN(CH₂)₆NCO, , . . - .

     

Kinetic equations with a complex balanced stationary point

A class of ordinary differential equations including nonideal nonisothermal kinetics is introduced. The behavior of their solutions for the system with a positive complex-balanced stationary point is studied. Sufficient conditions for this point to exist are described.

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, . . .

     

Catalytic activity of palladium-containing zeolites in the complete oxidation of benzene

The dependence of palladium distribution in zeolites on the temperature of thermal treatment has been established from the data on oxygen chemisorption and the statistical processing of electron micrographs. The influence of palladium distribution in zeolites on their catalytic activity towards the complete oxidation of benzene has been shown.

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- . .

     

Calculation of lower electronic states of the GaO and InO molecules by the SCF-Xα scattered wave method

In the overlapping atomic spheres approximation of the SCF-X SW method in different variants (spin-restricted, spin-unrestricted, and quasirelativistic), we have calculated fragments of the potential curves for the electronic ground state (X ₂) and the lower excited states (1 ², 2 ², 2 ², 3 ²) of the GaO and InO molecules. We have calculated the spectroscopic constants (T_e, _e, _ex_e, B_e, _e, R_e, A_e) for the molecules in the indicated states, the matrix elements for electronic transitions from the electronic ground state to the excited states, the Franck-Condon factors for the transitions 3 ², 2 ², 2 ² 1 ², X ² (v, v 10). For the InO molecule, we discuss the effect of relativistic corrections on the calculated molecular constants.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 22, No. 1, pp. 11–19, January–February, 1986.     

Some investigations on the photostabilization of polypropylene

The effect of substituted hydroxybenzophenone, benzotriazole and organic metal complexes on the photooxidation of PP films has been studied. It has been established that NiDBTC added in 1.0 wt. % is the most effective photostabilizer.While in the case of other stabilizers there is a slight increase in the Co content even during the induction period, with the application of NiDBTC practically no oxidation can be observed before the stabilizer completely disappears. This means that NiDBTC inhibits the photooxidation of the polymer even in low concentration.

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, - . , Ni, 1,0 . %. . , Ni , . , Ni .

     

Excited Π states of divinyl chalcogenides and chalcophenes

Electron transitions in divinyl chalcogenides (CH₂=CHXCH=CH₂, where X is S, Se, or Te) have been analyzed using UV absorption spectra of dialkyl and alkyl vinyl chalcogenides. The following relations for the orbital energies are found: ^* < ^* < ^* < ^* for Te and ^* < ^* < ^* < ^* for S and Se. For chalcophenes, a correlation between the energy of the excited state (E ^*) of specific symmetry, the ionization potential (I) and the electron affinity (EA) is obtained:E ^*=const+(I+EA)/2. The electron affinity of divinyl chalcogenides is estimated. The correlation between the excited ^* states of divinyl chalcogenides and chalcophenes is discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 831–835, May, 1994.     

State of components in group IV A metal modified nickel catalysts prepared via decomposition of metal complexes

The dispersity and state of components in (M+Ni)/SiO₂ catalysts (M=Ge, Sn, Pb), prepared via the reduction of anchored complexes, have been studied by chemisorption and electron microscopy. High dispersity of metallic particles (6Å) has been revealed. According to Mössbauer spectroscopy, in (Ni+Sn)/SiO₂ catalysts reduced at 873 K, a partial reduction of Sn to the metallic state is observed.

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(M+Ni)/SiO₂ ( M=Ge, Sn, Pb), , , - . 873 ° K (Sn+Ni)/SiO₂ , , , 6 Å.