Browsing by Author "Ivanov I."

Now showing 1 - 4 of 4
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    Crystallization of type I chloramphenicol acetyltransferase: An approach based on the concept of ionic strength reducers
    (2000-01-01) Andreeva A.; Borissova B.; Mironova R.; Glykos N.; Kotsifaki D.; Ivanov I.; Krysteva M.; Kokkinidis M.
    Chloramphenicol acetyltransferase (CAT) is responsible for bacterial resistance to chloramphenicol. It catalyzes inactivation of the antibiotic by acetyl-group transfer from acetyl CoA to one or both hydroxyl groups of chloramphenicol. Type I CAT possesses some unique properties which are not observed in other CAT variants. Type I CAT overexpressed in Escherichia coli was purified and crystals with a resolution limit of 2.22 Å have been obtained using a novel procedure which is based on the concept of 'ionic strength reducers'. The crystals have the symmetry of space group P1 and unit-cell parameters a = 96.46, b = 113.86, c= 114.21 Å, α= 119.9, β= 94.1, γ = 98.6°. These dimensions are consistent with four to six trimers per unit cell, corresponding to a solvent fraction ranging from 65 to 47%.
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    Selective adsorption of thiophene and its polyaromatic derivatives from fuels on pyrolyzed rice husks: A thermodynamic study
    (2019-01-01) Uzunova S.; Uzunov I.; Atanasova G.; Angelova D.; Ivanov I.
    This work presents the study of the adsorption affinity of biobased adsorbent prepared by pyrolysis of rice husks (PRH) toward thiophene (T), benzothiophene (BT) and dibenzothiophene (DBT) in single and mixed solutions in a batch mode using a model fuel and real diesel. A degree of desulfurization higher than 40 % is observed in case of an initial sulfur concentration in the fuel not higher than 1000 ppm. The adsorption selectivity of PRH from single solutions of the three aromatic sulfur heterocycles (ASHs) decreases following the line: T > BT > DBT. The theoretical equilibrium adsorption capacity of PRH in regard to the three sulfur compounds adsorption is determined using the adsorption isotherms of Langmuir, Freundlich, Dubinin-Radushkevich and Temkin. The calculated values of the thermodynamic parameters, such as the Gibbs energy, ΔG0, the entropy, ΔS0, and the enthalpy, ΔH0, indicate that the adsorption in PRH-ASHs-fuel system proceeds at normal and high temperature values through simultaneous physical and chemical interactions.
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    Simultaneous immobilization of uricase and peroxidase to copolymer of acrylonitrile with acrylamide
    (2002-01-01) Ivanov I.; Yotova L.
    A method for individual and simultaneous covalent immobilization of uricase and peroxidase to copolymer of acrylonitrile with acrylamide was presented. The immobilized enzymes revealed slight change in their pH optima and an appreciable increase in their temperature optima, Km and activation energy compared to data for the soluble enzymes. A substrate inhibition of the soluble and immobilized peroxidase at high substrate concentrations was established. A little glass column packed with immobilized enzymes was used for manual uric acid determination in blood sera. The method was characterized by high analytical precision and reproducibility. The developed method is economical (the enzyme-carrier conjugate may be used more than 200 times) and easier to perform than the manual methods utilizing soluble enzymes. The established manual method for uric acid determination in biological fluids is suitable for clinical laboratory diagnostics and for work in small clinical laboratories. © 2002 Taylor and Francis Group, LLC.
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    Stress analysis of insulating glass units under transient thermal loadings
    (2018-01-01) Ivanov I.; Velchev D.; Penkova N.; Krumov K.; Iliev V.
    Finite element stress analysis of glass panels in insulating glass units (IGU) is carried out based on the temperature field determined by serial numerical simulations of the conjugate heat transfer varying with the ambient temperature and solar radiation during a year. The hermetically sealed gas cavity between glass panels is modelled by finite elements and the stress is analyzed when the gauge pressure is maximum positive or negative due to gas temperature change at different seasons. The influence of the primary and secondary sealant of IGU is discussed. The combination of loadings due to thermal expansion and to cavity pressure is analyzed. Important conclusions about IGU structure and its durability are drawn.