Browsing by Author "Stoyanova-Ivanova A."
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Item BENDING ELASTICITY OF PHOSPHOLIPID BILAYERS CONTAINING AN AMPHIPATHIC PEPTIDE WITH LOW MAMMALIAN CYTOTOXICITY(2022-01-01) Vitkova V.; Stoyanova-Ivanova A.; Jaber S.; Naydenova E.; Danalev D.Peptide mimetics imitate natural peptides’ structure but they could be specifically designed to be more selective concerning their toxicity to mammalian cells. In most cases this specificity is due to their ability to form α-helix in amphipathic environment. In addition, the specific activity depends on the ability of final structure to penetrate cell membrane. Being responsible for the cell integrity and compartmentalization, biomembranes also play a major role in cellular processes, in which the membrane deformations are important. In the present study we probe peptide-membrane interactions for a shortened amino acid sequence KLAKLAK-NH2 of an antimicrobial peptide with apoptotic effect. The bending rigidity of model lipid bilayers is measured by flicker spectroscopy of quasispherical unilamellar vesicles monitored and analyzed in phase contrast light microscopy. At high peptide concentrations ∼ 30 µmol/L and peptide-to-lipid total molar ratios ∼ 0.90 bilayer stacking formation is observed. A reduction of the bending constant is reported at peptide-to-lipid total molar ratio ∼ 0.80. The membrane softening indicates peripheral peptide orientation at the lipid bilayer, which is considered a prerequisite for channel formation. Based on KLAKLAK-NH2 effect on the membrane bending elasticity we provide an evaluation of the peptide partition coefficient characterizing its affinity to POPC bilayers. The acquired results might be helpful in efforts to further tailor the pharmacokinetic properties of antimicrobial peptides in combination with strengthened stability towards enzymatic degradation.Item Influence of the alkaline electrolyte bath on the oxygen content of REBa2Cu3OY (RE = Y, Gd) superconducting ceramics(2013-12-17) Georgieva S.; Stoyanova-Ivanova A.; Nedeltcheva T.The oxygen stability of the REBa2Cu3Oy (RE = Y, Gd; y = 6,5+δ) superconducting ceramics previously immersed in strong alkaline electrolyte bath have been studied. The samples were placed in solution of 7M KOH (pH 13) for 96 h and then the oxygen content was determined by spectrophotometry and XRD-spectrometry. The results obtained by the both methods are identical in the limit of random errors. Consequently the electrolyte treatment of the samples does not influence the oxygen content. The critical temperature and the structure of the ceramics do not change also and these superconducting ceramics can be used in the electrotechnical devices.Item Interaction of KLAKLAK-NH2 and Analogs with Biomimetic Membrane Models(2024-03-01) Vitkova V.; Antonova K.; Petkov O.; Stoyanova-Ivanova A.; Jaber S.; Ivanova V.; Naydenova E.; Danalev D.Background: Specifically designed peptide mimetics offer higher selectivity regarding their toxicity to mammalian cells. In addition to the α-helix conformation, the specific activity is related to the peptide’s ability to penetrate the cell membrane. The alterations in lipid membrane properties were addressed in the presence of the peptide KLAKLAK-NH2 and analogs containing β-alanine, strengthening the antibacterial activity and/or naphtalimide with proven anticancer properties. Methods: The molecular interactions of the peptide mimetics with POPC bilayers were studied using FTIR-ATR spectroscopy. The thermal shape fluctuation analysis of quasispherical unilamellar vesicles was applied to probe the membrane bending elasticity. The impedance characteristics of bilayer lipid membranes were measured using fast Fourier-transform electrochemical impedance spectroscopy. Results: A lateral peptide association with the membrane is reported for β-alanine-containing peptides. The most pronounced membrane softening is found for the NphtG-KLβAKLβAK-NH2 analog containing both active groups that corroborate with the indications for 1,8-naphthalimide penetration in the lipid hydrophobic area obtained from the FTIR-ATR spectra analysis. The β-alanine substitution induces strong membrane-rigidifying properties even at very low concentrations of both β-alanine-containing peptides. Conclusions: The reported results are expected to advance the progress in tailoring the pharmacokinetic properties of antimicrobial peptides with strengthened stability towards enzymatic degradation. The investigation of the nonspecific interactions of peptides with model lipid membranes is featured as a useful tool to assess the antitumor and antimicrobial potential of new peptide mimetics.Item Phase formation and microstructure in Re1-xCax Ba2Cu3Oz (Re=Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics(2006-03-01) Stoyanova-Ivanova A.; Terzieva S.; Staneva A.; Mikli V.; Traksmaa R.; Dimitriev Y.; Kovachev V.We have investigated the influence of Ca-substitution and different rare earths on the microstructure of RE1-xCax Ba2Cu3O7-δ (RE= Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics. Scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy have been used to study the microstructure and the chemical composition of the samples. A correlation was established between the polycrystalline microstructure and phase formation depending on the additive content. We observed that calcium is distributed uniformly in the crystals. The formation of minor impurity phases improved the sintering conditions. © Central European Science Journals Warsaw and Springer-Verlag Berlin Heidelberg 2006.Item Phase formation and microstructure in Re1-xCax Ba2Cu3Oz (Re=Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics(2006-03-01) Stoyanova-Ivanova A.; Terzieva S.; Staneva A.; Mikli V.; Traksmaa R.; Dimitriev Y.; Kovachev V.We have investigated the influence of Ca-substitution and different rare earths on the microstructure of RE1-xCax Ba2Cu3O7-δ (RE= Y, Eu, Er; x=0, 0.2, 0.3) superconducting ceramics. Scanning electron microscopy, X-ray microanalysis and energy dispersive spectroscopy have been used to study the microstructure and the chemical composition of the samples. A correlation was established between the polycrystalline microstructure and phase formation depending on the additive content. We observed that calcium is distributed uniformly in the crystals. The formation of minor impurity phases improved the sintering conditions. © Central European Science Journals Warsaw and Springer-Verlag Berlin Heidelberg 2006.Item Spectrophotometric method for rapid estimation of the oxygen stoichiometry in YBCO superconducting samples(2013-03-01) Georgieva S.; Nedeltcheva T.; Vladimirova L.; Stoyanova-Ivanova A.Detailed investigations on the possibility for determination of the oxygen stoichiometry in YBa2Cu3Oy superconducting samples by measuring the absorbance of the colored Co(III)-EDTA and Cu(II)-EDTA complexes in one solution have been performed. The absorptivity of the complexes at pH 2.5 and 4.9 as well as Beer's law have been studied. The content of the solution for the sample dissolving has been established. The stoichiometry of the oxidation-reduction reaction between Cu(III) from the sample and Co(II) in the solution has been proved. The possibility to dissolve the sample in air has been tested. As a result a simple and rapid spectrophotometric method for the determination of the δ non-stoichiometric oxygen coefficient in YBa 2Cu3O6.5+δ samples has been developed without precise measurement of the sample mass. The standard deviation of the δ- and y-coefficient results is 0.017. The reproducibility of the y-values in relative units (s r ) is 0.2%. The accuracy of the results has been confirmed by the method, which is based on the absorbance measurement of the I 3 - -starch compound. © 2012 Versita Warsaw and Springer-Verlag Wien.Item Structural investigation of tellurium based thin films(2018-01-01) Ivanova V.; Trifonova Y.; Lilova V.; Mikli V.; Stoyanova-Ivanova A.Tellurium based materials possess a low temperature phase change transition. Sometimes they cannot be obtained in an amorphous state under normal conditions. Thus they can be used as an active element in phase change memory devices. Our study is dedicated to the synthesis of tellurium based materials using the systems (GeTe3)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol % and (GeTe4)100-xInx, x = 5 mol %, 10 mol %, 15 mol % and 20 mol %, the deposition of thin films on their ground and the investigation of their morphology, topology and structure aiming to obtain new knowledge for evaluation of the possibility for application of these materials in phase change memory devices.Item Synthesis and superconducting properties of Nd0.33 Eu0.08Gd0.58Ba2Cu3 Oz materials(2008-03-01) Stoyanova-Ivanova A.; Terzieva S.; Shivachev B.; Mikli V.; Vladimirova L.We have studied the effect of the ratio of different rare-earth element on the superconducting properties and phase formation of (Nd0.33Eu0.08Gd0.58)Ba2 Cu3Oz ceramic. Bulk NEG samples were prepared using the solid-state reaction process. The superconducting transition for Nd0.33Eu0.08Gd0.58Ba2 Cu3Oz is Tc ≈ 90 K and the value of the critical current density (Jc) is 13.9 A/cm2 at 77 K under zero magnetic fields. This value is twice as high when compared with the (Jc) value of YBCO systems (J3 = 7.31 A/cm2). The obtained bulk sample was used for the production of superconducting Ag-sheathed tapes by OPIT method including hot rolling. The critical current density of the obtained tape (337 A/cm2) is one order higher than the one of the bulk sample. © Versita Warsaw and Springer-Verlag Berlin Heidelberg 2008.