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EPR Study of Radical Intermediates of H-transfer from Oxygeneous, Carboneous and Nitrogeneous Donors
Marešová, Renata ; Mazúr, Milan (referee) ; Stopka, Pavel (referee) ; Omelka, Ladislav (advisor)
Within the PhD. thesis the problems of H-transfer from selected types of phenols, aromatic secondary amines and compounds with acidic CH bond were investigated. This process was initiated by the action of redox agents, mostly PbO2, in nonpolar solvents. The abstraction of hydrogen atom results in the formation of radical intermediates, which were detected by EPR spectroscopy. Because in most cases, highly unstable radicals were produced, it was neccessary to apply the technique of indirect detection, so called spin trapping. This method enables to interpret the character of generated radicals on the basis of EPR parameters of radical adducts, which are formed by their reaction with suitably chosen compounds, spin traps, which are added to the reaction system. As spin traps, the aromatic nitrosocompounds were preferentially used. Due to the fact that EPR spectra of radical adducts were in most cases very complex, their interpretation was possible only using the computer simulation. Based on this approach, new information about the character of addition of phenoxyl tradicals to aromatic nitrosocompounds, stability of alkyl substituents in methyl substituted phenols and character of C-radicals, generated from substituted coumarines and beta-diketones, was obtained. Simultaneously, the small ability of nitrogen radicals, primary products of the splitting of NH bond, to enter the reaction with spin traps was proved by aromatic secondary amines.
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Characterisation of chokeberry and saskatoon berry fruits
Burdějová, Lenka ; Omelka, Ladislav (referee) ; Vítová, Eva (advisor)
Diploma thesis deals with the characterization of chokeberry (Aronia melanocarpa) and saskatoon berry (Amelanchier alnifolia) fruits. The theoretical part focuses on the botanical classification, chemical composition, curative effects and the use of chokeberry and saskatoon berry in the food industry. Further, the attention is also devoted to a review of methods for the determination of important antioxidants, specialized on the determination of polyphenols, anthocyanins, ascorbic acid and total antioxidant activity. The experimental part of the work deals with the determination of selected parameters in different varieties of chokeberry and serviceberry, harvested in 2012 and 2013 using spectroscopic and chromatographic techniques with an emphasis on effect of solvent (50% ethanol, destilled water, DMSO). Specifically total content of polyphenols and anthocyanins, color characteristics using the CIE L*a*b* system and the ability of the sample to reduce the iron are determined using the UV-VIS-NIR spectroscopy. Further ascorbic acid concentration in individual extracts is quantified by High performance liquid chromatography coupled with a diode array detector and the total antioxidant activity is characterized by electron paramagnetic resonance involving •DPPH and ABTS•+ assays.
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EPR study of radical reactions initiated by the decomposition of selected types of peroxy compounds
Krkošková, Petra ; Kučerík, Jiří (referee) ; Omelka, Ladislav (advisor)
The products of the decomposition of selected types of peroxo compounds in the presence of redox agents (Pb and Co compounds) were investigated by EPR method. Besides some commercial peroxides the study was performed with peroxo compounds of Luperoxide group (Luperox 101, Luperox 256, Luperox 531). For the detection of the decomposition products the technique of spin-trapping using nitrosobenzene was applied. EPR spectra of radical adducts formed by the reaction of the reactive oxygenous radicals with nitrosobenzen having the character of stable nitroxyl radicals were analyzed. Their EPR parameters were obtained by simulation method. Besides the addition to nitrosobenzene the generated oxygen centered radicals were proved also on the basis of their reaction with model compounds (Santonox R; 2,6–ditercbutyl–4–methylphenol; diphenylamine).
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Influence of pH on polyectrolyte-surfactant system
Mourycová, Jana ; Omelka, Ladislav (referee) ; Pekař, Miloslav (advisor)
In this bachelor thesis, was studied influence of pH on aggregation polyelectrolyte-surfactant system using fluorescence spectroscopy and pyrene as fluorescent probe was used. As polyelectrolyte, hyaluronan with different molecular weight was used. Two surfactants cationic surfactant cetyltrimethylamonium bromide (CTAB) and nonionic surfactants polyoxyethylene (20) (TWEEN 20) were selected for interaction with hyaluronan. In the pH range from 3 to 10, no signifact changes in the aggregate non-polar core was observed. This mean system is stable against changes in the observed pH range. At system with higher concentrations of cationic surfactant (CTAB) and higher molecular weight hyaluronan occurred phase separation and gel exclusion was observed.
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Radical Reactions of the Decomposition of N-H, O-H and O-O Bonds Initiated by Homogeneous and Heterogeneous Redox Agents
Majzlík, Petr ; Mazúr, Milan (referee) ; Stopka, Pavel (referee) ; Omelka, Ladislav (advisor)
The Ph.D. thesis was focused on EPR study of redox reactions of selected types of phenols, secondary amines and diperoxy coumpounds. Within the study some redox agents were employed in nonpolar, in some special cases also in polar solvents. EPR spectra of generated radical products were interpreted using spectral simulation. Study of radical reactions under participation of phenols was preferentially concentrated on the behavior of para methyl substituted phenols, where the instability of this substituent in relation to the applied redox agents was evaluated. The tendency towards the abstraction of hydrogen from methyl group, representing the paralell mechanism to the hydrogen abstaction from phenolic OH group was proved. The generated benzyl radicals were undirectly detected in the form of adducts with aromatic nitroso compounds. These adducts undergo the consecutive transformations, leading to the formation of new types of phenoxy radicals. The study of the decomposition of NH bonds was performed with substituted N,N´-paraphenylenediamines, 1-anilino-1-phenylpentane-3-ones and amino substituted 1,3-dimethyluracils. By the oxidation with 3-chloroperbenzoic acid the corresponding nitroxyl radicals were prepared. In the case of 1-anilino-1-phenylpentane-3-ones the aminyl radicals were prepared by the oxidation with PbO2, which existence was evidenced by spin trapping method with nitrosobenzene. In the framework of the investigation of the decomposition of -O-O- bonds the oxygen centred radicals, generated from peroxidic compounds of Luperox type using selected redox agents were detected by spin trapping method. The analysis of EPR spectra documented that primary alkoxyl radicals undergo the consecutive fragmentation, which leads in the presence of oxygen to the formation of secondary alkoxyl radicals. The generation of oxygen centred radicals during the decomposition was indirectly proved in the presence of model phenolic compounds, where due to the abstraction of hydrogen from phenolic OH group the phenoxyl radicals are formed.
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EPR Study of Radical Reactions of Secondary Amines in Liquid Phase
Šafaříková, Lenka ; Lehocký,, Marián (referee) ; Mazúr, Milan (referee) ; Omelka, Ladislav (advisor)
In the framework of Ph.D. thesis the evaluation of radical reactions of four groups of secondary amines R1–NH–R2 in the presence of some selected types of agents was performed using EPR spectroscopy. First group was represented by peroxoagents (3-chloroperbenzoic acid, tBuO2 radicals), the second group involved compounds of PbIV+ (PbO2, Pb(OAc)4) functioning as hydrogen-abstracting agents. In the presence of peroxyagents the formation of corresponding aminoxyl radicals R1–NO–R2 was demonstrated. In the case of surplus of tBuO2 radicals these radicals enter the consecutive reactions which products are new types of secondary aminoxyls. By the interpretation of reaction mechanism the intermediary formation of nitrones was assumed. These function in later steps of the reaction as spin traps for tBuO2 radicals. By the study of radical mechanism of amines R1–NH–R2 initiated by PbIV+ agents was proved, that these compounds hydrogen atom not only from –NH– group under formation of aminyl radicals, but also from C–H bonds in substituents R1, R2 (the formation of C-radicals). Because both groups of the radicals formed are characterized by high reactivity, their identification was possible only using spin-trapping method. In the course of the investigation of radical reactions of secondary amines the big amount of experimental EPR spectra was obtained. These exhibited in many cases very high hyperfine structure. Due to this fact it was necessary to perform the detailed evaluation on the basis of spectral simulation. Except of some special cases using this approach the interpretation of majority of registered EPR spectra was interpreted and EPR parameters were summarized in tables.
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Radical products of the oxidation of selected phenols and amines
Holubcová, Petra ; Mazúr, Milan (referee) ; Omelka, Ladislav (advisor)
In the framework of diploma thesis, radical products of the oxidation of selected paramethyl phenols and secondary amines were investigated. In the case of para-methyl phenols using EPR spectroscopy the reaction mechanism was proved, where the abstraction of hydrogen from para-methyl group is involved. In this way benzyl radicals are formed, which can be identified by spin-trapping technique using nitroso compounds. The adducts formed undergo the consecutive rearrangement, which leads to the formation of the corresponding nitrones. In addition to phenols, the radical products of the oxidation of some secondary amines using peroxy radicals and peroxy acids were studied. In both cases new types of nitroxyle radicals were generated and the EPR parameters were determined by spectral simulation.
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Radical intermediates generated by the splitting of X-H bond in different types of H-donors
Šafaříková, Lenka ; Mazúr, Milan (referee) ; Omelka, Ladislav (advisor)
In the frame of submitted diploma thesis dealing with the radical products of the decomposition of different X-H bonds, primary attention was focused on the detection of radical intermediates from decomposed N-H bonds. This access was realized with secondary amines of N-alkylaniline group, as well as with other structures R1-NH-R2. The aim was to confirm the formation of unstable aminyl radicals R1-N•-R2, which is possible by using spin-trapping method. In the series of experiments, where the compounds of lead and cobalt, as well as stable radical DPPH were applied as initiators of the decomposition, the unambiguous evidence for aminyl radicals was found only in the case of N-alkylanilines. With other secondary amines the detection of aminyl radicals is an open problem, because their adducts with nitrosobenzene are in very low concentration. Besides the study of the decomposition of N-H bonds also the products of the decomposition of phenolic O-H bonds, as well as products of the decomposition of C-H bonds in alkylsubstituents in phenols was studied.
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Characterisation of alcoholic beverages by means of spectrospic methods
Fajtl, Zdeněk ; Omelka, Ladislav (referee) ; Polovka, Martin (advisor)
The bachelor’s project is focused on the problem of determination of thermo-oxidative stability of mixed Radler - type drinks with low alcohol content. The project consists of both, theoretical and practical parts. The theoretical part describes the basic technological procedures used in the production of lager-type beer and also mixed drinks of Radler type. It also deals with the issue of antioxidants and/or oxidative processes with specific focus on beer and its components. The author also pays attention to spectroscopic methods, which are recently extensively applied to characterize different qualitative and quantitative parameters of foods; predominantly UV-VIS, IR and EPR spectroscopy. These methods were chosen with respect to the topic being studied. In the experimental part, the application of EPR spectroscopy and spin traps methods for the characterization of thermo-oxidative stability of the selected group of commercially available samples of Radler-type beer is presented. The values of the induction periods which were taken as a measure of thermo-oxidative stability, are compared, both for the group of Radler-type beers under study and for the commercially available samples of typical lager-type beers of identical producers, which were taken as for reference.
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