Mercury (Hg) has been listed as a global high priority pollutant by many international organizations due to its mobility and persistence in the environments and high toxicity to organisms. This research was conducted with the aims to determine: (i) total Hg content (THg) and its spatial distribution in sediments of river Tisa along the river course, (ii) possible sources of THg and (iii) degree of THg pollution in sediments from the river Tisa through different criteria. Total Hg in the sediments ranged from 0.07 to 0.49 mg kg-1, with mean ? S.D. value of 0.26 ? 0.10 mg kg-1. The highest mean value of THg (0.30 mg kg-1) was found in the lower stream, while the lowest (0.13 mg kg-1) was found in the tributary. According to Principal Component Analyses (PCA) strong positive loading of metals in all parts of the river Tisa is mainly controlled from the same sources. These sources are related to anthropogenic activities based on calculated Enrichment Factor (EF) values. Total Hg are higher than background value. According to the Republic of Serbia official standard, THg values of river Tisa sediments were within the range of maximum permissible values. Compared with National Oceanic and Atmospheric Administration (NOAA) guideline, 80.49% of sediment samples indicated that THg in the river Tisa sediments represented minimal and possible risk towards the living organisms. Integrating the results of pollution assessment, it could be concluded that THg in river Tisa sediments in Serbia demonstrates considerable contamination according to Geoaccumulation Index (Igeo), and Contaminant Factor (CF), and high pollution risk according to Potential Environmental Risk Index (PERI).

The sequential extraction procedure was applied for partitioning of metals in river sediments collected along the course of the river Tisa (Serbia). Eight elements (Sb, Sn, As, Cd, Cu, Hg, Pb and Zn) from twenty-one sampling site were analyzed using the modified BCR sequential extraction procedure in combination with ICP-OES. The results of sequential extraction, statistical analyses and calculation of EF and lithogenic and anthro?pogenic ratio of metals are similar. In the river Tisa sediments Sn, Cd, Hg, Pb and Zn are of anthropogenic origin, while As, Cu and Sb are of lithogenic and anthropogenic origin. The sediments from the river Tisa show high risk for Cd, medium risk for Hg and Zn, low risk for Sn, As, Cu and Pb, whereas Sb does not show the risk for the aquatic environment.

Cytochrome P450 3A4 is the most significant enzyme in metabolism of medications. Flavonoids are common secondary plant metabolites found in fruits and vegetables. Some flavonoids can interact with other drugs by inhibiting cytochrome P450 enzymes. Thus, the objective of this study was to determine inhibition kinetics of cytochrome P450 3A4 by flavonoids: acacetin, apigenin, chrysin and pinocembrin. For this purpose, testosterone was used as marker substrate, and generation of the 6β-hydroxy metabolite was monitored by high performance liquid chromatography coupled with diode array detector. IC50 values, inhibition constants, and rates of inhibition were determined. IC50 values ranged between 0.6 and 11.4 μM. The strongest inhibitor was chrysin (IC50 0.6 μM, inhibition constant 0.6 μM, inhibition rate constant 0.065 min–1, inhibition efficacy 0.108 min–1 μM–1). Compared to other flavonoids analyzed, chrysin’s inhibitory effect can be attributed to the hydrophobic nonsubstituted B ring, as well as rigidity of the structure. When foods rich in chrysin are consumed, e.g. honey and propolis, chrysin can cause food-drug interactions. Further in vitro studies are needed to determine the reactive intermediate responsible for inactivation of cytochrome P450 3A4 enzyme, as well as in vivo studies to determine possible clinical significance of this inhibition.

: We construct bijections between certain energetically favorable resonance-like structures in several classes of benzenoid compounds and some well- known combinatorial structures enumerated by Catalan and related numbers. In that way we derive explicit formulas for the n umber of such structures in the considered classes of compounds. We also list some open problems and indicate some possible d irections for future research.

We introduce non-Abelian Kuramoto model on \begin{document}$ S^3 $\end{document} in the most general form. Following an analogy with the classical Kuramoto model (on the circle \begin{document}$ S^1 $\end{document} ), we study some interesting variations of the model on \begin{document}$ S^3 $\end{document} that are obtained for particular coupling functions. As a partial case, by choosing "standard" coupling function one obtains a previously known model, that is referred to as Kuramoto-Lohe model on \begin{document}$ S^3 $\end{document} . We briefly address two particular models: Kuramoto models on \begin{document}$ S^3 $\end{document} with frustration and with external forcing. These models on higher dimensional manifolds have not been studied so far. By choosing suitable values of parameters we observe different nontrivial dynamical regimes even in the simplest setup of globally coupled homogeneous population. Although non-Abelian Kuramoto models can be introduced on various symmetric spaces, we restrict our analysis to the case when underlying manifold is the 3-sphere. Due to geometric and algebraic properties of this specific manifold, variations of this model are meaningful and geometrically well justified.

Extracurricular activities as a variety of organizational forms of students gathering in free extracurricular time at elementary school have predominantly cultural, artistic, sporting, technical, recreational or scientific character (Cindrić, 1992). Through them, students meet their needs and develop the culture of using their leisure time. Thanks to its pedagogic potential, chess is increasingly used as an educational tool and taught in elementary schools around the world. This trend begins to follow Croatia, where chess in many schools is carried out as an extracurricular and/or extracurricular activity. In this paper we will present and analyze the advantages that chess has for the elementary school students and empirically determine the occurrence of chess as an extracurricular activity in the schools in the City of Zagreb. Also, based on the results of the qualitative research, the views of the chess teachers regarding advantages of chess implementation, ways of determining the interests of students, and teaching methods used in chess teaching will be presented.

Izvannastavne aktivnosti kao različiti organizacijski oblici okupljanja učenika u slobodno izvannastavno vrijeme u školi imaju pretežito kulturnoumjetničko, športsko, tehničko, rekreacijsko ili znanstveno obilježje (Cindrić, 1992). Kroz njih učenici zadovoljavaju svoje potrebe i razvijaju kulturu korištenja slobodnog vremena. Zahvaljujući svom pedagoškom potencijalu, šah se sve više implementira u kurikulume škola i poučava na svim razinama obrazovanja. Ovaj trend počinje pratiti i Hrvatska, u kojoj se šah u mnogo škola provodi kao izvannastavna i/ili izvanškolska aktivnost. U ovome radu prikazat će se i analizirati prednosti koje bavljenje šahom ima za učenike osnovnih škola te će se empirijskim istraživanjam utvrditi pojavnost izvannastavne aktivnosti šaha u školama Grada Zagreba. Temeljem rezultata kvalitativnog istraživanja prikazat će se stavovi voditelja šaha kao izvannastavne aktivnosti o prednostima njegove implementacije, načinima utvrđivanja interesa te o načinima rada voditelja i polaznika/učenika.