Browsing by Author "Niksirat, Pantea"

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    Complex formation of antitumor active dirhodium(II) acetate with small molecules of biological interest
    (2012) Niksirat, Pantea; Jalilehvand, Farideh
    Dirhodium(II) acetate, consisting of Rh2(CH3COO)4 complexes, is a compound with significant antitumor activities and limited side effects. Although the antitumor studies are mostly focused on its interaction with DNA, the mechanism is not recognized yet. In addition to DNA, other biological molecules such as amino acids can interact with this complex. In this dissertation the interaction of Rh2(CH3COO)4 with the amino acids cysteine, methionine and imidazole, which is the side chain of the amino acid histidine, have been investigated using different spectroscopic techniques including EXAFS, multinuclear NMR, IR, ESI-MS and UV-Vis spectroscopy. A pink solid complex Rh2(CH3COO)4(lm)2 was formed from the reaction of Rh2(CH3COO)4 and imidazole (Im) in the mole ratio 1 :2, where the imidazole ligands were coordinated through the axial positions of the Rhi(CH3COO)4 complex with the Rh­N bond distance 2.25 ± 0.02 A. In a freshly prepared aqueous solution of Rh2(CH3COO)4 and methionine (Met) in the mole ratio 1:2, axial coordination of methionine ligands occurs m Rhi(CH3COO)4(Met)1.2 complexes, with the average bond distances of 2.03 ± 0.02 A, 2.47 ± 0.04 A and 2.40 ± 0.02 A for the Rh-O, Rh-S and Rh-Rh bonds, respectively. Replacement of acetate in equatorial sites with methionine takes place over a period of 2 - 4 weeks, when the solution is saturated with methionine. The composition of the complexes gradually changes from Rhi(CH3COO)4(S-Meth to [Rh2(CH3COO)4.n(S,O­Met)n](CH3COO)n (n = 2 - 4), with peaks at 4685 ppm and 4616 ppm in the 103Rh NMR spectrum, more shielded than the resonance 8 (103Rh) = 7476.2 ppm for the hydrated Rhi(CH3COO)4(H2Oh complex, with water molecules as axial ligands. An EXAFS spectrum measured after 10 months of Rh2(CH3COO)4 dissolved in saturated methionine solution gives the average bond distances 2.03, 2.29 and 2.55 ± 0.02 A for the Rh-O, Rh­S and Rh-Rh bonds, respectively, assuming [Rh2(CH3COO)4_n(Met)n] (n = 2 - 4) species. The shorter Rh-Sand longer Rh-Rh distances, relative to those of Rhi(CH3COO)4(Met)1_2, confirm the equatorial coordination of the methionine ligands. By dissolving Rh2(CH3COO)4 in ethanethiol a Rh2(CH3COO)4(EtSHh complex is formed, with Rh-Rh and Rh-S bond distances of 2.41 ± 0.02 A and 2.53 ± 0.02 A. The UV-Vis spectra of Rh2(CH3COO)4 dissolved in water, ethanethiol, saturated methionine solution and 0.1 M / 1.0 M imidazole solutions show peak maxima at Amax = 588, 556, 537 and 521 nm, respectively, which imply that the ligand field strength for these axially coordinated ligands (X) in Rh2(CH3COO)2_4X2 is: X = H2O < EtSH (thiol) < methionine (thioether) < imidazole (Nsp2). Ethanethiol has a single functional group (-SH), and in its reaction with Rhi(CH3COO)4, the Rh-Rh bond remains intact. However, the chelating thiol-containing ligands cysteine (H2Cys) and N-acetylcysteine (H2NAC), are capable of breaking the Rh­Rh bond in their reaction with Rhi(CH3COO)4, to form mononuclear and binuclear Rh(III) species, respectively. In the solid [Rh(iii)HCys)(Cys)(H2O)].H2O compound, cysteine coordinates to Rh(III) ion through its thiol, amine and / or carboxylate groups, with average bond distances of Rh-(N/O) 2.17 ± 0.02 A and Rh-S 2.37 ± 0.02 A. In the solid [Rh111i(HNACh(NACh(H2O)4].2H2O compound, the only available coordination sites of N-acetylcysteine are its thiol and carboxylate groups. The oxidation state +3 for the rhodium ions was confirmed by Rh L3-edge XANES and magnetic susceptibility measurements.
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    ItemOpen Access
    The Effects of Maximum Residue Limits on Trade: A Case of Canadian Wheat Export
    (2020-11-05) Niksirat, Pantea; McQuillan, Kevin
    Wheat is Canada’s most produced crop, and Canada is one of the major wheat producers in the world. Canadian wheat production well exceeds the domestic demands, and thus more than half of the wheat production is being exported. With climate change and the increase in global food insecurity, Canada can play an essential role to feed the world. The success of the Canadian wheat industry depends on exports and market access. In addition to customary tariffs, non-tariff measures can affect trade. Maximum residue limits (MRLs) are a class of non-tariff measures. MRLs are the maximum levels of pesticide residues that remain in food and are deemed legally permitted. Codex Alimentarius Commission, an international standard-setting of the United Nations, vii establishes MRLs by applying science-based risk assessments. Following the Codex MRLs is voluntary, and countries can choose to develop their national MRLs as long as they abide by scientific methodologies that do not impede trade. States can decide to follow Codex or national MRLs. In either case, the domestic agricultural products, as well as the importing commodities, must meet the MRL requirements. The problem of MRLs has increased recently since countries started to develop national MRLs. The increasing number of unharmonized national MRLs create trade issues because by following the national MRLs, producers and farmers do not necessarily follow the importing countries’ regulatory requirements. A trade barrier occurs when the importing country’s MRLs are more stringent than the exporting countries’ MRLs. In addition, missing MRLs and pesticide bans add additional trade barriers. The complexity of the MRLs setting provides an opportunity for nationalistic and protectionist approaches to limit trade to promote national productions. The current literature has not reached a consensus on the effects of MRLs on trade. This report is the first quantitative analysis of the impacts of MRLs on Canadian wheat exports. In this study, the trade effects of five MRLs of major pesticides in wheat production were studied for 2018. A time-series analysis was also performed between 2014 and 2018. The gravity model and a bilateral stringency index were utilized for the econometric analysis. Although no wheat violations were reported for Canada in the last few years, the theoretical results show low MRLs can potentially act as trade barriers for Canadian wheat exports. Further studies are required to assess the exact values of the MRL impacts on trade.