| Abstract | Katehin je flavonoid najviše prisutan u zelenom čaju koji se ističe po antioksidativnom učinku te štiti organizam od štetnih utjecaja oksidativnog stresa. Neki od mehanizama antioksidativnih učinaka katehina su: hvatanje slobodnih radikala, keliranje metalnih iona uključenih u proces nastajanja slobodnih radikala, induciranje antioksidativnih enzima, inhibicija pro-oksidativnih enzima, utjecaj na signalne puteve. U ovom je diplomskom radu ispitana kinetika i mehanizam reakcije katehina i DPPH• radikala radi boljeg razumijevanja reakcijskog mehanizma te općenito reakcija katehina i štetnih radikala. Istraživanje kinetike i mehanizma reakcije katehina i DPPH• provedeno je u smjesi otapala dioksan-voda (0,95:0,05 v/v) te u acetonitrilu. Konstanta brzine reakcije katehina i DPPH• u smjesi otapala dioksan-voda (0,95:0,05 v/v) iznosi 2,27 mol-1 dm3 s-1 te je konstanta brzine reakcije u acetonitrilu 6 puta veća od konstante brzine u smjesi otapala dioksan-voda (0,95:0,05 v/v) te iznosi 12,8 mol-1 dm3 s-1, pri temperaturi 25°C. Izmjerene su konstante brzine za reakciju katehina i DPPH• u smjesi otapala dioksan-voda (0,95:0,05 v/v) na različitim temperaturama od 9 – 40°C. Na temelju dobivenih rezultata prikazane su Arrheniusove i Eyringove ovisnosti konstante brzine o temperaturi iz kojih su određeni aktivacijski parametri. Utvrđene vrijednosti Arrheniusovih aktivacijskih parametara – predeksponencijalnog faktora (A) i energije aktivacije (Ea) su: A = 2,39 ∙ 105 mol-1 dm3 s-1 i Ea = 28,9 kJ mol-1. Utvrđene vrijednosti Eyringovih aktivacijskih parametara – aktivacijske entalpije (ΔH‡), aktivacijske entropije (ΔS‡) i Gibbsove aktivacijske energije (ΔG‡) su: ΔH‡ = 26,39 kJ mol-1, ΔS‡ = 150,2 J K-1 mol-1 i ΔG‡ = 71,18 kJ mol-1. Određen je kinetički izotopni efekt (KIE) u smjesi otapala dioksan-voda (0,95:0,05 v/v) pri temperaturi 25°C, koji iznosi 3,67. Izmjereni KIE upućuje na reakcijski mehanizam HAT (hydrogen atom transfer) ili PCET (proton-coupled electron transfer). |
| Abstract (english) | Catechin is the flavonoid most present in green tea, which stands out for its antioxidant effect and protects the body from the harmful effects of oxidative stress. Some of the mechanisms of antioxidant effects of catechin include: capturing free radicals, chelation of metal ions involved in the process of free radical formation, induction of antioxidant enzymes, inhibition of pro-oxidative enzymes, influence on signaling pathways. In this thesis, the kinetics and mechanism of catechin and DPPH• radical reaction are examined in order to better understand the reaction mechanism and the reactions of catechin and harmful radicals in general. Research of the kinetics and mechanism of catechin and DPPH• reaction was performed in a solvent mixture of dioxane-water (0.95:0.05 v/v) and in acetonitrile. The determined rate constant for the catechin and DPPH• reaction in a solvent mixture of dioxane-water (0.95:0.05 v/v) is 2.27 mol-1 dm3 s-1 and the reaction rate constant in acetonitrile is 6 times higher than the rate constant in the solvent mixture dioxane-water (0.95:0.05 v/v), which is 12.8 mol-1 dm3 s-1, at a temperature of 25°C. Rate constants for the catechin and DPPH• reaction in a solvent mixture of dioxane-water (0.95:0.05 v/v) at different temperatures ranging from 9 to 40 ° C were measured. Based on the obtained results, the Arrhenius and Eyring plots of the rate constants against temperatures are presented, from which the activation parameters were determined. The determined values of Arrhenius activation parameters – pre-exponential factor (A) and activation energy (Ea) are: A = 2.39 ∙ 105 mol-1 dm3 s-1 and Ea = 28.9 kJ mol-1. The determined values of Eyring activation parameters – enthalpy of activation (ΔH‡), entropy of activation (ΔS‡) i Gibbs energy of activation (ΔG‡) are: ΔH‡ = 26.39 kJ mol-1, ΔS‡ = 150.2 J K-1 mol-1 and ΔG‡ = 71.18 kJ mol-1. The kinetic isotope effect (KIE) was determined in a mixture of solvent dioxane-water (0.95:0.05 v/v) at a temperature of 25°C, which is 3.67. The measured KIE refers to the reaction mechanism HAT (hydrogen atom transfer) or PCET (proton electron transfer). |
