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Hidroksitirozol i homovanilinski alkohol smatraju se snažnim antioksidansima koji svojim antioksidativnim učincima ostvaruju niz biološki povoljnih učinaka koji se intenzivno proučavaju. Međutim, u literaturi nema veliki broj podataka o kinetičkim obilježjima i mehanizmu njihovih antioksidativnih reakcija. Prema strukturi molekule hidroksitirozol i homovanilinski alkohol razlikuju se u tome što hidroksitirozol u orto-položaju ima hidroksilnu skupinu, dok homovanilinski alkohol ima metoksi-skupinu, što uvelike utječe i na karakteristike njihovih reakcija. U ovom radu spektrofotometrijski su određeni kinetički parametri reakcija hidroksitirozola i homovanilinskog alkohola s DPPH● u otapalu 1,4-dioksan:voda = 99:1 (v/v). Određene su konstante brzine reakcija, Arrheniusovi i Eyringovi aktivacijski parametri, kinetički izotopni efekt te izotopne razlike aktivacijskih parametara. Konstanta brzine reakcije hidroksitirozola i DPPH● u iznosu od 5,46 mol-1 dm3 s-1 znatno je viša od konstante brzine reakcije homovanilinskog alkohola i DPPH koja iznosi 0,31 mol-1 dm3 s-1. Dobivena razlika u konstantama brzina reakcija upućuje na veću antioksidativnu aktivnost hidroksitirozola, što je s obzirom na razlike u strukturi i očekivano. Na veću antioksidativnu aktivnost hidroksitirozola upućuju i dobiveni iznosi Gibbsove aktivacijske energije od 69,00 kJ mol-1 za hidroksitirozol u odnosu na 82,49 kJ mol-1 za homovanilinski alkohol. Određen je i kinetički izotopni efekt (KIE) 16,88 za hidroksitirozol i 14,29 za homovanilinski alkohol. Navedeni iznosi KIE upućuju na efekt tuneliranja jer izlaze izvan granice maksimalnog mogućeg kinetičkog izotopnog efekta prema semiklasičnoj teoriji. Prisustvo KIE omogućuje eliminaciju uzastopnog elektron transfer/proton transfer (ET/PT) reakcijskog mehanizma. Koristeći dobivene iznose Gibbsove aktivacijske energije provedena je i termokemijska analiza (TKA). Na temelju provedene TKA eliminiran je i proton transfer/elektron transfer (PT/ET) mehanizam. Prema TKA, najvjerojatniji je mehanizam reakcije u kojemu dolazi do usklađenog prijelaza elektrona i protona (PCET). Dobiveni kinetički i termodinamički parametri mogu doprinijeti razumijevanju reakcija antioksidansa i fenola te pomoći rasvijetliti kinetičke i mehanističke karakteristike tih reakcija. Hydroxythyrosol and homovanillyl alcohol are considered to be strong antioxidants that, with their antioxidative effects, achieve a series of biologically favorable effects that are intensively studied. However, there is not a large amount of data in the literature on kinetic features and the mechanism of their antioxidant reactions. The molecular structure of these two molecules differs in that hydroxytyrosol has a hydroxyl group at the ortho-position, while at the same position homovanillyl alcohol has a methoxyl group. This difference also largely affects the characteristics of their reactions. In this thesis, the kinetic parameters of the reactions of the mentioned phenols were determined in a reaction with DPPH● in a reaction solvent mixture consisting of 1,4-dioxane:water = 99:1 (v/v). Reaction rate constants, Arrhenius and Eyring activation parameters, kinetic isotope effect, and isotopic differences of activation parameters were determined using the spectrophotometrical method. The reaction rate constant for reaction of hydroxytyrosol and DPPH● with the value of 5,46 mol-1 dm3 s-1 is significantly higher than the reaction rate constant of homovanillyl alcohol and DPPH●, which is 0,31 mol-1 dm3 s-1. Larger reaction rate constant for hydroxytyrosol reaction points to a higher antioxidant activity of hydroxytyrosol, which was also expected considering structural differences. The obtained values of Gibbs activation energy of 69,00 kJ mol-1 for hydroxytyrosol and 82,49 kJ mol-1 for homovanillyl alcohol also indicate the higher antioxidant activity of hydroxytyrosol. Both investigated phenols showed high values of kinetic isotope effect (KIE), 16,88 for hydroxytyrosol and 14,29 for homovanillyl alcohol. Both specified values of KIE point to the tunneling effect since they are outside the limit of the maximum possible kinetic isotope effect according to semiclassical theory. Moreover, the presence of KIE allows the elimination of the sequential electron transfer/proton transfer (ET/PT) as a possible reaction mechanism. Additionally, thermochemical analysis was also performed using the experimentally determined Gibbs activation energy.. Based on the performed thermochemical analysis and the comparison of the obtained Gibbs activation energies with the standard reaction Gibbs energies, the proton transfer/electron transfer (PT/ET) consecutive mechanism was also eliminated. According to the thermochemical analysis, the most probable reaction mechanism is proton-coupled electron transfer (PCET). Obtained kinetic and thermodynamic parameters can contribute to the understanding of the reactions of antioxidans and phenols and help us to ellucidate kinetic and mechanistic characteristics of these reactions. |
