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Propolis is one of the richest sources of plant phenolics (flavonoids and phenolic acids), which are widely recognized as rather strong antioxidants. The aim of our work was to use colored stable free radical (DPPH · and ABTS ·+) spectrophotometric and thin-layer chromatographic (TLC) assays to study the antioxidative behavior of the phenolics (caffeic acid, galangin and pinocembrin) most commonly present in Croatian propolis samples obtained from different Croatian regions. We propose a mathematical model providing a more sophisticated interpretation of the obtained results and a new parameter named antioxidative efficiency (AOE) is introduced. The kinetic behaviour of chosen standards determined by spectrophotometric assays follows the exponential decrease of the absorption curve. Explained numerically, AOE represents the absolute value of the first derivative of an absorbance curve in the point A 0 /e (where A 0 is the absorbance measured at t = 0 and e is the natural logarithm base). The advantage of this newly introduced parameter is that it provides an easy and accurate mutual comparison between the rates of antioxidative efficiency of different propolis samples. A TLC assay was only applicable in the case of the DPPH · radical. Dose-response curves were described using a linear function with AOE expressed as a coefficient of the slope. The chromatographic method was shown to be very rapid, reliable and easy-to-perform.

Abstract. Kinetics and mechanisms of the oxidation of methoxyurea and N-methylhydroxyurea were studied in neutral and basic aqueous solutions. The obtained pH dependences of the oxidation rates indicate that for both hydroxyureas the reactive species are the deprotonated ones. The second order rate constants, the activation enthalpies and the activation entropies for the reactions of methoxyurea (O-methylhydroxyurea) and N-methylhydroxyurea anions with Fe(CN) 6 3− at 25 o C, I = 2 mol dm −3 (NaClO 4 ) were determined as (5.06 ± 0.01)  10 2 mol −1 dm 3 s −1 , (1.92 ± 0.02)  10 4 mol −1 dm 3 s −1 , 27 ± 1 kJ mol, and 107 ± 4 J mol −1 K −1 , respectively. The pK a value of methoxyurea at 25 o C and 2 mol dm −3 ionic strength was determined kinetically as 12.7 ± 0.1 and the thermodynamic parameters for the deprotonation reaction were determined as Δ a H = 43 ± 1 kJ mol, and. When the kinetic results are compared with the data reported for hydroxyurea, an inverse dependence of the rate constants on the pK a of the hydroxyurea derivatives at 25 o C is observed. Such unexpected behaviour has been explained by the ab initio calculations and NBO analysis of HOMOs for all three hydroxyureates. (doi: 10.5562/cca1799)

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The molecular structure of the title compound, also known as 2-thiothymine [systematic name: 2,3-dihydro-5-methyl-2-thioxopyrimidin-4(1H)-one], C(5)H(6)N(2)OS, is similar to that of thymine, with only small changes in the ring structure, apart from a significant difference at the substitution site [S=C = 1.674 (1) A]. The molecules are connected by hydrogen bonds, with N-H.O = 2.755 (2) A and N-H.S = 3.352 (1) A. The hydrogen-bond network is different from that in thymine, since it involves all the donor and acceptor atoms.

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