In the course of this research, ascordic acid, dihydroxyfumaric acid (DFH4) and ten of its newly obtained derivates were investigated in the modified DPPH• assay using the stopped-flow method. Fig.1. shows five most potent antioxidants synthesized from DFH4. Obtained curves demonstrate the existence of three types of kinetic behaviors: fast, mixed (the fast stage is followed by a slower stage, which can be visually distinguished) and slow kinetic behavior. The kinetics is determined both by the molecular structure of the studied compound and by the stability of the antioxidant radical formed following the reaction with DPPH•. Ascorbic acid is the only compound exhibiting rapid reaction with the radical, which is determined by the presence of active functional groups, which allow the rapid transfer of hydrogen atoms. Analysis of the obtained kinetic curves shows that DFH4, ester 2, acid 12 and diol 13 (Fig.1) show mixed kinetics. Upon interaction with DPPH•, a rapid decrease in radical concentration is observed in the first 3-10 seconds (1 min for ester 2), but the steady state is reached in a longer time (several minutes), compared to ascorbic acid. Anilide 5 and fumaramide 6 exhibit a slow kinetics in the DPPH• assay, with a reaction time of 45-60 minutes. O O OH OH O O 2 dimethyl 2,3-dihidroxifumarate HO O OH NH O O 5 (E)-methyl 2,3-dihydroxy-4-oxo-4(phenylamino)but-2-enoate HO O OH O O O H2N N OH HO O HN NH O N 2 6 + N fãrã solvent, temp. camerei, 8h 77% 2,3-dihydroxy-N1,N4-di(pyridin-2-il)fumaramide OH HO N NH O OH 12 (E)-3-(1H-benzo[d]imidazol-2-il)-2,3dihydroxiacrylic acid OH HO N NH N HN 13 (E)-1,2-di(1H-benzo[d]imidazol-2-il)etene-1,2-diol Fig.1. Most potent obtained derivatives of DFH4