Heterocyclic thiols as antioxidants: Why Ovothiol C is a better antioxidant than ergothioneine

Spectral, acid-base, and redox properties of 4-mercaptoimidazoles were investigated by pulse radiolysis in aqueous solutions. Thiyl radicals of 1-methyl-5-ethyl-4-mercaptoimidazole (MEMI) have weak absorption band at 330 nm, ε = 300 ± 60 M−1 cm−1. Because the ionic strength variation from 0.01 to 0.1 M in the pH range from 3 to 14 does not influence the rate constant of the radical decay, it is concluded that the MEMI thiyl radical is neutral. At pH 7, the reduction potential of the MEMI radical, E7 = 0.45 V, is lower than E7 = 0.48 V of the Trolox C radical, which means that MEMI may restitute vitamin E under physiological conditions (assuming similar reduction potentials of Trolox C and vitamin E radicals). Because pKa = 10.3 of the SH group in MEMI is lower than pKa = 11.9 of the OH group of Trolox C, the redox equilibrium with Trolox C is reversed at pH 13, and E13(MEMI-radical) = 0.29 ± 0.04 V is determined against E13(Trolox C - radical) = 0.19 V. In contrast to extraordinary electron donating properties, MEMI is only a moderately good H-atom donor. k(·CH3 + MEMI) = (1.5 ± 0.3) × 105 M−1 s−1 in neutral media is considerably lower than k(·CH3 + GSH) = 5 × 107 M−1 s−1, which is explained by the zwitterionic structure of MEMI. The ability of MEMI to act as antioxidant in biological systems is further demonstrated by its ability to efficiently scavenge superoxide and linoleate peroxyl radicals. On the basis of the two-fold increase in the rate of inhibition of radiation-induced oxidation of linoleic acid by MEMI and vitamin E (1:1) as compared with that by MEMI alone, it is concluded that MEMI shows synergism with vitamin E.