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 b and at 330 nm, epsilon = 300 +/- 60 M(-1) cm(-1). Because the ionic st rength 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 potenti al of the MEMI radical, E(7) = 0.45 V, is lower than E(7) = 0.48 V of the Trolox C radical, which means that MEMI may restitute vitamin E un der physiological conditions (assuming similar reduction potentials of Trolox C and vitamin E radicals). Because pK(a) = 10.3 of the SH grou p in MEMI is lower than pK(a) = 11.9 of the OH group of Trolox C, the redox equilibrium with Trolox C is reversed at pH 13, and E(13)(MEMI-r adical) = 0.29 +/- 0.04 V is determined against E(13)(Trolox C - radic al) = 0.19 V. In contrast to extraordinary electron donating propertie s, MEMI is only a moderately good H-atom donor. k(.CH3 + MEMI) = (1.5 +/- 0.3) x 10(5) M(-1) s(-1) in neutral media is considerably lower th an k(.CH3 + GSH) = 5 x 10(7) M(-1) s(-1), which is explained by the zw itterionic structure of MEMI. The ability of MEMI to act as antioxidan t in biological systems is further demonstrated by its ability to effi ciently scavenge superoxide and linoleate peroxyl radicals. On the bas is of the two-fold increase in the rate of inhibition of radiation-ind uced oxidation of linoleic acid by MEMI and vitamin E (1:1) as compare d with that by MEMI alone, it is concluded that MEMI shows synergism w ith vitamin E.