Free radical reactions of the phenothiazine, metiazinic acid

Abstract

Absolute rate constants have been measured for the reaction of the anionic conjugate base of metiazinic acid, MZ^–, in the pH range 8–11, with hydrated electrons (k 1.5 × 10⁹ l mol^–1 s^–1), and for the oxidation of MZ^– by OH˙, Br₂^–˙, (SCN)₂^–˙, CO₃^–˙, TI(OH)₂, N₃, ˙˙CH₂CHO, (CH₃SCH₃)₂⁺˙, CH₃SSCH₃⁺˙ and lipoate⁺˙. The k values for these oxidations are found to be in the 10⁹–10¹⁰ l mol^–1 s^–1 range, except for k(˙CH₂CHO + MZ^–) 2 × 10⁸ l mol^–1 s^–1. The oxidation of metiazinic acid leads to MZ^±˙ radical zwitterions with yields ranging from 50 to 100% with the minimum value referring to the oxidation by ˙CH₂CHO. The MZ^±˙ radical zwitterion is characterized by two strong optical absorption bands with maxima at 530 (ε 11 000 l mol^–1 cm^–1) and 270 nm (51 500). Other primary oxidation products are indicated, but only a transient absorption around 350 nm (OH˙ adduct and/or hydrogen abstraction product) could qualitatively be identified in the reaction of metiazinic acid with hydroxyl radicals.