Photoreactivity of biologically active compounds. XVI. Formation and reactivity of free radicals in mefloquine

The formation and reactivity of the tripler state and free radicals of mefl oquine hydrochloride (MQ) have been investigated by pulse radiolysis and fl ash photolysis. The excited triplet, cation radical and anion radical have been produced and their absorption characteristics determined. The triplet- triplet absorption spectrum of MQ showed a maximum at 430 nm, with a molar absorption coefficient of 3600 M-1 cm(-1) and the quantum yield for intersy stem crossing was determined to be close to unity. Deactivation of the trip ler, in the absence of oxygen, led to the formation of MQ cation and/or ani on radicals. The molar absorption coefficient of the cation radical at 330 nm was determined to be 2300 M-1 cm(-1), whilst that for the anion radical was 2400 M-1 cm(-1) at 620 nm and 3600 M-1 cm(-1) at 350 nm. The molar abso rption coefficients of the proposed neutral radical at 320 nm and 520 nm we re 4000 M-1 cm(-1) and 1300 M-1 cm(-1) respectively. The quantum yield for the formation of singlet oxygen, sensitized by MQ tripler, was determined t o be close to unity. Aqueous solutions of MQ were found to photoionize to y ield hydrated electron and cation radical of MQ in a biphotonic process. Th e influences of pH, buffer concentration, oxygen concentration and addition of sodium azide on the formation and reactivity of the transients were eva luated. The reactions between MQ and solvated electrons and superoxide anio n were also studied. (C) 2000 Elsevier Science S.A. All rights reserved.