Anthralin‐derived Transients—ii. Formation of the Radical by Spontaneous Fragmentation of Both Singlet and Triplet States of the 10,10′‐dehydrodimer: Radical Pair Multiplicity Effects

Abstract

The singlet and triplet states of the anthralin (1,8-dihydroxy-9-anthrone) dehydrodimer have been produced selectively in benzene via pulsed laser excitation and pulse radiolysis respectively. The lifetime of S1 is less than or equal to 30 ps, that of T1 short but unspecified. Both states fragment spontaneously to yield a pair of anthralin radicals. The singlet radical pair predominantly undergoes geminate recombination within the solvent cage. In contrast, the corresponding triplet radical pair undergoes essentially exclusive cage escape to give the anthralin free radical (lambda max 370, 490 and 720 nm) which recombines under normal diffusive conditions. Both recombination processes lead, at least in part, to one or more species which have been assigned as tautomeric forms of the original dimer. The anthralin free radical in benzene is insensitive to the vitamin E model 6-hydroxy-2,2,5,7,8-pentamethylchroman and reacts only slowly with oxygen.

Detection and characterization of 9-anthron-10-yl radicals formed by antipsoriatic and tumor-promoting 9-anthrones in aqueous buffers

“…These results indicate that additional factors are also important in determining the biological activity of this class of compounds. In contrast to results reported from thermolysis, photolysis, or pulse radiolysis studies in organic solvents, in which AN' was best obtained from anthralin dimer (36)(37)(38), attempts to generate AN' by autoxidation or photolysis of the dimer in aqueous buffers did not produce significant amounts of this radical.…”

“…Because irradiation of anthralin or its dimer in various solvents has been shown to produce AN' (36)(37)(38), the effect of UV irradiation upon the formation of the DBNBSanthron-10-yl spin adduct was also investigated. In contrast to results obtained in organic solvents by others (36)(37)(38), irradiation of aqueous solutions (10% MegSO; pH 7.5 and pH 10) containing anthralin dimer (200 µ ) and DBNBS (1.0 mM) did not produce significant amounts of spin adducts (data not shown). Due to the significant effect of pH on DBNBS adduct formation, a more thorough investigation of the pH dependence of adduct formation was undertaken.…”

“…10, '-Disubstituted anthralin analogs that cannot oxidize at the 10-position are also not active against psoriasis (32) or as tumor promoters (16). Therefore, it has been suggested that an anthralin radical (i.e., the 1,8-dihydroxy-9-anthron-10-yl radical, AN*; Scheme 1,3) formed during the oxidation process, or the reactive oxygen species produced from the oxidation process are the mediators of these biological responses (4,23,33,36).…”

“…AN* has been generated by subjecting either anthralin or its dimer to thermolysis (120 °C) or photolysis in xylene and characterized by EPR spectroscopy (37,38), and by pulsed laser excitation or pulse radiolysis of anthralin (29) or its dimer (36) in benzene. However, the role of these radicals in mediating the biological effects of 9-anthrones is still uncertain.…”

Anthralin: Primary Products of Its Redox Reactions

“…It has been claimed that anthralin reacts with oxygen to yield the superoxide radical anion, but there is strong evidence that auto-oxidation of anthralin can only occur in its enol form ( 1 - enol ) or in the anthralyl anion 2 - (Scheme ). ,, Therefore, it was proposed that these two species react with oxygen, producing superoxide and the radical cation of 1 - enol or the anthralyl radical, 2 • , respectively. The role of 2 • in the therapeutic process is not known, but it is often questioned because 2 • is a particularly stable radical, as shown by its low reactivity toward oxygen or vitamin E …”