Radiation chemistry of cyanine dyes: oxidation and reduction of merocyanine 540

Abstract

Merocyanine 540 (MC) shows promise as a treatment for certain types of leukemia. It is shown that MC readily undergoes one-electron reduction under pulse radiolytic conditions. The {pi}-radical anion, produced by reduction with hydrated electrons and 2-hydroxypropyl radicals, disproportionates rapidly (k = 1.9 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1}) under anaerobic conditions but reduces O{sub 2} to superoxide ions (k = 1.6 {times} 10{sup 9} M{sup {minus}1}s{sup {minus}1}) in aerated solution. The dye reacts with trichloromethylperoxyl radicals (k = 9 {times} 10{sup 8} M{sup {minus}1}s{sup {minus}1}) to form several products, one of which is believed to be an adduct formed by addition of CCl{sub 3}OO{sup {sm bullet}} to the bridgehead carbon atom of the benzoxazole subunit. This species decays via first-order kinetics (k = 4.0 {times} 10{sup 3} s{sup {minus}1}) under pulse radiolytic conditions to form cleavage products. A second primary product is believed to arise from addition of CCl{sub 3}O{sub 2}{sup {sm bullet}} to the polymethine chain to form an {alpha}-amino carbon-centered radical capable of reducing O{sub 2} to superoxide ions. Preliminary studies indicate that the breakdown products are cytotoxic and could be important intermediates for the known antiviral activity of MC.