A reinvestigation of the reaction of coumarins with superoxide in the liposomal bilayer: Correlation between depth and reactivity

Afri et al. reported in this journal (Free Radic. Biol. Med. 32:605-618; 2002) that a direct relationship exists between the depth of alkanoylcoumarins 1 within the liposomal lipid bilayer and the rate at which they undergo superoxide-mediated saponification. These results were based on a correlation between the ¹³C NMR chemical shift of polarizable carbonyl carbons and the E_T(30) polarity they sense. Subsequent studies challenged these results, however, demonstrating that, in conjugated ketones and aldehydes, charge separation influences the E_T(30) polarity measured. To elucidate whether this is true for conjugated esters such as coumarins as well, the nonconjugated analogs 3,4-dihydrocoumarins 11 and 15 were intercalated within DMPC liposomal bilayers and their relative locations within the liposomal bilayer were determined. The length of the alkyl chain substituted at C-4 and C-10 influences the depth of the substrates within the liposome. The location of these 3,4-dihydrocoumarins corresponds well with the conjugated analog coumarin 1-confirming the validity of the abovementioned results of Afri et al. The lack of substantial charge separation in the coumarin 1 system presumably results from the "swamping-out" effect of the ester oxygen. Instead of 1,3-delocalization of charge, typical of conjugated systems, delocalization of the nonbonding pair on the ester oxygen predominates.