Can superoxide organic chemistry be observed within the liposomal bilayer?

2-(Dimethylamino)fluorene (la) and 5-benzoyloxy-2,3,7,8,12,13,17,18-octaethylporphyrin (4) react with superoxide anion radical (generated from KO2/18-crown-6 polyether) in aprotic media. Yet, when incorporated into the lipid bilayer of dimyristoyl phosphatidylcholine liposomes, these two substrates are inert to superoxide, generated enzymatically (xanthine oxidase/acetaldehyde) or radiolytically (60Co or 137Cs source/formate solution). On the other hand, 7-acetoxy-4-methylcoumarin (6), which reacts with superoxide in aprotic media yielding the corresponding 4-methylumbelliferone (7), also gives the same product when incorporated within the liposomal bilayer and reacted with radiolytically or enzymatically generated superoxide. In the latter case, the reaction is inhibited by SOD. NMR studies indicate that in contradistinction to the highly lipophilic la and 4, which presumably lie well within the lipid bilayer, 7 lies in a highly polar region of the bilayer. These results suggest that superoxide anion does not penetrate deep into the liposomal bilayer; nevertheless, superoxide reactions can, indeed, be observed, provided the active site of the substrate lies at or near the lipid-water interface.