Photooxidation of soybean apoleghemoglobin with protoporphyrin IX, heme and dye

Soybean apoleghemoglobin a was irradiated with visible light in the presence of different sensitizers to probe the heme environment of the protein. With protoporphyrin IX as sensitizer, specific photooxidation of histidine-92 and histidine-61 occurred. Irradiation of oxyleghemoglobin and cyanleghemoglobin resulted in photooxidation of histidine-92, while addition of methylene blue caused both histidine-92 and histidine-61 to be oxidized. Apoleghemoglobin, irradiated in the presence of rose bengal or methylene blue, lost tryptophan-128 in addition to the two histidines.