4-hydroxy-2,2,6,6-tetramethylpiperidine-N-oxide (TEMPOL), 4-OH-TEMP and PpIX disodium were purchased from Sigma-Aldrich (St. Louis, MO, USA). Phosphate buffered saline pH 7.2 (PBS) was purchased from the Invitrogen Corporation (Carlsbad, CA, USA), and L-histidine was purchased from Wako Pure Chemical Industries, Ltd. (Osaka, Japan). All other reagents were of analytical grade.