Singlet oxygen generation of porphyrins, chlorins, and phthalocyanines

The production of singlet oxygen was measured indirectly for three classes of photosensitizers: porphyrins (Photofrin II, TPPS4), chlorins (MACE, DACE), and a phthalocyanine (CASPc). Buffered solutions of sensitizers and singlet oxygen acceptors were irradiated with a CW dye laser and the oxygen depletion was monitored electrochemically with a Clark-type microelectrode. A comparison of oxygen-depletion rate constants and quantum efficiencies yields the order of efficiency of the sensitizers: TPPS4 greater than MACE greater than PII greater than DACE greater than CASPc. For singlet oxygen acceptors the order was: furfuryl alcohol greater than imidazole greater than tryptophan. CHO cell suspensions were also used as acceptors. Here the order of efficiency (per absorbed photon) was PII greater than MACE approximately CASPc. Expressed in terms of oxygen depletion per cell the order was CASPc approximately PII greater than MACE. When performing cell clonogenicity studies the order of efficiencies, expressed as percentage cell kill per unit weight of sensitizer, was CASPc greater than PII greater than MACE approximately DACE. The discrepancy between the efficiencies of sensitizers to generate singlet oxygen and their cytotoxicity was explained in terms of photodegradation (for the chlorins), intracellular localization (for PII), and contributions from a Type I mechanism (for CASPc).