Photodynamic-induced cytotoxicity by hypericin (HYP) was studied on three human melanoma cell lines: one pigmented cell line (G361) and two amelanotic cell lines (M18 and M6). No significant variation in the rate of uptake and in the maximum level of HYP incorporation for the different cells was observed. In the dark, no cytotoxicity was observed in the range 0-10-6 M HYP for the three cell lines. Amelanotic cells were found to be more sensitive than pigmented cells to irradiation of HYP with visible light (lambda > 590 nm). In addition, for the three cell lines HYP-induced photocytotoxicity was found to be drug-dose and light-dose dependent. Under the conditions used, thiobarbituric acid-reacting substances (TBARs) were significantly increased in amelanotic cells after irradiation (P < 0.0001). By contrast, the amount of TBARS remained unchanged in pigmented cells. Antioxidant defenses including enzymes and glutathione (GSH) were assayed before and after HYP photosensitization. Significantly increased total SOD activity was observed after photosensitizaton for amelanotic cells (P < 0.05), while glutathione peroxidase (GSHPx) and catalase (Cat) activities but also GSH levels were significantly decreased (P < 0.01). In pigmented cells a significantly increased Cat activity was found (P < 0.05), whereas GSHPx was unaffected after irradiation. It can be inferred that (a) HYP may be an effective PDT agent for melanoma and (b) there is a relationship between melanin content and sensitivity to HYP phototoxicity in human melanoma cells.
The quenching rate constants of singlet oxygen O2(1Δg) luminescence at 1.27 μm by 29 amines and aromatic
hydrocarbons were measured in acetonitrile and benzene. Quenching occurs via reversible charge transfer
with the formation of an exciplex with a partial (δ ≈ 0.2 esu) electron transfer. An estimate of the intersystem-crossing rate constant for the exciplex of ca. 1010 s-1 is obtained, and a modest decrease with increasing
exciplex energy is noted. The data predict and it is confirmed that charge transfer plays a significant role in
the nonradiative decay of singlet oxygen in solvents as difficult to oxidize as toluene-d
8 and mesitylene. It
is also confirmed that N,N,N‘,N‘-tetramethyl-p-phenylene diamine quenches O2(1Δg) nearly exclusively by
full electron transfer in D2O, but electron transfer is not observed in other solvents.
Quenching of the lowest n,n* triplet states of various diketones and acetone by molecular oxygen was studied by time-resolved ' 0 2 luminescence and nanosecond laser photolysis in solution. The singlet oxygen production efficiency SA in benzene was 0.28 for acetone and 0.28-0.58 for a-diketones. The quenching rate constants of a-diketone triplets by oxygen (kT) are very low, (2-9) x lo8 M-' s-l. A decrease in I 0 2 lifetime was observed upon excitation of a-diketones at laser pulse energies 21 mJ and is explained by quenching of singlet oxygen by acylperoxy radicals. Rate constants of ' 0 2 quenching by different peroxy and acylperoxy radicals kq were estimated to be "(1-5) x 1O' O M-' s-l, near the diffusion-controlled limit.
The quenching of the triplet state of hypericin by acceptors of electronic energy and donors and acceptors of electrons was studied by nanosecond laser photolysis. The energy of triplet hypericin is E T ) 13 350 cm -1 , and for acceptors with E T < 13 300 cm -1 the observed energy transfer rate constant declines with increasing exothermicity. Quenching of triplet hypericin by amines in acetonitrile and electron acceptors in DMSO occurs via electron transfer. Spectroscopic and kinetic properties and the quantum yield of hypericin radical ion formation were established. The rate constant profiles for electron transfer (in both directions) are consistent with an excited-state reaction occurring before electron transfer. The rate constant of the interaction of reduced hypericin with O 2 was estimated to be 3.8 × 10 8 M -1 s -1 in acetonitrile.
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