THE EFFECT OF BIOLOGICAL SUBSTRATES ON THE ULTRAFAST EXCITED-STATE DYNAMICS OF ZINC PHTHALOCYANINE TETRASULFONATE IN SOLUTION

Zinc phthalocyanine tetrasulfonate (ZnPcS4), a potential photosensitiz er for photodynamic therapy (PDT), has been studied using femtosecond laser spectroscopy, The excited-state dynamics in water have been foun d to be fast (<80 ps) and dominated by intermolecular aggregation. Sin ce the proposed mechanism for PDT is energy transfer from the triplet excited state of the photosensitizer to triplet O-2 creating singlet O -2, the short lifetime is expected to be unfavorable for producing sin glet O-2, This leads to the suggestion that the presence of biological substrates may have an effect on the excited-state dynamics, To test this hypothesis, the lifetimes of the excited states of ZnPcS4 have be en directly measured in the presence of a model membrane, n-hexadecylt rimethylammonium bromide (CTAB). The excited-state dynamics of ZnPcS4 in buffer solutions and with human serum albumin (HSA) have also been measured, The presence of HSA and CTAB increases the excited-state lif etime significantly relative to that observed in water, The longer lif etime of ZnPcS4 in CTAB (>1 ns) indicates that the micellar surface fa vors monomer formation, By increasing the excited-state lifetime, the surface substantially increases the photosensitizing potential of ZnPc S4.