EXCITED TRIPLET-STATE PHOTOPHYSICS OF THE SULFONATED ALUMINUM PHTHALOCYANINES BOUND TO HUMAN SERUM-ALBUMIN

The binding of the sulphonated aluminum phthalocyanines to human serum albumin (HSA) in aqueous phosphate buffer solution at 25 degrees C ha s been studied by measuring the properties of the triplet excited stat es of these dyes, The triplet lifetimes were measured by triplet-tripl et absorption flash photolysis. The triplet lifetime of the disulphona ted AlS2Pc (2.5 mu M) varies from 500 +/- 30 mu s in the absence of pr otein to 1100 mu s and longer with HSA concentrations above 100 mu M. Under identical conditions, the maximum triplet lifetimes of the mono- , tri- and tetrasulphonated compounds bound to HSA are shorter than th ose for the disulphonated species. The increase in the triplet state l ifetimes is attributed to the ability of the bulk aqueous phase to int eract with the sensitizer at the site of binding; the site of binding being dependent on the degree of sulphonation. For AlS2Pc and AlS3Pc a t all HSA concentrations, and regardless of the degree of sulphonation , all the triplet state decay profiles follow simple pseudo-first-orde r kinetics. The exponential decay of the triplet phthalocyanine at all PISA concentrations is ascribed to the rapid association and dissocia tion of the phthalocyanine-HSA complex on the time-scales of the tripl et state lifetimes, A simplified one-step binding model is utilized to describe the results. The association of AlS1Pc with HSA results in s ubstantial quenching of the triplet state quantum yield, and a more co mplex model is required to analyze the results. The tetrasulphonated c ompound (AlS4Pc) binds to the protein at a site where it experiences s ome protection from the aqueous phase. (C) 1997 Elsevier Science S.A.