Excited triplet state photophysics of the sulphonated aluminium phthalocyanines bound to human serum albumin

• Published in: Journal of photochemistry and photobiology. B, Biology Biology, 1997-03, Vol.38 (1), p.10-17
• Main Authors: Foley, Mary S.C., Beeby, Andrew, Parker, Anthony W., Bishop, Steven M., Phillips, David
• Format: Article
• Language: English
• Subjects: Aluminum; Dynamic binding equilibrium; Fluorescent Dyes - chemistry; Fluorescent Dyes - metabolism; Human serum albumin; Humans; Indoles - chemistry; Indoles - metabolism; Kinetics; Models, Chemical; Organometallic Compounds - chemistry; Organometallic Compounds - metabolism; Photochemistry; Photodynamic therapy; Phthalocyanine; Radiation-Sensitizing Agents - chemistry; Radiation-Sensitizing Agents - metabolism; Serum Albumin - chemistry; Serum Albumin - metabolism; Spectrophotometry, Atomic
• ISSN: 1011-1344; 1873-2682
• DOI: 10.1016/S1011-1344(96)07434-9

The binding of the sulphonated aluminum phthalocyanines to human serum albumin (HSA) in aqueous phosphate buffer solution at 25°C has been studied by measuring the properties of the triplet excited states of these dyes. The triplet lifetimes were measured by triplet-triplet absorption flash photolysis. The triplet lifetime of the disulphonated AlS 2Pc (2.5 μM) varies from 500 ± 30 μs in the absence of protein to 1100 μs and longer with HSA concentrations above 100 μM. Under identical conditions, the maximum triplet lifetimes of the mono-, tri- and tetrasulphonated compounds bound to HSA are shorter than those for the disulphonated species. The increase in the triplet state lifetimes is attributed to the ability of the bulk aqueous phase to interact with the sensitizer at the site of binding; the site of binding being dependent on the degree of sulphonation. For AlS 2Pc and AlS 3Pc at all HSA concentrations, and regardless of the degree of sulphonation, all the triplet state decay profiles follow simple pseudo-first-order kinetics. The exponential decay of the triplet phthalocyanine at all HSA concentrations is ascribed to the rapid association and dissociation of the phthalocyanine-HSA complex on the time-scales of the triplet state lifetimes. A simplified one-step binding model is utilized to describe the results. The association of AlS 1Pc with HSA results in substantial quenching of the triplet state quantum yield, and a more complex model is required to analyze the results. The tetrasulphonated compound (AlS 4Pc) binds to the protein at a site where it experiences some protection from the aqueous phase.