Photophysical studies on the interaction of two water-soluble porphyrins with bovine serum albumin. Effects upon the porphyrin triplet state characteristics

The interaction of two metal-free water soluble porphyrins (PPh): meso-tetrakis (p-sulfo-natophenyl)porphyrin (TPPS4) and meso-tetrakis( 4-N-methyl-pyridiniumyl)porphyrin (TMPyP), with bovine serum albumin (BSA) was investigated in the pH range from 4.0 to 8.5 using the flash-photolysis technique in comparison with results obtained by optical absorption, fluorescence and resonance light scattering. It was found that in the presence of BSA, TPPS4 can exist in aqueous solutions as free monomers, aggregates and/or monomers bound to a single BSA molecule and monomers inside the BSA aggregates, while TMPyP does not form aggregates at binding. Binding to BSA transforms the profile of the triplet decay curve from monoexponential to biexponential form with the component lifetimes and relative amplitudes depending on binding characteristics. The triplet lifetime of a bound porphyrin monomer is longer than that of a free one. The aggregation of TPPS4 observed at [TPPS4]/[BSA] > 1 reduces the T-T absorption since the lifetimes of the aggregate excited states are very short and/or quantum yield of the aggregate triplet state is very low. The porphyrin binding to BSA reduces the quenching constants of the porphyrin triplet states by molecular oxygen due to obstacles produced by binding. This effect is especially pronounced for the porphyrin molecules located inside BSA aggregates formed around the porphyrin molecules in excess BSA. (C) 1998 Elsevier Science S.A. All rights reserved.