Aggregation Kinetics of Meso-tetrakis(4-sulfonatophenyl) Porphine in the Presence of Proteins: Temperature and Ionic Strength Effects

The kinetics of J-aggregation was studied through UV/Vis spectroscopy for meso-tetrakis(4-sulfonatophenyl) porphine—TSPP at pH = 2.0 using the protein human serum albumin as template. The effect of protein concentration on the kinetics was monitored by the appearance of the J-aggregate band at 486 nm and the simultaneous decrease of the monomer absorption at 434 nm. A simple equation based on the X ^k Y reaction, where k may be assumed as a pseudo-first-order rate, fits well both the J-aggregation formation and monomer transformation. Temperature dependence of the reaction rate follows an Arrhenius behavior up to T = 38°C, but above this value the dependence is inverted. This temperature seems to reflect the mid-point for the thermal denaturation of HSA. Ionic strength effect clearly exposes the prevalence of the electrostatic nature of this J-aggregation, and using a semi-empirical equation an estimate of the interaction length between TSPP-Na⁺ of 4.5. an was obtained in good agreement with crystallographic data. Absorption band shift and bandwidth were used to estimate the number of monomers in the J-aggregate unit that leads to spectral changes, and a number around 6–7 was found. There is no apparent growth of J-aggregate taking into account the invariance of the bandwidth of J-aggregate band with time at any of the temperatures studied.