Optical absorption spectroscopy study on the interaction of 5,10,15,20-Tetrakis(N-Methyl-4-Pyridyl)Porphyrin with human serum albumin

Abstract

The interaction of 5,10,15,20-Tetrakis (N-Methyl-4-Pyridyl) Porphyrin (TMPyP), a water soluble porphyrin, with human serum albumin (HSA) has been investigated in 5 mM phosphate buffer, pH=7.0, at various temperatures by optical absorption spectroscopy. The spectral pattern at various TMPyP/ HSA molar ratios represented three distinct stages in this process. Aggregate formation of TMPyP on HSA at high TMPyP/ HSA molar ratios was designated stage I and aggregate formation of HSA around porphyrin at low TMPyP/ HSA molar ratio was designate stage III. The appearance of an isosbestic point at intermediate TMPyP/HSA molar ratios represented the existence of an equilibrium between the TMPyP:HSA complex and TMPyP:2HSA complex (stage II). Based on the absorption data a binding model has been proposed and used to analyze the binding process at stage(II). The binding constants for formation of TMPyP:2HSA from TMPyP:HSA have been estimated to be 4.9 × 104 and 3.5 × 104 M-1 at 27 and 37°C, respectively. Values of -24.6 kJ/mol and 20.0 J/K were determined for enthalpy and entropy, respectively using the Vont Hoff relation. It was concluded that both entropy and enthalpy changes play important roles in the binding process.