Solution Properties of Cu(II) Complex of a Moderately Hydrophobic Water-Soluble Porphyrin and Investigation of Its Interaction with Human Serum Albumin

The association behavior of Cu(II) complex of 5,10,15,20-tetrakis(4-N-benzyl-pyridyl) porphyrin (Cu(II)TBzPyP) in aqueous solution at various ionic strength was studied by optical absorption and resonance light scattering (RLS) spectroscopies. The results show that Cu(II)TBzPyP exists as a monomer at low ionic strength and ill-defined aggregates at high ionic strength. The binding of the Cu(II)TBzPyP to human serum albumin (HSA) at 0.005Mphosphate buffer, pH 7.0 and 27_C has been also studied by optical absorption and RLS spectroscopies. The optical absorption spectral patterns of Cu(II)TBzPyP at various concentration of HSA represent two distinct stages in the process of interaction. The existence of an isosbestic point in first titration stage can be related to the equilibrium of free Cu(II)TBzPyP with that of Cu(II)TBzPyP:HSA complex. The aggregation of HSAmolecules around porphyrin has occurred in the second titration stage. The analysis of binding process by calculation on absorption data led us to estimate binding constant for formation of HSA:Cu(II)TBzPyP complex. The RLS spectra of Cu(II)TBzPyP at various concentration of HSA do not show any aggregation of Cu(II)TBzPyP in the presence of HSA, which certified the results of UV-Vis studies. The fluorescence emission of HSA chromophore was quenched due to the porphyrin binding. The process of quenching has been analyzed by Stern-Volmer equation. Hence the binding constant of Cu(II)TBzPyP to HSA has also been estimated as the Stern-Volmer quenching constant, which is in good agreement with the result of UV-Vis studies.