The association behavior of Cu(II) complex of 5,10,15,20-tetrakis(4-N-benzyl-pyri- dyl)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)TBzPyPexists as a monomer at low ionic strength and ill-defined aggre- gates at high ionic strength. The binding of the Cu(II)TBzPyP to human serum albumin (HSA) at 0.005 Mphosphate buffer, pH 7.0 and 27 C has been also studied by optical ab- sorption and RLS spectroscopies. The optical absorption spectral patterns of Cu(II)TBzPyP at various concentration of HSA represent two distinct stages in the pro- cess of interaction. The existence of an isosbestic point in first titration stage can be re - lated to the equilibrium of free Cu(II)TBzPyP with that of Cu(II)TBzPyP:HSAcomplex. 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 esti- mate 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.
