Binding studies of aristololactam-β-d-glucoside and daunomycin to human serum albumin

Abstract

The interaction of the plant alkaloid aristololactam-β-D-glucoside (ADG) and the anticancer agent daunomycin (DAN) with human serum albumin (HSA) was investigated. Absorption and steady-state fluorescence spectroscopy, steady state fluorescence anisotropy, circular dichroism and isothermal titration calorimetry techniques have been exploited to characterize the binding phenomena. Absorbance and fluorescence quenching experiments revealed the formation of a strong complex of DAN and HSA, and comparatively weaker complex between ADG and HSA. Spectroscopic analysis suggested the binding affinity of ADG to HSA to be of the order of 10⁴ M⁻¹ and that of DAN–HSA to be of the order of 10⁵ M⁻¹. Fluorescence quenching data suggested a static quenching mechanism in both cases at the ground state. Three dimensional fluorescence and circular dichroism data are consistent with a conformational change in the protein on binding of ADG and DAN. The calorimetric study revealed exothermic binding of both drugs which was favored by negative standard molar enthalpy and standard molar entropy contributions. ADG was found to be a weaker binder to HSA compared to DAN. Detailed comparative biophysical aspects of the binding are presented.