The interaction of chlorogenic acid (CGA) with human serum albumin (HSA) was studied from the view-point of thermodynamics and mechanism of binding at pH 6.0. The association constants (Ka) for the HSA-CGA interaction at 10, 25 and 40 degrees C were 6.0 x 10(4), 9.0 x 10(3) and 2 x 10(4) M-1, resulting in delta G of -6.21, -5.80, -6.32 kcal/mol, respectively. These high Ka-values showed that the interaction between CGA and HSA is strong, endothermic and entropically driven. Binding of chlorogenic acid induces conformational change in HSA as indicated by quenching of fluorescence emission intensity along with a red shift in the emission maxima from 338 to 350 nm. This suggested the involvement of the lone tryptophan residue in the region of binding. Far-ultraviolet circular dichroic data showed a decrease in the alpha-helical content of HSA from 56 to 50% upon binding of CGA. These data are also supported by the decrease in the apparent Tm of HSA by 4 degrees C upon binding of CGA causing destabilization of the HSA molecule. The kinetics of the interaction involves a single step in the binding, and the kinetic curve attains equilibrium within 180 +/- 5 s. Data on caffeic acid (CA) and quinic acid (QA), which are the hydrolysis products of the bidentate CGA molecule, indicate that CA interacts more strongly than CGA. CA binds with an association constant of 8 x 10(4) M-1 and with a maximum number of binding sites of four. Microcalorimetric investigation of the interaction of these ligands with HSA suggests that the strength of binding follows the order CA >> CGA >>> QA with a single class of binding sites. The effect of temperature on the binding of CGA to HSA showed that the interaction is dominated by hydrophobic forces and hydrogen bonding.