Fluorescence quenching of human serum albumin by xanthines

A study of the fluorescence quenching of human serum albumin (HSA) by caffeine, theophylline and theobromine, based on temperature dependence, has shown that it is predominantly static. This quenching mechanism is due to the formation of a xanthine-HSA non-fluorescent complex. The Stern-Volmer equation let us determine the association constants. It seems that the quenching of the protein fluorescence depends on the number and position of the methyl groups. The temperature dependence of the association constant is used to estimate the values of the thermodynamic parameters involved in the interaction of the drugs with HSA. All three binding processes are exothermic and probably hydrophobic, and hydrogen bonds play a significant role in the stabilization of such complexes. The enthalpy and entropy changes observed appear to compensate each other to produce a relatively small Gibbs free energy.