Kinetics and thermodynamics of bovine serum albumin interactions with Congo red dye

To optimize the therapeutic applications of Congo red (CR), a potential inhibitor of protein aggregation, the kinetics and thermodynamics of the interactions between CR and a model protein need to be understood. We used surface plasmon resonance (SPR) and fluorescence techniques to determine the dynamics and thermodynamic parameters for the formation of complexes between CR and bovine serum albumin (BSA). CR interacts with BSA through a transition complex; the activation energy for association (E_act(a)) was determined to be 35.88kJmol^-1, while the activation enthalpy (ΔH^‡), entropy (ΔS^‡), and Gibbs free energy (ΔG^‡) are 33.41kJmol^-1, 0.18Jmol^-1K^-1, and 33.35kJmol^-1, respectively. When this intermediate transforms into the final CR-BSA complex, the entropy of the system increases and part of the absorbed energy is released; this process is associated with a reverse activation energy (E_act(d)) of 20.17kJmol^-1, and values of ΔH^‡, ΔS^‡, and ΔG^‡ of 17.69kJmol^-1, -162.86Jmol^-1K^-1, and 66.25kJmol^-1, respectively. A comparison of the SPR and fluorescence results suggests that there is more than one site where BSA interacts with CR.