Interaction of bovine serum albumin with a psychotropic drug alprazolam: Physicochemical, photophysical and molecular docking studies

The interaction between alprazolam (Alp) and bovine serum albumin (BSA) has been investigated under physiological conditions by UV-vis, steady state as well as time-resolved fluorescence, circular dichroism (CD) spectroscopic and molecular docking studies. The binding constant K of Alp to BSA was found to be 1.8×105 L mol−1 from absorption data. Fluorometric studies suggested the formation of the Alp-BSA complex, while time-resolved fluorescence studies showed that the binding of Alp by BSA was mainly static and the effective rate constant is found to be 2.33×1013 L mol−1 s−1. According to the modified Stern-Volmer equation, the Stern-Volmer quenching constants (KSV) between Alp and BSA at four different temperatures 295, 303, 308, 313 K were obtained to be 1.19×105, 1.05×105, 0.99×105 and 0.90×105 L mol−1, respectively. The change in enthalpy (ΔH) and entropy (ΔS) were calculated to be −11.66 and 57.64 J mol−1 K−1, respectively, indicating that the interaction was hydrophobic in nature. Site marker competitive experiments suggested that the binding of Alp to BSA primarily took place in sub-domain IIA, whereas the binding distance (r) between Alp and the tryptophan residue of BSA was obtained to be 1.87 nm by Förster's theory of non-radiative energy transfer. The conformational studies by CD spectroscopy showed that the presence of Alp decreased the α-helical content of BSA and induced the unfolding of the polypeptide of the protein. The change in conformation was also supported by excitation-emission matrix spectroscopy (EEMS) studies. The molecular docking experiment supports the above results and effectively proves the binding of Alp to BSA.