Sulfadiazine binds and unfolds bovine serum albumin: an in vitro study.

Sulfonamide derivatives, such as sulfadiazine (SD) are used as antibiotics and, very recently, anti-amyloid properties of these have been reported. We have evaluated binding of SD with bovine serum albumin (BSA) followed by unfolding of protein. Studies were accomplished at physiological conditions of temperature (37 °C) and pH (7.4), employing UV, fluorescence, circular dichroism (CD) and Fourier transform infra-red (FTIR) spectroscopies. In presence of drug, UV spectrum of BSA was altered from the spectrum of native BSA due to the interaction between albumin and drug. Excitation of protein at 295 nm showed that fluorescence quenching of BSA by SD is a result of the formation of SD–BSA complex. The data were analyzed using Stern–Volmer and Lineweaver–Burk methods. From both methods it was evaluated that the quenching involved in BSA–SD binding was static. BSA had only one binding site for SD. Synchronous fluorescence spectra have shown a red shift and advocated that hydrophobicity around both Trp and Tyr residues was decreased. CD results revealed that the conformation of macromolecule remain undisturbed at low concentrations (up to 20 μM of the SD) and there was small perturbation in the secondary structure from 20 to 50 μM of SD followed by a large change and consequent unfolding on further increase in the drug concentration. Both synchronous and CD measurements were consistent to each other. FTIR spectra revealed the shifting of amide I band which is also an indication of conformational change of the protein.