Probing the interaction of a globular protein with a small fluorescent probe in the presence of silver nanoparticles: spectroscopic characterization of its domain specific association and dissociation

Photoinduced intramolecular charge transfer produces a polar excited state in the small fluorescent probe 1-anilino-8-naphthalene-sulfonate (ANS) rendering the resulting emission sensitive to the medium polarity. The binding of ANS to proteins has been well characterized in the literature. The probe binds to the less polar hydrophobic, restricted proteinous environment of bovine serum albumin (BSA) resulting in a blue shift of the emission maximum with an increase in emission intensity. However, upon addition of silver nanoparticles to the probe-BSA system, there was found to be an interaction between the nanoparticle and protein resulting in a weakening of the probe-BSA binding and an associated conformational change of the protein in order to accommodate the silver nanoparticle. This has been hitherto undeciphered. Studies using site markers of flufenamic acid and phenylbutazone coupled with other spectroscopic techniques, REES, time resolved studies, FTIR, and SEM imaging studies have shed light on the interaction of silver nanoparticles with the probe bound protein. ANS was observed to move from its binding site in the protein to accommodate the silver nanoparticle surface which involved conformational changes of the protein near the probe binding site. The association of the probe to the protein and its dissociation from the sites on introduction of silver nanoparticles has thus been investigated using spectroscopic studies.