Probing BSA binding to citrate-coated gold nanoparticles and surfaces

The interaction of bovine serum albumin (BSA) with gold colloids and surfaces was studied using zeta-potential and quartz crystal microbalance (QCM) measurements, respectively, to determine the surface charge and coverage. The combination of these two measurements suggests that BSA binding to gold nanoparticles and gold surfaces occurs by an electrostatic mechanism when citrate is present. The binding of BSA to bare gold is nearly two times greater than the binding of BSA to a citrate-coated gold surface, suggesting that protein spreading (denaturation) on the surface may occur followed by secondary protein binding. On the other hand, binding to citrate-coated gold surfaces can be fit to a Langmuir isotherm model to obtain a maximum surface coverage of (3.7 +/- 0.2) x 10(12) molecules/cm(2) and a binding constant of 1.0 +/- 0.3 microM(-1). The zeta-potential measurements show that the stabilization of colloids by BSA has a significant contribution from a steric mechanism because the colloids are stable, even at their isoelectric point (pI approximately 4.6). To be consistent with the observed phenomena, the electrostatic interactions between BSA and citrate must consist of salt-bridges, for example, of the carboxylate-ammonium type, between the citrate and the lysine on the protein surface. The data support the role of strong electrostatic binding but do not exclude contributions from steric or hydrophobic interactions with the surface adlayer.