Kinetics of Fluorophore Formation in Bovine Serum Albumin-Gold Complexes

We revisit the prevailing hypothesis that the red fluorophore (lambda(em) = 640 nm) in the bovine serum albumin (BSA) gold (Au) compound is a Au-25 nanoduster. To examine the hypothesis, we investigated the kinetics of Au binding in this compound. In addition to the specific Au(III) binding sites in BSA, we found a significant degree of nonspecific Au (III) binding on the BSA surface. Time-course of the emergence of the red fluorescence was measured in detail for a range of pH, temperature, and concentration of Au(III) with respect to BSA. The red fluorophore formation was a slow yet dynamic process, which was consistent with the pH-induced equilibrium transition in the conformation of BSA. Notably, the kinetic rate of the fluorophore formation was not strongly dependent on the concentration of Au(III). Incorporating the existence of multiple specific and nonspecific binding sites, we propose a new model of the red fluorophore formation mechanism based on Langmuir-type adsorption of Au to BSA, as an alternative to the single-site nucleation model of Au-25 nanoclusters.