Conformational Change-Induced Fluorescence of Bovine Serum Albumin-Gold Complexes

We report new findings on the red fluorescent (lambda(em) = 640 nm) bovine serum albumin (BSA)-gold (Au) compound initially described by Xie et al. (J. Am. Chem. Soc. 2009, 131, 888889) as Au-25 nanoclusters. The BSA-Au compounds were further reducible to yield nanoparticles, suggesting that these compounds were BSA-cationic Au complexes. We examined the correlations between BSA conformations (pH-induced as well as denatured) and the resulting fluorescence of BSA-Au complexes, to understand the possible cationic Au binding sites. The red fluorescence of the BSA-Au complex was associated with a particular isoform of BSA, the aged form (pH > 10) of the five pH-dependent BSA conformations, while the other conformations, expanded (pH < 2.7), fast (2.7 < pH < 4.3), normal (4.3 < pH < 8), and basic (8 < pH < 10) did not result in red fluorescence. There could be internal energy transfer mechanisms to produce red fluorescence, deduced from excitationemission map measurements. The ensemble minimum number of Au(III) per BSA to yield red fluorescence was <7. We illustrate the presence of multiple specific Au binding sites in BSA, and present an interpretation of the fluorescence of the BSA-Au complex, alternative to a single-site nucleation of a neutral Au-25 nanocluster.