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

We report new findings on the red fluorescent (λ_em = 640 nm) bovine serum albumin (BSA)–gold (Au) compound initially described by Xie et al. (J. Am. Chem. Soc. 2009, 131, 888–889) as Au₂₅ 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 excitation–emission 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 Au₂₅ nanocluster.