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

Abstract

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

The Effect of Fatty Acids and BSA Purity on Synthesis and Properties of Fluorescent Gold Nanoclusters

“…7, 4.3, 8, and 10 [8,11,12]. According to the recent paper [13], only BSA in the Aged form (pH > 10) interacting with Au ions forms the red fluorescent species (i.e., multiple chromophores such as Au-complexes and AuNCs, which are present on different binding sites of BSA).…”

“…Concerning metal cations, four binding sites are described in albumins: N-terminal site (NTS, or "Asp fragment"), vicinity of Cys34 (the only cysteine residue in BSA that does not form disulfidic bridges), multi-metal binding site (MBS or site A) at the interface of domains I and II, and site B [15]. Two of these sites, NTS and Cys34, were examined as possible binding or nucleation sites of gold (0 or III) [13]. Based on intentional cysteine residues blocking, it was evidenced [13,15] that all cysteine residues, not specially Cys34, served as the best binding sites for gold ions in albumin.…”

Hard X-ray excited optical luminescence from protein-directed Au∼20 clusters

“…25 The amino acid compositions of the proteins are almost similar, particularly for the binding and reduction of Au 3+ to produce the gold clusters. 26 The PL quantum yield upon visible light excitation (470 nm) was the highest in lactoferrin-Au, while the PL lifetime was the shortest. XPS also showed that the amount of Au(I) in the lactoferrindirected cluster was slightly higher than for the other clusters; indeed, Schneider et al reported the absence of Au(I) in BSAdirected Au 8 clusters.…”

NIR‐Emitting Gold Nanoclusters–Modified Gelatin Nanoparticles as a Bioimaging Agent in Tissue

“…So, after 3 h of reaction with gelatin at 60 °C under alkaline conditions, Au (III) has been reduced partially to Au (I) and Au (0) to form AuNCs. However, Au (III)–protein complex has been assigned recently by Dixon and Egusa to also contribute to the fluorescence . By virtue to the electron‐donating groups in gelatin, Au (I) as an intermediate reduced state of gold has been produced, with a further reduction to Au (0).…”