Quantitation, Visualization, and Monitoring of Conformational Transitions of Human Serum Albumin by a Tetraphenylethene Derivative with Aggregation-Induced Emission Characteristics

Human serum albumin (HSA) is a major protein component of blood plasma, and its assay is of obvious value to biological research. We, herein, present a readily accessible fluorescent bioprobe for HSA detection and quantitation. A nonemissive tetraphenylethene derivative named sodium 1,2-bis[4-(3-sulfonatopropoxyl)phenyl]-1,2-diphenylethene (BSPOTPE) is induced to emit by HSA, showing a novel phenomenon of aggregation-induced emission (AIE). The AIE bioprobe enjoys a broad working range (0-100 nM), a low detection limit (down to 1 nM), and a superior selectivity to albumins. The fluorescent bioassay is unperturbed by the miscellaneous bioelectrolytes in the artificial urine. The AIE luminogen can also be used as a rapid and sensitive protein stain in gel electrophoresis for HSA visualization. Utilizing the AIE feature of BSPOTPE and the Forster resonance energy transfer from HSA to BSPOTPE, the unfolding process of HSA induced by guanidine hydrochloride is monitored, which reveals a multistep transition with the involvement of molten globule intermediates. Computational modeling suggests that the AIE luminogens dock in the hydrophobic cleft between subdomains IIA and IIIA of HSA with the aid of hydrophobic effect, charge neutralization, and hydrogen bonding interactions, offering mechanistic insight into the microenvironment inside the hydrophobic cavity.