Coumarin-Tethered 1,2,3-Triazole Based Fluorescent "Turn-ON" Probe via Metal-Free Click Chemistry for Selective Sensing of Serum Albumin: Quantitative Detection of HSA in Urine

Abstract

A series of novel highly substituted coumarin-based 1,2,3-triazoles (CTZs) was synthesised by a metal-free organo-click reaction. The environmentally benign, atom-economical condition generated the desired products in moderate to excellent yield. All derivatives exhibited good binding affinity toward bovine serum albumin (BSA), with binding constants in the order of Kb ̴ 104 L mol⁻¹. Among them, compound 3f, a nitro analogue of CTZ, demonstrated a selective fluorescence “turn-on” response upon binding to both BSA and human serum albumin (HSA). Fourier transform infrared (FT-IR) and circular dichroism (CD) spectroscopy analyses revealed that the compound 3f induced significant conformational perturbations in both of the serum albumin secondary structures, particularly a reduction in the α-helical fraction. Site marker displacement experiments revealed that 3f binds to the bilirubin-binding site (subdomain IB) of both serum albumins. Isothermal titration calorimetry (ITC) established that the binding of 3f to both serum albumins was exothermic, suggesting that the hydrogen bonding and van der Waals interactions were the predominant factors responsible for the binding. Molecular docking studies further supported the selective binding of 3f within the bilirubin-binding pocket of both BSA and HSA through hydrogen-bonding. Finally, to demonstrate the practical applicability of 3f, we have successfully detected and quantified HSA levels in a real human urine sample. Our findings highlight the potential of 3f as a simple, selective, and cost-effective fluorescent probe for the clinical monitoring and quantification of serum albumin levels.