Metal-free multicomponent spirocyclization based on isatin and quinoline toward spirooxazine for rapid, sensitive, and selective detection of human serum albumin

Seven spirooxazine derivatives (M1–M7) with potential pharmacological activity were synthesized through a metal-free multicomponent spirocyclization of alkyne, isatin, and quinoline. The obtained spirooxazines have typical aggregation-caused quenching characteristics. M3 only showed a real-time and high-selective fluorescent enhancement response to human serum albumin (HSA), while there was almost no response to bovine serum albumin and other proteins. Experiments and simulations have shown that M3 enters the hydrophobic cavity of HSA and forms various noncovalent interactions with amino acid residues, which restricts the intramolecular motion and enhances fluorescence emission of M3. In addition, M3 has the same emission intensity in pure solution, simulated serum, and real human serum at the same HSA concentration, indicating its excellent anti-interference ability and advantages in accuracy and convenience. These results show that spirooxazine provides an effective scaffold for selective HSA recognition and offers a useful strategy for the discrimination of highly homologous proteins.