Insights into intramolecular charge transfer fluorescent probes for recognizing human serum albumin

Abstract Development of powerful intramolecular charge transfer (ICT) fluorescent probes that can recognize, detect, and visualize human serum albumin (HSA) is important in the fields of biology and biomedicine. Three ICT probes, namely, butyl gallate (BG), isobutyl gallate (IBG), and isoamyl gallate (IAG), which have simple structures, were selected and used to detect HSA. The probes selectively recognized HSA in aqueous solutions by forming stable complexes with the protein. All of the probes exhibited different changes in fluorescence, and HSA underwent significant changes in fluorescence intensity and decay time. We hypothesized that the probes formed electromagnetic coupling with the whole HSA. Not all biomolecules can offer favorable environment for electromagnetic coupling due to their complex structures. The probes shared the same main binding site (located in Sudlow site II) in HSA. The probes and warfarin fluorescence intensity evidently increased in different hydrophobic cavities of HSA. This finding indicated the lack of limit to the location of the probes. A more suitable electromagnetic environment in protein made the ICT probes all had better response ability in HSA than in BSA. Y-type alkyl chains in IBG and IAG were beneficial for their responsiveness. The binding and response abilities of a specific hydrophobic cavity of a protein should be comprehensively considered when designing a novel fluorescent probe.