Solution structure of a peptide-conjugated aggregation-induced-emission (AIE) fluorophore bound to a protein.

Here, we present a class of peptide-conjugated fluorophores based on aromatic thioethers with aggregation-induced emission (AIE) properties for specific recognition of small proteins and also the first solution NMR structure of an AIE ligand specifically bound to a protein. Binding of the hPin1-WW domain as a model protein resulted in a significant fluorescence increase, making these AIE-peptide fluorophores suitable as direct readout probes for specific protein interactions. Autofluorescence and protein-binding properties were elucidated as a function of peptide length and fluorophore concentration. While an AIE fluorophore with two conjugated peptides shows an enhanced fluorescence increase upon protein binding, a one-armed derivative enabled us to solve the structures of the free AIE-peptide and its complex. In the absence of a protein, the AIE-peptide conjugate shows a curled-up conformation that still allows rotation of the aromatic rings of the AIE-core, thus resulting in low auto-fluorescence. In contrast, the AIE-peptide unfurls upon binding, allowing the hPin1-WW domain to interact with its peptide recognition sequence. The aromatic rings of the AIE-core form additional contacts with the protein surface, which restricts their motions and thus results in the observed fluorescence increase.