A peptide-based fluorescent ratiometric sensor for quantitative detection of proteins

A ratiometric fluorescent sensor was obtained by solid-phase synthesis of a peptide singly labeled at its N-terminus with a 3-hydroxychromone (3HC) derivative, an environmentally sensitive fluorophore with a two-band emission. The construct contains the binding site recognized by an antibody fragment, scFv1F4_Q34S, with nanomolar (nM) affinity. The dye only marginally affected the kinetic and equilibrium binding parameters of the scFv-peptide interaction, as measured by surface plasmon resonance. On interaction with the antibody fragment, the sensor showed up to 47% change in the ratio of its two emission bands, indicating an enhanced screening of the 3HC fluorophore from bulk water. Competition with two unlabeled peptides of different lengths led to a dynamic displacement of the construct governed by the relative binding constants. Calibration showed that the response is proportional to the ratio of scFv1F4_Q34S to labeled peptide. The detection limit of scFv1F4_Q34S was 15 nM. In a more complex medium (100 μg/ml bovine serum albumin), the scFv could be detected in the 50- to 100-nM range. This work demonstrates that, with the perspective of further improvements of the dye spectroscopic properties, fluorescent ratiometric sensing based on small synthetic peptides represents a promising tool for quantitative target detection.