Protein Design Provides Lead(II) Ion Biosensors for Imaging Molecular Fluxes around Red Blood Cells

Metalloprotein design and semiconductor nanoparticles have been combined to generate a reagent for selective fluorescence imaging of Pb2+ ions in the presence red blood cells. A biosensor system based on semiconductor nanoparticles provides the photonic properties for small molecule measurement in and around red blood cells. Metalloprotein design was used to generate a Pb2+ ion selective receptor from a protein that is structurally homologous to a protein used previously in this biosensing system. Parameters for the Pb2+ ion binding site were derived from crystallographic structures of low molecular weight Pb2+ ion complexes that contain a stereoactive lone pair. When the designed protein was produced and attached to ZnS-coated CdSe nanoparticles, two Pb(NO3)(2)-associated binding events were observed (2-fold emission decrease; K-A1 = 1 x 10(9) M-1; K-A2 = 3.5 x 10(6) M-1). The fluorescence response had a 100 pM Pb(NO3)(2) detection limit, while no response was observed with Ca2+ ions (10 mM), Zn2+ ions (100 mu M), or Cd2+ ions (100 mu M). Metal ion selectivity presumably comes from the coordination geometry selected to favor lone pair formation on Pb2+ ions and electrostatically disfavor tetrahedral coordination. Replacement of ZnS-coated CdSe with ZnS-coated InGaP nanoparticles provided similar biosensors (100 pM limit of detection; K-A1 = 1 x 10(9) M-1; K-A2 = 1 X 10(7) M-1) but with excitation/emission wavelengths longer than the major absorbance of red blood cell hemoglobin (> 620 nm). The InGaP nanoparticle-based biosensors provided a 5 nM Pb(NO3)(2) detection limit in the presence of red blood cells. The modularity of the biosensor system provides exchangeable Pb2+ ion detection around red blood cells.