Bioconjugation of (CdSe)ZnS Quantum Dots Using a Genetically Engineered Multiple Polyhistidine Tagged Cohesin/Dockerin Protein Polymer

We have constructed bioconjugates consisting of genetically modified cohesin/dockerin protein polymers combined with (CdSe)ZnS colloidal quantum dots. This recombinant protein contains fusions of Clostridium thermocellum cellulosomal cohesin and dockerin domains and a C-terminal 6x-histidine tag. These unique cohesin/dockerin monomeric building blocks (ca. 60 kDa) were allowed to self-assemble, yielding oligomers and polymers, which were subsequently characterized by high-pressure size exclusion chromatography (HPSEC). The C-terminal 6x-His tags from each monomer facilitate binding to the quantum dot surface chemistry while mix the protein polymers with water-soluble QDs at neutral pH. Using HPSEC, we were able to fractionate the reaction mixture into two major distributions of bioconjugate species. Scanning transmission electron microscopy (STEM) and photoluminescence spectroscopy (PL) were employed to characterize the components from these Chromatographic fractions. The fraction containing the larger bioconjugates contained clusters of quantum dots surrounded by protein polymers with an estimated radius of 190 ± 30 Å and an apparent molecular weight of 8 000 ± 3 000 kDa. The STEM images from the fraction containing the smaller species were amenable to detailed analysis and graphical simulation that revealed species containing one, two, or three quantum dots surrounded by 10,15, or 18 protein monomers, respectively. Our data demonstrate strong binding coefficients not only between the protein monomers to form polymers, but also with the (CdSe)ZnS colloidal quantum dots and thus provides a method of producing stable, water-soluble luminescent quantum dot bioconjugates. PL spectroscopic analysis shows that the samples from both Chromatographic fractions have strong excitonic emission with a peak at ca. 2.2 eV (562 nm).