A fluorescence resonance energy transfer-derived structure of a quantum dot-protein bioconjugate nanoassembly

Abstract

The first generation of luminescent semiconductor quantum dot (QD)-based hybrid inorganic biomaterials and sensors is now being developed. It is crucial to understand how bioreceptors, especially proteins, interact with these inorganic nanomaterials. As a model system for study, we use Rhodamine red-labeled engineered variants of Escherichia coli maltose-binding protein (MBP) coordinated to the surface of 555-nm emitting CdSe-ZnS core-shell QDs. Fluorescence resonance energy transfer studies were performed to determine the distance from each of six unique MBP-Rhodamine red dye-acceptor locations to the center of the energy-donating QD. In a strategy analogous to a nanoscale global positioning system determination, we use the intraassembly distances determined from the fluorescence resonance energy transfer measurements, the MBP crystallographic coordinates, and a least-squares approach to determine the orientation of the MBP relative to the QD surface. Results indicate that MBP has a preferred orientation on the QD surface. The refined model is in agreement with other evidence, which indicates coordination of the protein to the QD occurs by means of its C-terminal pentahistidine tail, and the size of the QD estimated from the model is in good agreement with physical measurements of QD size. The approach detailed here may be useful in determining the orientation of proteins in other hybrid protein-nanoparticle materials. To our knowledge, this is the first structural model of a hybrid luminescent QD-protein receptor assembly elucidated by using spectroscopic measurements in conjunction with crystallographic and other data.maltose-binding protein ͉ three-dimensional structure ͉ nanotechnology ͉ nanocrystal T he burgeoning field of nanotechnology promises to revolutionize many scientific fields, and the first generation of functional hybrid nanomaterials exploring the interface between biology and materials science is now being developed and prototyped (1-3). One exciting avenue of biomaterials research involves protein-nanomaterial composites (2-4). Proteins lend many of their unique properties to these hybrid materials, such as: assisting in ordered self-assembly processes such as that of Pd nanoparticles assembled on tubulin or viral assembly of orientated nanowires (5, 6), engendering exquisite biorecognition properties such as the receptors used in hybrid nanocrystal biosensors (7), and catalyzing useful electrochemical and cleavage reactions (2, 8). Of critical importance in developing these materials is a fundamental understanding of how proteins or bioreceptors interact with inorganic nanomaterials.The unique properties of luminescent colloidal semiconductor nanocrystals or quantum dots (QDs) have recently been incorporated into hybrid functional nanoassemblies. Cadmium selenide-zinc sulfide (CdSe-ZnS) core-shell QDs, in particular, have exceptional photochemical stability and relatively high quantum yields, as well as broad excitation and size-tunable photoluminescence spectra with narrow emission bandwidt...

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