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...

Retention of known geometry, with regard to mean atomic positions, has proved useful in the refinement of macromolecules. In structures with a paucity of diffraction data and large displacements of the atoms from their mean positions, it is also of value to restrain the thermal factors to be consistent with known stereochemistry. This paper presents a technique for accomplishing this by restraining the variances of the interatomic distributions (which are functions of the mean atomic positions and the thermal parameters) to suitably small values. This procedure allows meaningful anisotropic refinement of macromolecules to be carried out with low-resolution diffraction data. Anisotropic thermal parameters obtained in this way should prove useful in understanding the dynamics of the biological functions of macromolecules.

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This report describes two related methods for decorating cowpea mosaic virus (CPMV) with luminescent semiconductor nanocrystals (quantum dots, QDs). Variants of CPMV are immobilized on a substrate functionalized with NeutrAvidin using modifications of biotin-avidin binding chemistry in combination with metal affinity coordination. For example, using CPMV mutants expressing available 6-histidine sequences inserted at loops on the viral coat protein, we show that these virus particles can be specifically immobilized on NeutrAvidin functionalized substrates in a controlled fashion via metal-affinity coordination. To accomplish this, a hetero-bifunctional biotin-NTA moiety, activated with nickel, is used as the linker for surface immobilization of CPMV (bridging the CPMVs' histidines to the NeutrAvidin). Two linking chemistries are then employed to achieve CPMV decoration with hydrophilic CdSe-ZnS core-shell QDs; they target the histidine or lysine residues on the exterior virus surface and utilize biotin-avidin interactions. In the first scheme, QDs are immobilized on the surface-tethered CPMV via electrostatic attachment to avidin previously bound to the virus particle. In the second strategy, the lysine residues common to each viral surface asymmetric unit are chemically functionalized with biotin groups and the biotinylated CPMV is discretely immobilized onto the substrate via NeutrAvidin-biotin interactions. The biotin units on the upper exposed surface of the immobilized CPMV then serve as capture sites for QDs conjugated with a mixture of avidin and a second protein, maltose binding protein, which is also used for QD-protein conjugate purification. Characterization of the assembled CPMV and QD structures is presented, and the potential uses for protein-coated QDs functionalized onto this symmetrical virion nanoscaffold are discussed.