Surface Plasmons Utilized to Achieve High-Density Nanolithography
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RESEARCH/RESEARCHERS
Orientation of Protein Determined in QD-Bioconjugate Assembly One class of hybrid nanomaterials that bridges the interface between materials science and biology is the protein–nanomaterial composite. Beneficial protein properties imparted by design to such nanocomposites include self-assembly assistance, biorecognition, and catalysis. The interaction between protein bioreceptors and the inorganic nanocomponents is fundamentally important. Recently, quantum dots (QDs) were bound to maltose-binding proteins (MBPs) to make solution-phase biosensors that function by changes in fluorescence resonance energy transfer (FRET). Although the biosensors’ activities were evident, the nature of the MBP-QD interaction was unknown. In the work discussed here, I.L. Medintz, J.H. Konnert, and co-workers at the Naval Research Laboratory in Washington, D.C., have developed a FRET-based modeling technique to determine the orientation of MBP coordinated to the QD surface. As reported in the June 29 issue of the Proceedings of the National Academy of Sciences (p. 9612), the researchers prepared MBP-QD assemblies from CdSe-ZnS core–shell QDs and six different MBP mutants in which single cysteine substitutions were spatially distributed on the protein surface and labeled with rhodamine red (RR) dye. The core–shell QDs were made water soluble by using bidentate dihydrolipoic acid to replace the organic capping shell left over from the colloidal synthesis process. MBP, an ellipsoidal protein with dimensions of about 30 Å × 40 Å × 65 Å, coordinates to the Zn-S surface of each spherical QD (core–shell diameter, ~60 Å) by a C-terminal oligohistidine segment. The researchers determined from FRET efficiency data the distances, di , from the energy-donating QD to the six different RR-acceptor locations. These distances were used in conjunction with the MBP crystal structure to model the orientation of MBP with respect to the QD surface using a method the researchers liken to a nanoscale global positioning system, that is, triangulating a point on the globe from orbiting satellites. The protein’s dye-acceptor positions relative to a spherical QD are analogous to satellite positions about the earth. The researchers said that this approach should, in principle, allow determination of the QD orientation with respect to MBP with as few as four non-coplanar locations instead of the 21 parameters that define the six MBP-dye acceptor positions and the QD center. The researchers, however, resorted to an iterative method that minimized the mean-square deviations of the 606
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Figure 1. Refined maltose-binding protein (MBP) orientation relative to a quantum dot (QD). (a) Side view with all six rhodamine red structures positioned (in red). The refined distances to the center of the QD are shown in yellow. The distance from the nearest MBP atom is shown in green. (b) View of the final refined orientation of MBP relative to the QD, with QD surface sulfur atoms in teal and zinc atoms in pink. The red shell shows the estimated outer rad
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