Kinetics of metal-affinity driven self-assembly between proteins or peptides and CdSe-ZnS quantum dots

We present a molecular characterization of metal-affinity driven self-assembly between CdSe-ZnS core-shell quantum dots (QDs) and a series of proteins and peptides appended with various length polyhistidine tags. In particular, we investigated the kinetics of self-assembly between surface-immobilized QDs and proteins/peptides under flow conditions, as well as between freely diffusing QDs and proteins/peptides (solution phase). In the first configuration, QDs were immobilized onto functionalized substrates and then exposed to dye-labeled peptides/proteins. Using evanescent wave excitation, we assessed self-assembly by monitoring the time-dependent changes in the dye fluorescence. In solution, the kinetics of self-assembly was monitored via energy transfer between QDs and dye-labeled proteins/peptides. These measurements allowed determination of the kinetic parameters, including the association and dissociation rates (k(on) and k(off)) and the apparent binding constant (K-d). We find that self-assembly is rapid with an equilibrium constant K-d(-1) approximate to 1 nM for solution self-assembly, confirming that metal-affinity interactions provide QD bioconjugates that are functional and stable.