Surface Association of Ubiquitin with CdTe and InP/ZnS Quantum Dots in Aqueous Buffer

Detailed knowledge of the molecular interaction of protein-quantum dot (QD) association facilitates the development of new applications in nanoscience. In this work, we determined possible interactions responsible for the association of ubiquitin with two different water-soluble QDs. We used nuclear magnetic resonance (NMR) spectroscopy to identify the interaction sites of QDs in ubiquitin in a residue-specific manner. Using CdTe and InP/ZnS QDs, we monitored and compared their association with wild-type ubiquitin and also with ubiquitin-K48C. The affected residues of wild-type ubiquitin on CdTe and InP/ZnS QD binding were similar to the interacting sites of ubiquitin with other nanoparticles. Further, dimerization of ubiquitin was observed for ubiquitin-K48C at both QD surfaces. Ubiquitin-K48C could efficiently conjugate with CdTe QDs, whereas only a direct phase transfer of ubiquitin cysteine mutant resulted in the conjugation of the protein with InP/ZnS QDs. Solid-state NMR further revealed that the protein has to be restructured for binding with InP/ZnS QDs. In summary, we provide a comprehensive understanding of the protein-QD interaction using different ubiquitin samples and QDs.