Examining the Transient Dark State in Protein-Quantum Dot Interaction by Relaxation-Based Solution NMR

We probed the "dark" state involved in the protein-quantum dot (QD) interaction using a relaxation-based solution nuclear magnetic resonance (NMR) approach. We examined the dynamics and exchange kinetics of the ubiquitin-CdTe model system, which undergoes a fast exchange in the transverse relaxation time scale. We applied the recently developed dark-state exchange saturation transfer (DEST), lifetime line broadening (ΔR₂), and exchange-induced chemical shift (δ_ex) solution NMR techniques to obtain a residue-specific binding behavior of the protein on the QD surface. The variation in the estimated ¹⁵N-R₂^bound values clearly shows the dynamic nature of bound Ub. Upon mapping the amino acid residues showing a faster relaxation rate on the electrostatic potential surface of the protein, we have determined that the interaction is preferably electrostatic, and the amino acid residues involved in binding lie on the positively charged surface of the protein. We believe that our experimental approach should provide more in-depth knowledge to engineer new hybrid protein-QD systems in the future.