Small-molecule ligands strongly affect the Förster resonance energy transfer between a quantum dot and a fluorescent protein

Abstract

We report herein the study of Förster resonance energy transfer (FRET) between a CdSe/ZnS core/shell quantum dot (QD) capped with three different small-molecule ligands, 3-mercaptopropionic acid (MPA), glutathione (GSH), and dihydrolipoic acid (DHLA), and a hexa-histidine (His₆)-tagged fluorescent protein, mCherry (FP). The Förster radius (R₀) and the corresponding donor–acceptor distances (r) for each of the QD–FP FRET systems were evaluated by using the Förster dipole–dipole interaction formula. Interestingly, both the FRET efficiency (E) and r were found to be strongly dependent on the capping small-molecule ligands on the QD surface, where E ≈ 85% was obtained at a FP : QD copy number of 2 : 1 for the MPA capped QD, while that for the DHLA capped QD was <25% under the same conditions. A molecular model was proposed to explain the possible reasons behind these observations. The dissociation constants (K_ds) and kinetics of the self-assembled QD–FP systems were also evaluated. Results show that the QD–FP self-assembly process is fast (completes in minutes at low nM concentrations), strong (with K_d ≈ 1 nM) and positively cooperative (with the Hill coefficient n > 1), suggesting that the QD–His₆-tagged biomolecule self-assembly is a facile, effective approach for making compact QD-bioconjugates which may have a wide range of sensing and biomedical applications.