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

We report herein the study of the 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 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 >80% was obtained at a FP:QD copy number of 2:1 for MPA capped QD, while that for the DHLA capped QD was < 25% under the same condition. A molecular model was proposed to explain the possible reasons behind these observations. The dissociation constants (Kds) 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 concentration), strong (with Kd ~ 1 nM) and positively cooperative (with Hill coefficient n >1), suggesting that the QD-His-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.