Fluorescence Resonance Energy Transfer Between Quantum Dot Donors and Dye-Labeled Protein Acceptors

TLDR

The study develops a QD–MBP conjugate that allows tuning of spectral overlap and enables a single donor to interact with multiple acceptors. The authors assembled CdSe–ZnS quantum dots with Cy3‑labeled maltose‑binding protein via noncovalent self‑assembly, enabling precise donor–acceptor spacing (<100 Å) and systematic measurement of FRET efficiency as a function of spectral overlap and dye loading. Efficient nonradiative exciton transfer was observed, with FRET efficiency strongly dependent on spectral overlap and donor–acceptor distance estimates agreeing with structural dimensions.

Abstract

We used luminescent CdSe−ZnS core−shell quantum dots (QDs) as energy donors in fluorescent resonance energy transfer (FRET) assays. Engineered maltose binding protein (MBP) appended with an oligohistidine tail and labeled with an acceptor dye (Cy3) was immobilized on the nanocrystals via a noncovalent self-assembly scheme. This configuration allowed accurate control of the donor−acceptor separation distance to a range smaller than 100 Å and provided a good model system to explore FRET phenomena in QD−protein−dye conjugates. This QD−MBP conjugate presents two advantages: (1) it permits one to tune the degree of spectral overlap between donor and acceptor and (2) provides a unique configuration where a single donor can interact with several acceptors simultaneously. The FRET signal was measured for these complexes as a function of both degree of spectral overlap and fraction of dye-labeled proteins in the QD conjugate. Data showed that substantial acceptor signals were measured upon conjugate formation, indicating efficient nonradiative exciton transfer between QD donors and dye-labeled protein acceptors. FRET efficiency can be controlled either by tuning the QD photoemission or by adjusting the number of dye-labeled proteins immobilized on the QD center. Results showed a clear dependence of the efficiency on the spectral overlap between the QD donor and dye acceptor. Apparent donor−acceptor distances were determined from efficiency measurements and corresponding Förster distances, and these results agreed with QD bioconjugate dimensions extracted from structural data and core size variations among QD populations.