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A two-photon excitation fluorescence cross-correlation assay for a model ligand-receptor binding system using quantum
J L Swift1, R Heuff, D T Cramb
1Department of Chemistry, University of Calgary, Calgary AB, T2N 1N4, Canada.
Biophysical Journal
|November 22, 2005
Summary
Quantum dots enhance two-photon excitation fluorescence cross-correlation spectroscopy (TPE-XCS) for studying molecular interactions. This novel labeling method overcomes spectral crosstalk, enabling precise measurement of ligand-receptor binding kinetics.
Area of Science:
- Biophysics
- Spectroscopy
- Materials Science
Background:
- Two-photon excitation fluorescence cross-correlation spectroscopy (TPE-XCS) is effective for studying interactions between labeled molecules, particularly ligand-receptor systems.
- Traditional TPE-XCS using organic dyes suffers from spectral crosstalk between detection channels, complicating data analysis.
- Quantum dots offer narrower emission spectra, potentially alleviating crosstalk issues in TPE-XCS.
Purpose of the Study:
- To investigate the use of quantum dots as fluorescent labels for both ligand and receptor in TPE-XCS.
- To demonstrate a novel approach for TPE-XCS using solely quantum dots, applicable to various biomolecular interactions.
- To present a modified analysis for extracting pharmacological parameters like the equilibrium dissociation constant (Kd) from TPE-XCS data.
Main Methods:
- Utilized two-photon excitation fluorescence cross-correlation spectroscopy (TPE-XCS).
- Employed semiconductor quantum dots functionalized with biotin (emission lambda(em) = 605 nm) and streptavidin (emission lambda(em) = 525 nm) as distinct labels.
- Developed a modified expression for fractional occupancy based on auto- and cross-correlation decays.
Main Results:
- Achieved a quantum dot-based TPE-XCS measurement yielding an average equilibrium dissociation constant (Kd) of 0.30 ± 0.04 x 10(-9) M for the biotin-streptavidin system.
- Determined the off-rate coefficient (k(off)) for quantum dot dissociation to be 5 x 10(-5) s(-1).
- Observed a slightly higher off-rate compared to the native biotin-streptavidin system, attributed to the quantum dots' size and restricted motion.
Conclusions:
- Quantum dots are a viable and advantageous alternative to organic dyes for labeling in TPE-XCS, minimizing spectral crosstalk.
- The developed method allows for direct and accurate extraction of binding parameters (Kd, k(off)) from TPE-XCS data.
- This quantum dot-based TPE-XCS approach shows promise for studying a wide range of biomolecular interactions.
