Spectral Shift.
Detect molecular interactions by monitoring changes in a fluorophore’s emission spectrum.
When to use Spectral Shift.
When a fluorescently labeled target binds to a ligand, the local chemical environment around the fluorophore changes. This environmental change causes a subtle shift in the fluorophore’s emission wavelength—a phenomenon that can be detected and quantified to measure binding affinity under isothermal conditions.
Spectral Shift is ideal when you need to:
When to use Spectral Shift.
When a fluorescently labeled target binds to a ligand, the local chemical environment around the fluorophore changes. This environmental change causes a subtle shift in the fluorophore’s emission wavelength—a phenomenon that can be detected and quantified to measure binding affinity under isothermal conditions.
Spectral Shift is ideal when you need to:
- Measure interactions from µM to mM Kd’s
- Screen large libraries in microwell format (hundreds to thousands of compounds)
- Collect high-quality binding data with limited amounts of sample
- Characterize ternary complexes for TPD or bispecifics
- Measure interactions in complex matrices like cell lysates
The science behind Spectral Shift.
Fluorophore emission depends on the local environment
Fluorophores are exquisitely sensitive to their chemical surroundings. The wavelength at which they emit light depends on factors like:
Polarity: Hydrophobic environments shift emission differently than polar, aqueous environments
pH: Protonation state affects emission properties
Proximity to other molecules: Nearby amino acids, ligands, or solvent molecules influence the emission spectrum
When a labeled target protein binds to a ligand, the binding event can alter the environment around the fluorophore. Even if the fluorophore isn’t directly at the binding site, conformational changes or changes in solvent accessibility can shift its emission spectrum.
Ratiometric detection quantifies nanometer-scale spectral shifts
The emission spectrum shift caused by binding is often very small—sometimes less than a nanometer. Conventional fluorescence plate readers can’t reliably detect such subtle changes.
Spectral Shift technology uses a ratiometric approach. Instead of trying to measure the absolute emission maximum, it measures fluorescence intensity simultaneously at two wavelengths flanking the emission peak.
For red fluorophores (commonly used in Spectral Shift assays), these wavelengths are typically 650 nm and 670 nm.
When the target is unbound, the ratio of emission at these two wavelengths (F670/F650) has a baseline value. When the target binds to a ligand, the emission spectrum shifts slightly. This shift changes the ratio—one wavelength increases relative to the other.
By plotting this ratio against ligand concentration, you can generate a binding curve and calculate the Kd.
Spectral Shift excels with challenging interactions.
Spectral Shift excels with challenging interactions.
Multimeric complexes
Ternary complexes formed by PROTACs or molecular glues involve three components (target protein, degrader, E3 ligase). Spectral Shift can measure both binary interactions (target-PROTAC, E3-PROTAC) and ternary complex formation (target-PROTAC-E3) with the same assay conditions.
Intrinsically disordered proteins
Proteins lacking stable structure are difficult to study with methods requiring immobilization or specific conformations. Spectral Shift works in free solution and doesn't require a folded structure, letting IDPs bind in their native un-restricted conformations.
Covalent ligands
Irreversible binders form covalent bonds with their targets. Spectral Shift can measure these interactions, whereas methods relying on equilibrium binding struggle. Taking measurements in-solution also negates the need for chip-regeneration.
Molecules difficult to immobilize
Surface-based methods like SPR require immobilization of one binding partner. Some molecules (membrane proteins in detergent, large complexes, certain small molecules) are difficult or impossible to immobilize. Spectral Shift works in solution, avoiding this limitation.
Better show than tell. See how DLS generates information about your sample.
Measuring binding affinity with Spectral Shift.
In a Spectral Shift experiment, you prepare samples containing a constant concentration of fluorescently labeled targe and a dilution series of ligand.
Samples can be measured in either microwell plates or capillaries, depending on the instrument.
The fluorescence ratio (F670/F650) is measured for each ligand concentration. Plotting this ratio against ligand concentration produces a dose-response curve. The Kd is determined by fitting the data to a binding model based on the law of mass action.
The Kd represents the ligand concentration at which 50% of the target molecules are bound. Lower Kd values indicate tighter binding.
Additionally, you can determine binding stoichiometry based on the shape of the binding curve, or characterize cooperativity.
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Frequently Asked Questions
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