Determining the Thermodynamic and Kinetic Association of a DNA Aptamer and Tetracycline Using Isothermal Titration Calorimetry
August 23rd, 2022
The present protocol describes the use of Isothermal Titration Calorimetry (ITC) to analyze the association and dissociation kinetics of the binding between a DNA aptamer and tetracycline, including sample preparation, running standards and samples, and interpreting the resulting data.
This protocol allows researchers to determine the binding affinities of aptamers against targets free in solution, while providing insight into the mechanism of binding. Because ITC measures changes in heat due to binding, neither aptamer nor ligand needs to be labeled or functionalized, as would be the case for techniques such as SPR. New users should pay close attention to matching buffers across all binding components.
Additionally, be aware that the technique has high sample requirements that may be difficult to meet for larger ligands such as proteins. To begin, activate the membrane of the dialysis column, fill with PBS buffer, equilibrate for 10 minutes at room temperature, and centrifuge at 5, 000 times G for 15 minutes. Remove the buffer and load 500 microliters of aptamer samples into the column.
Centrifuge at 5, 000 times G and repeat it four times to exchange the original buffer for PBS. Collect the dialyzed DNA aptamer using a pipette and transfer it to the new 1.5-milliliter tube. Collect the last flow-through buffer to dissolve tetracycline.
Ensure that the buffer for the experiment in the syringe matches the buffer in the reference cell. Determine the aptamer concentration again using a UV-visible spectrometer. Use the last exchange buffer to adjust the concentration to 40-micromolar tetracycline and two-micromolar aptamer.
Fold the DNA aptamer by heating at 90 degrees Celsius for 10 minutes, cooling at four degrees Celsius for 10 minutes, and then returning to room temperature for 20 minutes. Degas the folded aptamer and dialyzed tetracycline using a degassing station or vacuum pump for 25 minutes to eliminate dissolved gases. After cleaning up and confirming the cleanliness of the machine, test the accuracy of the machine with a standard kit that includes EDTA and calcium chloride using the default program and following the instructions from the manufacturer.
Set up the running parameters. Check the required volumes using a running program calculator. With this running parameter, perform ITC measurement with 230 microliters of 40-micromolar tetracycline in the ITC syringe and 485 microliters of two-micromolar aptamer in the ITC sample cell using the ITC.
Load the dialyzed tetracycline syringe plates and the folded aptamer into the sample cell, avoiding bubbles, using a pipette. Start running the ITC instrument by clicking on the Start button on the software. Open the data analysis software by double clicking to start analyzing the data.
Open the path of the saved raw data to know the tendency of binding. Open the Modeling tab and use different binding models to find the best fit for the data curve. Then, the software automatically calculates the ITC thermogram and various thermodynamic parameters, including enthalpy, entropy, free energy, equilibrium binding constant, and stoichiometry.
Collect the thermodynamic parameters determined from the data and fitting model information. Create a report including pictures of the ITC thermogram and various thermodynamic parameters. The aptamer selected by Kim, et al binds to tetracycline with binding affinities of disassociation constant one equals 13 micromolar and disassociation constant two equals 53 nanomolar.
The fitting model and stoichiometry from ITC reflect that the aptamer binds to tetracycline through a 2:1 binding ratio with the sequential binding model. The thermodynamic parameters determined by ITC measurement for site two indicated that enthalpy overcoming relatively significant entropic loss drives the strong binding. The enthalpy-driven binding with entropy loss relates to nucleic acid conformational changes, which have been reported as binding behaviors between RNA and a small molecule.
The aptamer and ligand must be in the same buffer. The aptamer should be refolded if developed with refolding, and samples must be degassed. Addressing these considerations will ensure that only aptamer-ligand interactions contribute to the measured signal.
Due to the size of the aptamer target, an appropriate follow-up method could be microscale thermophoresis to confirm the measured binding affinity free in solution.
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Overview
This protocol describes the use of Isothermal Titration Calorimetry (ITC) to analyze the binding kinetics of a DNA aptamer and tetracycline. It provides detailed instructions for sample preparation, running standards and samples, and interpreting the resulting data.
Key Study Components
Area of Science
- Biochemistry
- Molecular Biology
- Analytical Chemistry
Background
- ITC measures heat changes during binding interactions.
- It allows for the analysis of binding affinities without labeling the aptamer or ligand.
- Matching buffers across all components is crucial for accurate results.
- The technique has high sample requirements, particularly for larger ligands.
Purpose of Study
- To determine the binding affinities of aptamers against targets in solution.
- To gain insights into the binding mechanism of aptamer-ligand interactions.
- To provide a detailed protocol for researchers using ITC.
Methods Used
- Activation of the dialysis column membrane.
- Equilibration with PBS buffer.
- Centrifugation to prepare samples.
- Loading aptamer samples into the column for analysis.
Main Results
- Successful measurement of binding kinetics between the aptamer and tetracycline.
- Insights into the thermodynamics of the binding process.
- Establishment of a reliable protocol for ITC experiments.
Conclusions
- ITC is an effective method for studying aptamer-ligand interactions.
- The protocol can be adapted for various aptamer-target systems.
- Understanding binding kinetics is essential for applications in drug development.
Frequently Asked Questions
Executive Industry Relevance
Quantitative characterization of aptamer-small molecule binding is critical for target validation and mechanistic de-risking in early drug discovery. Isothermal Titration Calorimetry (ITC) enables label-free, solution-phase measurement of binding affinity, stoichiometry, and thermodynamic parameters, supporting predictive confidence in aptamer selection. This capability informs go/no-go decisions and portfolio triage for nucleic acid-based therapeutic and diagnostic programs.
Strategic Applications in Biopharma R&D
Early Discovery & Target Validation
- Enables direct measurement of aptamer-target binding affinity and mechanism in solution.
- Provides thermodynamic profiles (enthalpy, entropy, free energy) for mechanistic de-risking.
- Supports functional target validation by confirming specific molecular interactions.
- Facilitates predictive confidence in aptamer selection for downstream applications.
Screening & Assay Development
- Delivers quantitative binding constants and stoichiometry for assay standardization.
- Supports reproducibility and comparability across candidate aptamers and targets.
- Enables screening readiness by validating aptamer-target interactions without labeling.
- Provides data for platform reuse and scalability in assay development workflows.
Translational & Preclinical Research
- Aligns aptamer binding properties with disease-relevant small molecule targets.
- Ensures continuity from discovery to preclinical validation by confirming solution-phase binding.
- Reduces risk of late-stage failure due to mechanistic ambiguity in molecular recognition.
- Supports translational biomarker development when aptamers are used as detection reagents.
Pipeline & Workflow Integration
ITC-based binding analysis fits at the intersection of early discovery, lead identification, and assay development, providing foundational data for aptamer-target systems prior to preclinical advancement.
- Discovery Biology: Quantifies binding affinity and thermodynamics to support hypothesis testing and pathway clarification.
- Screening: Confirms assay readiness and reproducibility through direct, label-free measurements.
- Analytics: Generates quantitative outputs (Kd, ΔH, ΔS, stoichiometry) for robust comparison of aptamer candidates.
- Translational Research: Bridges discovery and preclinical phases by validating aptamer-target interactions in solution.
- Enterprise Reuse: Establishes a reusable analytical capability for diverse aptamer-target pairs across programs.
Operational & Enterprise Impact
- Scientific Value: Increases predictive confidence and reduces mechanistic ambiguity in aptamer-target validation.
- Operational Value: Standardizes binding measurements and supports reproducibility across teams.
- Strategic Value: Informs go/no-go decisions and enhances capital efficiency by de-risking early candidates.
- Portfolio Impact: Enables risk-adjusted prioritization and advancement of aptamer-based assets.
Implementation Considerations
- Requires expertise in calorimetry and quantitative data analysis for model fitting.
- Demands access to ITC instrumentation and compatible analytical software.
- Necessitates rigorous buffer matching and sample preparation to avoid artifacts.
- May require adaptation for larger ligands or limited sample availability.
- Sample degassing and aptamer refolding are critical for accurate measurements.
