# Protein–DNA Binding Affinity Calculator (Kd Estimate) Single-point 1:1 Kd estimate: Kd = [P] × (1−f)/f from a direct fraction or anisotropy-derived fraction, plus ΔG° = RT·ln(Kd) and affinity bands from very tight (<1 nM) to weak (>1 µM). Cantor & Schimmel / Heyduk & Lee / Lakowicz derived. Educational use only. > Canonical page: https://elysiatools.com/en/tools/protein-dna-binding-affinity - **Category:** Education - **Keywords:** protein dna binding affinity, kd calculator, dissociation constant estimate, fraction bound anisotropy, fluorescence anisotropy binding, binding free energy delta g, transcription factor dna binding, equilibrium dissociation constant, 1:1 binding model, molecular biology, lab calculator ## Overview The Protein–DNA Binding Affinity Calculator estimates a single-point dissociation constant (Kd) for a 1:1 binding model using either a direct bound fraction or fluorescence anisotropy values. It also calculates ΔG° from Kd and groups the result into an affinity band for educational use and experimental planning. ## Inputs - **Calculation Mode** (select): Enter the bound fraction directly, or derive it from anisotropy values. - **Protein Concentration (nM)** (number): Total protein concentration in the binding reaction (kept well above the labeled DNA). - **Fraction Bound (0–1)** (number): Fraction of labeled DNA in complex (fraction mode; strictly between 0 and 1). - **Measured Anisotropy (r)** (number): Anisotropy of the sample (anisotropy mode). - **Free-State Anisotropy (r free)** (number): Anisotropy of the unbound labeled DNA alone. - **Bound-State Anisotropy (r bound)** (number): Anisotropy of the fully bound complex (protein-saturated). - **Temperature (°C)** (number): Incubation temperature, used for ΔG° = RT·ln(Kd). - **Decimal Places** (number) ## When to use - Estimate Kd from one protein concentration and a measured fraction of labeled DNA bound. - Convert fluorescence anisotropy measurements into a bound fraction and estimate Kd. - Calculate the corresponding ΔG° and review whether the estimated affinity is very tight, strong, moderate, or weak. ## How it works - Choose Fraction bound or Fluorescence anisotropy as the calculation mode. - Enter the total protein concentration in nM and the temperature in °C. - For fraction mode, provide a bound fraction between 0 and 1; for anisotropy mode, enter the measured, free-state, and bound-state anisotropy values. - The calculator applies Kd = \[P\] × (1−f)/f, calculates ΔG° = RT·ln(Kd), and returns the estimated Kd, bound fraction, free energy, and affinity band. ## Use cases - Quickly estimate transcription factor or other protein–DNA binding affinity from an EMSA or related bound-fraction measurement. - Interpret fluorescence anisotropy readings when free and fully bound reference values are available. - Compare estimated Kd and ΔG° values during educational exercises or early experimental planning. ## Frequently asked questions ### What does this calculator estimate? It estimates a single-point Kd for protein–DNA binding under a 1:1 equilibrium model. ### Can I enter a bound fraction directly? Yes. Select Fraction bound and enter a value strictly between 0 and 1. ### How does anisotropy mode calculate the bound fraction? It uses f = (r − r free) / (r bound − r free), based on the measured, free-state, and bound-state anisotropy values. ### What does ΔG° represent here? ΔG° is the binding free-energy estimate calculated from Kd at the selected temperature. ### Is this a replacement for fitting a full binding curve? No. It is a single-point estimate; a complete titration should be fitted as a full curve for more robust analysis. ## Related tools - [Enzyme Activity Unit Converter (U ↔ katal)](https://elysiatools.com/en/tools/enzyme-unit-converter): Converts activity values exactly between U/mU/µU and katal/mkat/µkat/nkat/pkat (1 U = 16.67 nkat, 1 kat = 6×10⁷ U). 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