Binding Calculator

Calculate binding energy, binding affinity, dissociation constant (Kd), and Gibbs free energy (ΔG) for molecular interactions.

Calculation Type

Temperature (K)

Standard: 298 K (25°C)

Kon (association rate, M⁻¹s⁻¹)

Koff (dissociation rate, s⁻¹)

Kd = Koff / Kon

Kd (Dissociation Constant, M)

ΔG = RT ln(Kd)

Fraction Bound (θ)

Free Ligand Concentration [L] (M)

Kd = [L] × (1-θ)/θ

IC50 (M)

Substrate Concentration [S] (M)

Km (Michaelis constant, M)

Assay Type

Ki (Inhibition constant, M)

Inhibition Type

Kd (Dissociation Constant)

ΔG (Gibbs Free Energy)

Interpretation

Binding Parameters

Frequently Asked Questions

What is Kd (dissociation constant)?

Kd is the dissociation constant that quantifies the affinity between two molecules (e.g., ligand and receptor). A lower Kd indicates higher binding affinity. Kd = [L][R]/[LR], where [L] is free ligand concentration, [R] is free receptor concentration, and [LR] is the concentration of the complex.

How do you calculate binding affinity?

Binding affinity is inversely related to Kd: higher affinity = lower Kd. The standard free energy change ΔG = RT ln(Kd), where R = 1.987 cal/(mol·K) and T is temperature in Kelvin. More negative ΔG indicates stronger binding.

What is the relationship between Kd and binding strength?

Kd is inversely proportional to binding strength. A smaller Kd (nanomolar or picomolar range) indicates high affinity binding. Typical ranges:

μM (10⁻⁶): Weak binding
nM (10⁻⁹): Moderate to strong binding
pM (10⁻¹²): Very strong binding

How do I convert IC50 to Kd?

The conversion depends on assay type. For competitive inhibition: Kd = IC50 / (1 + [S]/Km). For non-competitive: Kd = IC50. The Cheng-Prusoff equation is commonly used: Ki = IC50 / (1 + [S]/Km).

What is Gibbs free energy (ΔG) in binding?

ΔG is the change in free energy upon binding. A negative ΔG indicates spontaneous binding. The relationship with Kd is: ΔG = -RT ln(1/Kd) = RT ln(Kd). At 298 K, ΔG = 1.987 × 298 × ln(Kd) cal/mol.