Try scattered plot anlysis for affinity of your ligand and receptor.

I would suggest using Scatchard plots analyses instead.

Hi Jin,

Let's denote Af as the free concentration of A (ligand), Bf is the free concentration of B (receptor), At is the total concentration of ligand, Bt is the total concentration of receptor, and AB is the concentration of AB complex. You have asked how to calculate AB/Bt. You need a binding model to take into account the possibilities for ligand and receptor. The simplest model is the one-to-one binding: 1 ligand can bind to 1 receptor site. In this case the following two equations hold: At = Af + AB, Bt = Bf + AB. As Kd = Af*Bf/AB, what you need to do is replacing Af with At - AB, and Bf with Bt-AB, then solve the equation for AB. This is a second order equation for AB. When solving with the well-known formula, only the root with the "-" can have chemical meaning, so finally %A bound to B is 100AB/Bt = [Kd+At+Bt - SQRT((Kd+At+Bt)2 - 4*At*Bt)]/(2*Bt) This so called quadratic formula is valid at any given concentration. In the case of At >> Bt (minimum 20x molar excess of ligand or more) than this formula drops to the hyperbolic binding function: AB/Bt = At/(At + Kd). You can do this calculation with any of the popular graphing softwares.

Hope this helps. Cheers: Karoly

the equation really helped! thank you!

We have developed a Web app exactly for this type of questions: just input the kd and concentrations to the app, the web app will plot the fraction of protein in bound state. It also includes the quadratic formular.

https://jiang.bio.purdue.edu/binding