What is binding?
“The first step in essentially all biological activities is a union between separate constituents into a merged entity.”
Using a KinExA assay for making Kd measurements is ideal because measurements are made without perturbing the solution equilibrium and with unmodified molecules, unfettered, in solution.
Measurement of free receptor concentration may also be done prior to the samples reaching equilibrium. Measurements taken as a system approaches equilibrium provide data for the kinetics of the reaction.
Cells with intact plasma membranes in a bioreactor/fermenter can be considered to act as tiny capacitors under the influence of an electric field.