Association Constant

What is the association constant?

The association constant is a kinetic parameter used when measuring the intermolecular interactions and binding events between two or more molecules. The association rate constant (k_a, k_on or on-rate) is the rate at which one molecule binds to the second to form a complex. It is recorded in moles (the standard quantification of number of molecules) per second: M^-1s^-1.

How do you measure the association constant?

When analyzing binding kinetics with some biosensors, such as the Creoptix WAVEsystem, one molecule will be immobilized to the biosensor surface (the chip) and is termed the ‘ligand.’ The second molecule is flowed past in a microfluidic flow and is termed the ‘analyte’. 

First, the baseline is established by flowing the buffer past the sensor with an immobilized ligand. After this, the sample (analyte) is flowed past. The analyte is associated with the ligand during the association phase. From here, the association constant can be calculated, as the rate at which the analyte binds to the ligand to form a complex at standard concentration. This is why it is expressed in moles per second.  

The resulting curve from a binding interaction measurement (a sensorgram).

Together, the association rate and dissociation rate (k_d) define the affinity (equilibrium dissociation constant, K_D) and determine the binding kinetics (the dynamic interaction between two molecules). The K_D is calculated as k_d/k_a and determines the analyte concentration at which half of the ligands are occupied by the analyte at equilibrium.

How do you measure association constants with the Creoptix WAVEsystem?

Accurate and efficient measurement of intermolecular interactions and binding events is a critical element of a wide variety of basic research, and an indispensable component of drug discovery programs. The Creoptix WAVEsystem is an optical biosensor that can measure the association constant as it measures binding kinetics.

Table from Creoptix WAVE | Optical Biosensor Platform

What other methods can I use to measure the association constant?

Besides the WAVEsystem – which uses grating-coupled interferometry (GCI), common methods to measure the association constant are biolayer interferometry (BLI) or surface plasmon resonance (SPR). On our technology comparison page, you can find out more about the features of BLI vs SPR and GCI.