Association constant

When analyzing binding kinetics with some biosensors such as the WAVEsystem, a Creoptix technology, 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’. 

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

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.  

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.

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 WAVEsystem is an optical biosensor that can measure the association constant as it measures binding kinetics.

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.