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Optical tools for studying biomolecular interactions

19 August 2020

Understanding binding specificity, affinity and kinetics is essential information when developing biological drugs. Surface plasmon resonance (SPR) and biolayer interferometry (BLI) are optical techniques used for investigating biomolecular interactions.

Importantly, both can be applied for label-free compounds and real-time measurements conducted to evaluate binding kinetics. The methods have much in common and it may be challenging to choose between them. On the other hand, sometimes they can be used for complementing each other.

The SPR phenomenon is used in a variety of applications in different fields and in drug development it has already decades of history as an analytical technique. The basis of the equipment consists of light source, sensor chip and detector. SPR is the phenomena, where in a certain angle the intensity of light is reduced when the light is reflected, in this case, from the sensor interface. Any biomolecular interactions taking place on the sensor will affect the refractory index and this utilised to obtain data about the interactions between the analytes and ligands. BLI utilises the interference pattern of light reflected from two surfaces. The readout is wavelength shift, which is affected by the changes in the biomolecular interactions. The technology has been available for drug discovery and development almost twenty years.

In SPR the ligand is attached on the sensor surface, which is in contact with a microfluidic system, where the analyte is available for binding. Instead of sensor chips, BLI utilises single use fiber-optic biosensors and there is no fluid flow, but instead the sensor tips are introduced directly to the sample located on a well-plate. In a similar manner with SPR, also in BLI the ligand is bound on the sensor tip and the analyte for the interaction is available in the sample.

The main differences when using these two techniques arise mainly from the throughput capacity. When using BLI, several samples can be analysed in parallel and the single use sensor tip is introduced to the sample allowing reusing the sample if necessary. In addition, the assay set up is typically faster. On the other hand, the system uses well-plates, with warming, and hence there is a risk of sample evaporation or re-binding interfering the analysis. In the SPR the sample comes through the microfluidic system to the sensor and is directed then to the waste, which differs from the BLI. In addition, the SPR system requires regeneration for reusing the sensor chip. Typically, the regeneration is performed by injecting acidic buffer to the system, which detaches the analyte, but the regeneration buffer should not cause loss of the ligand function.

The commonly investigated biomolecular interactions include antibody-antigen interactions to determine the antibody specificity. Antibody-FcRn interactions are responsible for the long in vivo half-life of antibodies and it is useful to cover also non-clinical species, in addition to the human receptor. Binding of the antibody to FcƴR may lead to antibody-dependent cellular cytotoxicity and typically interactions with several different human FcƴR are investigated.

BLI is undeniably faster and enables more screening type of studies and perhaps, SPR could serve as a tool for when the most accurate and consistent data is needed for lower number of samples. As each project has its own specific questions, it may be good to discuss with an experienced scientist for selecting the right way to go and to assist with the study design.

Written by 

Miia Kovalainen