Understanding molecular interactions—especially dynamics—can answer so many questions, it's no surprise that analysis of molecular interactions is required in many areas of research.

For example, in order to understand how signaling occurs in an organism, you may need to know how molecules and receptors interact. Or, during drug discovery, you may want to know whether a drug binds to another compound of interest, and how tightly. Binding affinity can tell us this, but we can learn more details by measuring binding kinetics.

The fields of application associated with binding dynamics are as broad as the techniques, and a variety of techniques can be used to study them. Here we compare two popular technologies, bli and spr.

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What is biofilm layer interference technology?

Biolayer interferometry (BLI) is a label-free, surface-based optical analysis technique. Unlike other biosensor technologies, BLI does not use microfluidics, but works by dipping the sensor tip into a sample/buffer. Light reflected from the fiber tip will exhibit a phase shift according to the refractive index near the tip surface. The reflected light interferes with the light reflected from the internal reference surface.

By using white light as a light source, a spectral interference pattern is recorded, which includes information about the refractive index near the tip surface. When biomolecules bind to the surface of a biofilm layer immersed in an experimental solution (eg, a sample), the refractive index profile and spectral pattern change.

Advantages of BLI

Easy to use

Virtually non-clogging for complex, viscous samples

Use of a reference tip minimizes volume effects

Relatively low cost

Label-free and real-time analysis

Disadvantages of BLI

Sensors are orders of magnitude less sensitive than SPR and GCI sensors

Limited precision when determining kinetic parameters

Limited ability to measure tight binders and fast association rates; measurement limited by diffusion

Limited ability to measure fast off-rates

What is Surface Plasmon Resonance?

Surface plasmon resonance (SPR) is another label-free optical analysis method – in fact, it was one of the first surface-based label-free analysis techniques to be employed. SPR detects refractive index changes caused by molecular interactions within an evanescent field near the sensor surface.

In these sensors, a metal film on a glass support is illuminated with light of a specific wavelength. Depending on the refractive index near the surface, so-called surface plasmons are excited at specific angles. Due to the lack of energy in the reflected beam, there is a "dip" in intensity when it hits the sensor.

By determining the location of the drop in real time, SPR measures the change in the refractive index of the metal surface. The analyte-containing solution is injected through the microchannel, and at least one reference flow cell is used to eliminate volume effects.

Advantages of SPR

Can measure tight binders and fast binding rates

Sample requires no labeling or modification before analysis

Low requirement on sample purity

High throughput and high sensitivity analysis

In situ, real time dynamics study under natural conditions

Disadvantages of SPR

Limited detection of fastest transition due to use of flow cells in series

Traditional microfluidics require careful maintenance due to clogging tendency

Limited ability to measure fast off-rates