The current biophysics toolbox is composed of a number of well-established methods that are continuously being enriched by novel techniques that are higher throughput to further our mechanistic understanding of low molecular weight drug candidates (Ottl J. Biophysics/label-free assays in hit discovery and verification. In M Cooper, LM Mayr (eds), Label-free Technologies for Drug Discovery. Wiley; 2011, pp. 155–169). Among the available biophysical methods, a number of techniques are based on biosensors where ligand interactions with immobilized biological molecules are monitored. Biosensor-based techniques vary in type and instrument design within each type, and they are gaining traction due to their label-free nature and the ability to provide real-time data (Daghestani et al., Sensors 2010;10:9630–9646).
A long-standing and perhaps one of the most widely used biosensor-based techniques is surface plasmon resonance (SPR). First demonstrated in the late 1960s and commercialized in the 1990s, SPR utilizes single transverse magnetic (TM) polarization for detection of binding events. In contrast, a more recently introduced evanescent technique (commercialized in 2000), dual polarization interferometry (DPI), allows measurements in both the TM and transverse electric (TE) modes, thus providing information not only on changes in refractive index but also in thickness. As a result, DPI offers the opportunity to probe structural changes that occur during binding in addition to the kinetics of the interactions.
Potentially, DPI could meet the challenging need to monitor the real-time interplay between binding mode of action (MoA) and structural change. In the past, DPI has been successfully shown to detect protein structural changes upon ion binding, such as Zn2+ (Fresquet et al., J Biol Chem 2007;282:34634). Herein, using calmodulin (CaM) as a model system, Coan et al.* demonstrated that DPI was capable of detecting protein structural change induced by small molecule ligand binding.