- Surface Plasmon Resonance (SPR) is a sophisticated and widely adopted analytical technique used to study biomolecular interactions in real time and without the need for labeling.
- At its core, SPR measures changes in the refractive index near the surface of a sensor chip, which is typically coated with a thin layer of gold. When a polarized light beam strikes this metal layer under conditions of total internal reflection, it excites surface plasmons—coherent oscillations of free electrons—at the metal-dielectric interface. This interaction leads to a drop in reflected light intensity at a specific angle, known as the resonance angle. When biomolecules bind to the sensor surface, the refractive index changes, causing a measurable shift in this angle.
- In a typical SPR experiment, one molecule (the ligand) is immobilized on the sensor surface, and its potential binding partner (the analyte) is flowed over the surface in a controlled manner. If binding occurs, the increase in mass at the surface alters the refractive index, producing a real-time response signal. This signal is displayed as a sensorgram, which plots response units (RUs) against time, allowing researchers to monitor the association and dissociation phases of the interaction. From this data, kinetic parameters such as the association rate constant (kₐ), dissociation rate constant (k_d), and the equilibrium dissociation constant (K_D) can be accurately derived.
- SPR is particularly valuable in drug discovery, where it helps in identifying and characterizing the interaction of small molecules with target proteins. It is also extensively used to investigate protein–protein, protein–DNA/RNA, and antibody–antigen interactions. Because it does not require labels or modifications to the analytes, SPR offers a closer representation of native interaction dynamics compared to techniques that depend on fluorescence or radioactivity.
- Despite its many strengths, SPR does have limitations. The immobilization of ligands on the chip surface can sometimes affect their native conformation or binding activity. Additionally, non-specific interactions or mass transport limitations may complicate data interpretation. Nevertheless, with careful experimental design and controls, SPR remains one of the most powerful techniques for studying molecular interactions in a quantitative and physiologically relevant manner.
- Overall, Surface Plasmon Resonance has become an indispensable tool in both basic and applied biosciences, offering unparalleled insights into the mechanics and kinetics of biomolecular recognition processes.
