Simply put, a label-free assay measures a protein-protein or a ligand-protein interaction without the use of labels or tags on the protein or ligand. There are many applications that measure protein-protein interactions, but they usually require a fluorescent, luminescent, radioactive, or some other kind of tag to be labeled on the protein(s). As one can imagine, those tags could alter the protein-protein binding mechanism, creating a system that is not representative of the native proteins. As an alternative, several label-free methods have arisen that allow the interaction of untagged proteins to be measured.
One of the best known examples of label free technology is BIAcore, which uses surface plasmon resonance (SPR) to measure protein-protein binding. Proteins are conjugated to a microchip, while the interacting protein or ligand is ‘flowed’ over the conjugated protein. Measuring a change in the SPR signal gives detailed information about this interaction, allowing binding constants and rates to be determined. A major drawback to BIAcore and other label-free technologies, though, is the requirement to use expensive, single-use instrumentation.
Recently a new label-free technique has been developed that utilizes a more common piece of laboratory equipment, a simple absorbance spectrophotometer. The SoPRano™ assay Gold Nano Rods from Pharma Diagnostics allow label-free assays to be performed in a microplate-based environment using an instrument that can capture a full UV/Vis absorbance spectrum. Taking advantage of the localized surface plasmon resonance (LSPR) found in Gold Nano Rods which manifests as a spectral curve, the SoPRano™ assay measures protein-protein interactions by monitoring the shift in the wavelength maximum, which is proportional to the size of the interacting protein.
For more information on how to perform the SoPRano™ Label-free assay on BMG LABTECH’s spectrometer-based microplate readers (PHERAstar FS, SPECTROstar Nano, FLUOstar Omega, POLARstar Omega, and SPECTROstar Omega), see the following application notes: