trFluor™ Tb goat anti-rabbit IgG (H+L) *Cross Adsorbed*

Figure 1. Time-resolved fluorescence energy transfer (TR-FRET) is the practical combination of time-resolved fluorometry (TRF) combined with Förster resonance energy transfer (FRET) that offers a powerful tool for drug discovery researchers. TR-FRET combines the low background aspect of TRF with the homogeneous assay format of FRET. The resulting assay provides an increase in flexibility, reliability and sensitivity in addition to higher throughput and fewer false positive/false negative results. FRET involves two fluorophores, a donor (such as trFluor Eu and trFluor Tb) and an acceptor. Excitation of the donor by an energy source (e.g. flash lamp or laser) produces an energy transfer to the acceptor if the two are within a given proximity to each other. The acceptor in turn emits light at its characteristic wavelength. The FRET aspect of the technology is driven by several factors, including spectral overlap and the proximity of the fluorophores involved, wherein energy transfer occurs only when the distance between the donor and the acceptor is small enough. In practice, FRET systems are characterized by the Förster's radius (R₀): the distance between the fluorophores at which FRET efficiency is 50%. For many FRET parings, R₀ lies between 20 and 90 Å, depending on the acceptor used and the spatial arrangements of the fluorophores within the assay. Through measurement of this energy transfer, interactions between biomolecules can be assessed by coupling each partner with a fluorescent label and detecting the level of energy transfer. Acceptor emission as a measure of energy transfer can be detected without needing to separate bound from unbound assay components (e.g. a filtration or wash step) resulting in reduced assay time and cost.