Figure 1 Europium trisbipyridine cryptate has an excitation peak at 337 nm and an emission peak at 620 nm.

The best choice for an acceptor molecule for the (Eu)K donor will complement the cryptate fluorescence energy spectrum, accept the transfer of energy efficiently, be stable in common assay media, and lend itself to appropriate chemistry for conjugation with biological components. For HTRF assays, the best choice for acceptor molecule is one of the main constituents of the phycobilisomes of red algae. These protein-pigment complexes absorb light and channel it to the cell’s photosystem. In the phycobilisome, the last component that transfers light is allophycocyanin (APC), a phycobiliprotein of 105 kD with an absorption band ranging from 600 to 660 nm and an emission maximum at 660 nm. APC, when modified by cross-linking, emits at 665 nm and is called XL665 (Fig. 2).