Promising fluorescent probes from phycobiliproteins

Phycobiliproteins are brilliant-colored and water-soluble pigment-protein complexes existing widely in cyanobacteria, red algae, and cryptophytes. They function as predominant lightharvesting complexes to absorb the sunlight from 480 to 660 nm, and efficiently transfer the energy to chlorophyll a. The phycobiliproteins are classified into three types based on their absorption spectra: phycoerythrin (PE), phycocyanin (PC), and allophycocyanin (AP). They are all composed of hetero-subunits (alpha) and (beta) and commonly exist in trimer ((alpha)(beta))3 or hexamer ((alpha)(beta))6 made up of equimolar monomer ((alpha)(beta)). One monomer contains two to five chromophore phycobilins. In cyanobacteria and red algae, phycobiliproteins assemble a macromolecular particle, phycobilisome. Efficient excitation energy coupling among the chromophores in the phycobiliprotein trimer/hexamer and among the phycobiliproteins in the phycobilisome gives them some special spectroscopic properties superior to organic fluorescent dyes. These properties make the phycobiliproteins become promising fluorescent probes used in various fields of biological investigation.