The phycobilins fluoresce at a particular wavelength, and are, therefore, often used in research as chemical tags, e.g., by binding phycobiliproteins to antibodies in a technique known as immunofluorescence.
[4] There are four types of phycobilins:[1] They can be found in different combinations attached to phycobiliproteins to confer specific spectroscopic properties.
In chemical terms, phycobilins consist of an open chain of four pyrrole rings (tetrapyrrole)[5] and are structurally similar to the bile pigment bilirubin,[6] which explains the name.
(Bilirubin's conformation is also affected by light, a fact used for the phototherapy of jaundiced newborns.
)[7] Phycobilins are also closely related to the chromophores of the light-detecting plant pigment phytochrome,[8] which also consist of an open chain of four pyrroles.