The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by
chlorophyll a.[2] Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light,[3] while those at greater depth often contain more of the phycobilins that can capture green light, which is relatively more abundant there.
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]
Phycocyanobilin (also known as phycobiliverdin), which is blue.
They can be found in different combinations attached to phycobiliproteins to confer specific spectroscopic properties.
Structural relation to other molecules
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.
Chlorophylls are composed of four pyrroles as well, but there the pyrroles are arranged in a ring and contain a metal atom in the center of it.