Phycobilin
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Through Chloroplast, Plant and Protein
The phycobilins are especially efficient at absorbing red, orange, yellow, and green light, wavelengths that are not well absorbed by chlorophyll a. Organisms growing in shallow waters tend to contain phycobilins that can capture yellow/red light, 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.
Types
There are four types of phycobilins:
- Phycoerythrobilin, which is red
- Phycourobilin, which is orange
- Phycoviolobilin (also known as phycobiliviolin) found in phycoerythrocyanin
- Phycocyanobilin (also known as phycobiliverdin), which is blue.
They can be found in different combinations attached to phycobiliproteins to confer specific spectroscopic properties.
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Structural relation to other molecules
In chemical terms, phycobilins consist of an open chain of four pyrrole rings (tetrapyrrole) and are structurally similar to the bile pigment bilirubin, which explains the name. (Bilirubin's conformation is also affected by light, a fact used for the phototherapy of jaundiced newborns.) Phycobilins are also closely related to the chromophores of the light-detecting plant pigment phytochrome, 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.