Nomenclature
Carotenes are carotenoids containing no oxygen. Carotenoids containing some oxygen are known as xanthophylls.
The two ends of the β-carotene molecule are structurally identical, and are called β-rings. Specifically, the group of nine carbon atoms at each end form a β-ring.
The α-carotene molecule has a β-ring at one end; the other end is called an ε-ring. There is no such thing as an "α-ring".
These and similar names for the ends of the carotenoid molecules form the basis of a systematic naming scheme, according to which:
- α-carotene is β,ε-carotene;
- β-carotene is β,β-carotene;
- γ-carotene (with one β ring and one uncyclized end that is labelled psi) is β,ψ-carotene;
- δ-carotene (with one ε ring and one uncyclized end) is ε,ψ-carotene;
- ε-carotene is ε,ε-carotene
- lycopene is ψ,ψ-carotene
ζ-carotene is the biosynthetic precursor of neurosporene, which is the precursor of lycopene, which, in turn, is the precursor of the carotenes α through ε.
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