The basic structure of the hydrophobic<unk>  molecule consists of a cyclic end group, a polyene side chain and a polar end group. The conjugated system formed by alternating C=C double bonds in the polyene side chain are responsible for the color of<unk> s (typically yellow, orange, or red). Hence, many<unk> s are chromophores. Alternation of side chains and end groups creates the various classes of<unk> s. First- and second-generation<unk> s are able to bind with several<unk>  receptors due to the flexibility imparted by their alternating single and double bonds. Third-generation<unk> s are less flexible than first- and second-generation<unk> s and therefore, interact with fewer<unk>  receptors. Fourth-generation<unk> , Trifarotene, binds selectively to the RAR-y receptor. It was approved for use in the US in 2019.
Retinoid
Retinol
Epoetin beta
Vitamer
Immunoisolate
Β-Carotene
Phytosteroid
Psymberin