Peridinin-chlorophyll-protein complex by the CATH database, which systematically classifies protein structures, as an "alpha solenoid" fold; however, elsewhere in the literature the term alpha solenoid is used for open and less compact helical protein structures. Photosynthetic dinoflagellates contain membrane-bound light-harvesting complexes similar to those found in green plants. They additionally contain water-soluble protein-pigment complexes that exploit carotenoids such as peridinin to extend their photosynthetic capacity. Peridinin absorbs light in the blue-green wavelengths (470 to 550 nm) which are inaccessible to chlorophyll by itself; instead the PCP complex uses the geometry of the relative pigment orientations to effect extremely high-efficiency energy transfer from
Carotenoid especially reds and purples, are due to other classes of chemicals. Carotenoids serve two key roles in plants and algae: they absorb light energy for use in photosynthesis, and they protect chlorophyll from photodamage. Carotenoids that contain unsubstituted beta-ionone rings (including beta-carotene, alpha-carotene, beta-cryptoxanthin and gamma-carotene) have vitamin A activity (meaning that they can be converted to retinol), and these and other carotenoids can also act as antioxidants. In the eye, lutein, meso-zeaxanthin, and zeaxanthin are present as macular pigments whose importance in visual function remains under clinical research in 2017. The basic building blocks of carotenoids are isopentenyl diphosphate