Protocyanin
{{Short description|Anthocyanin pigment}}
Protocyanin is an anthocyanin pigment that is responsible for the red colouration of roses, but in cornflowers is blue. The pigment was first isolated in 1913 from the blue cornflower (Centaurea cyanus),{{cite journal | author=Willstätter R & Everest RW | journal=Justus Liebigs Ann. Chem. | year=1913 | volume=401 | pages=189–232 | doi = 10.1002/jlac.19134010205 | title=Untersuchungen über die Anthocyane. I. Über den Farbstoff der Kornblume | issue=2 | url=https://zenodo.org/record/1427619 }} and the identical pigment was isolated from a red rose in 1915.{{cite journal | vauthors=Willstätter R, Mallison H | year=1915 | journal=Justus Liebigs Ann. Chem. | volume=408 | pages=147–162 | doi = 10.1002/jlac.19154080110 | title=Untersuchungen über die Anthocyane. X. Über Variationen der Blütenfarben | url=https://zenodo.org/record/1427639 }} The difference in colour had been explained as a difference in flower-petal pH, but the pigment in the blue cornflower has been shown to be a supermolecular pigment consisting of anthocyanin, flavone, one ferric ion, one magnesium and two calcium ions{{cite journal |vauthors=Shiono M, Matsugaki N, Takeda K | title=Structure of the blue cornflower pigment | year=2005 | journal=Nature | volume=436 | pages=791 | doi=10.1038/436791a | pmid=16094358 | issue=7052 |bibcode = 2005Natur.436..791S | s2cid=4312804 | doi-access=free }} forming a copigmentation complex.
The molecular formula of protocyanin complex is of the type of C366H384O228FeMg.