This is one of my fave plants that I see around, its pink to purple colours made of various pigments (as you will find out), stand out. And what causes these colours? "excitation electron transitions between the quantum mechanical orbitals associated with bonds" (again, read on and find out).
and thank you again Andre Jagger for your stunning illustrations, and fine work. Over to you.....
In this final part on C. argentea, the writer examines some of the rich and known biochemistry of Celosia species. The order Caryophyllales in which the family Amaranthaceae sits is unique in that some of the families possess pigments not based on the ketone containing; flavonoids, isoflavonoids and neoflavonoids [ anthocyanidins, anthoxanthins, flavanones, flavanonols, flavans]. Instead, another compound forms the basis of pigments in these families. It is a chemotaxonomic marker for some families of the Caryophyllales, this compound is an aldehyde and a carboxylic acid and is known as Betalamic Acid. The name derives from "Beet" as in Beetroot or Swiss Chard (Silver Beet) - these vegetables are now
in the family Amaranthaceae.
Brace oneself for the chemistry! Ooooook Andre....hit me.......
It is the aldehyde functional group of Betalamic acid (end of the sidechain of the piperidine / pyridine ring system NOT containing COOH) that creates all the action (like a movie action-thriller) and provides the wealth of pigments (and antioxidants) known as the Betalains. Amines 'substitute' with the aldehyde group creating iminium cations that form the basis for many Yellow to Orange pigments known as Betaxanthins. Similarly, Dopamine undergoes ring closure to form an equivalent to the iminium cations, known now as indolium cations, for the longest carbon chain in the naming scheme now resides in the indole ring system (formerly Dopamine), forming the Betacyanins - Red to Purple pigments with Beetroot as an example. The table in the lower left hand side of this picture shows examples of Betacyanins. Of these, two have been found as reddish pigments in Celosia species. These two are; Amaranthin and Isoamaranthin (the same as Amaranthin except the H and COOH group are switched front and back in the plane creating the reverse orientation locally - a stereoisomer - at carbon 15 in the enumeration presented in blue). The pink to purple colours of Celosia argentea are most likely the result of the pigments Amaranthin and Isoamaranthin under different acidity (pH) conditions and levels of glycosylation. The sidechain substituted for 'H' on the hydroxyl group in the Betacyanin backbone at R1 for Amaranthin and Isoamaranthin, is sophorobiuronic acid, basically sophorose (α-sophorose is a disaccharide - two glucose units with the unusual β-1,2 bond, Glc(β1-2)α-Glc the simplified carbohydrate chemistry name) with a carboxylic acid substitution in one of the glucose components. Sophorose forms a common glycosylation with many anthocyanidins too, forming many important anthocyanins, and not just the Betcyanindin derivatives. So there it is, a plausible explanation for the pink - purple pigmentation in C. argentea.
Finally, what causes the colour in these chemicals? The brief answer would be excitation electron transitions between the quantum mechanical orbitals associated with bonds (and possibly some non-bonding electrons) in these various colour producing chemicals, known as chromophores. Early in the discovery of Betalamic acid, itself a lemon-yellow chromophore, led to the discovery of the conjugated double bond structure, given by a parent compound 5-dimethylamino-penta-2,4-dienal, with it's σ-bonds, π-bonds and s and p hybridized orbitals. By analogy a similar chromophore backbone exists in the Betalains, with the backbone structure, commonly referred to as 1,7-diazaheptamethin system, being that of the parent, 5-dimethylamino-penta-2,4-diene-1-dimethyliminium cation. These backbones in the larger molecules (Betalamic Acid and Betalains) represent an amalgamation of resonance structures leading to the same kind of resonance hybridization seen in the "double" bonds of Benzene. This resonance plus the nature of the atomic orbitals, bonds and hybridization all happen to create an environment in which electronic transitions beteen energy states are sufficiently rich and interesting that it leads to the colour variation seen in these chromophores.
... and with that the writer concludes three months of work on C. argentea.
Thanks again Andre.