(BETACYANINS & BETAXANTHINS)
The ORDER:Caryophyllales encompasses not only the Amaranthaceae (Goosefoot Family) but also the families of Aizoaceae, Caryophyllaceae, Droseraceae, Francienaceae, Nyctaginaceae, Phytolaccaceae, Plumbaginaceae, Polygonaceae, Portulaceae and Tamaricaceae (as well as perhaps a dozen other families not represented in UK flora). The Caryophyllales are alone in the plant world for adopting colouring compounds based exclusively on Betalains rather than Anthocyanins (blue/purple Flavonoids). The only exception to this are the families
Caryophyllaceae (Carnation Family) and Molluginaceae from which Betalains are entirely absent. Betalain-like compounds have been discovered in carnivorous plants from the Drosera Genus (Sundew) and from the Nepenthes Genus (Pitcher Plants) which has led to both of these being re-classified into the order: Caryophyllale.
The Betalains and Anthocyanins are all pigments and seem to be mutually exclusive; plants adopting one never have both at once! But sometimes the Betalains co-exist with Anthoxanthins (but not anthocyanins). Anthocyanins are blue/red flavonoids, whereas Anthoxanthins are yellow/orange flavonoids. The Betalains in the Caryophyllales completely take over the role assumed by Anthocyanins in other plant families. Both class of compounds are highly coloured, but the anthocyanins dominate in all other plant orders. No one quite knows why this is, nor which came first and which was superseded by the other.The red coloured betalains are derived from Tyrosine, whereas the red coloured anthocyanins are derived from phenylpropanoids. The red coloured betalains serve as a complete functional substitute for anthocyanins, and so far no plant has ever been found to contain both types of red pigments.
The Betalains, on the other hand, are not flavonoids at all, but indole-derived pigments. The name Betalain given to this group of pigments is derived from the Genus Beta, which contains Beta vulgaris, hence the text is written here [It should rather be written under a page dedicated to the order: Caryophllanes, but your Author has created no such page; he doesn't classify Families into Orders. So the Author has put this section on Betalains under Beta vulgaris for want of a better place to put it].
Although both Anthocyanins and Betalains are water-soluble reddish pigments found in the vacuoles of plants, they have totally different chemical structures, the former based on
Flavonoids the latter on
Betalamic Acid, an alkaloidal dicarboxylic acid. Betalamic Acid is the essential structural element of all natural Betalains and their presence in plants belonging to the order: Caryophyllales is instrumental.
The Betalains are further sub-divided into two categories: The Betacyanins which are reddish to violet pigments; and the Betaxanthins, which are yellowish to orange pigments (the colour is within the Ancient Greek names for sky-blue and yellow).
The best studied Betalain is Betanin, the glycoside of Betanidin. Betalain, also called Beetroot Red because it is found within Beetroots (which is yet another member of the Beta vulgaris plants), is a deep red colour and is used as a food colouring agent (Permitted colouring E162), and it hydrolyses into the sugar glucose and its coloured aglycone Betanidin. Indeed, most Betalains are also glycosides, and hydrolyse into a sugar and a coloured aglycone portion.
Like many dyes and pigments, the actual colour of them is sensitive to pH, and it can be used as an indicator of acidity/alkalinity. Both molecules are polar, where there is a separation of charge and it is the partial sharing of a hydrogen atom which gives rise to resonance, the oscillation frequencies being reduced to then lie within the optical band which gives rise to the colour. The pH affects this resonance, so affecting the colour. In some people who are un-able to metabolise Betanin, eating beetroot results in their urine turning red. The Betalains are anti-oxidants and may have positive health benefits.
Betanidin and IsoBetanidin are stereoisomers (epimers) of one another, and both are present in Beet.
It is said that PreBetanin (which is a violet pigment) and NeoBetanin are also present, but these may just be artefacts resulting from the chemical analysis, but they are also probably present when the roots are damaged or when they are cooked).
Important betaxanthines (yellow/orange coloured betalains) present in plants include Vulgaxanthin, Miraxanthin II and Miraxanthin V, Portulaxanthin and Indicaxanthin. The most important difference structurally between Betacyanins and Betaxanthins is that Betacyanins retain an un-broken Indole moiety, whereas in Betaxanthins the indole moiety is either absent, broken, or replaced by an amino acid or amine or other small group.
Miraxanthin I, Miraxanthin II, Miraxanthin III and Miraxanthin V (all found in the mauve-coloured flowers of Four O'Clock Flower aka Marvel of Peru (Mirabilis jalapa). Miraxanthin V incorporates
Betalamic Acid and Dopamine in its structure. Miraxanthin I, below, has a sulfur atom within it and incorporates the amino acid
Methionine Sulfoxide, whereas Miraxanthin II incorporates the amino acid Aspartic Acid instead. Miraxanthin III incorporates Tyramine and Miraxanthin V a dopamine amine.
Humilixanthin is found both in a member of the Pokeweed plants. It is also said to be present in
Carrots, but this is very suspect; the Carrot Family (Apiaceae) is not in the order: Caryophyllales.
Indicaxanthin (which is yellow/orange and found both in Beets and in the cactus
Barbary Fig aka
Prickly Pear (Opuntia ficus-indica). It is a powerful anti-oxidant and incorporates the amino acid Proline.
Portulacaxanthine is found in the flowers of both
Common Puslane (Portulaca oleracea) and in
Moss-rose Purslane (Portulaca grandiflora) and in other plants of the
Portulaceae Family). Portulacaxanthin I incorporates
Hydroxyproline, Portulacaxanthin II a Tyrosine and Portulacaxanthin III a
Glycine amino acid.
Vulgaxanthin II is one of the major betaxanthins in Beetroot and in Swiss Chard. Like all betaxanthins it cannot be hydrolysed to the aglycone without degradation, and stability is not good. It exhibits anti-oxidant properties. After Betanin, Vulgaxanthin I and Vulgaxanthin II are the next most significant betalains within Beetroot. Vulgaxanthin I incorporates a
Glutamine moiety, whereas Vulgaxanthin II incorporates the amino acid Glutamic Acid.
Important Betacyanins (blue/red betalains) are Amaranthin (which is dark-red) and its stereoisomer IsoAmaranthine, which are found in species from the
Amaranthaceae Family. The garden plant
Love-lies-bleeding belongs to the
Amaranthaceae Family and contains large quantities of betacyanins, which the Hopi Indians used as a source of a red dye, 'Hopi Red Dye'. Amaranthin is a double glycoside containing glucose (shown in red) and a Glucuronic Acid (shown in orange) sugar units; it hydrolyses into
Glucose, Glucuronic Acid and Betanidin.
Muscapurpurin is a betacyanin with an intense purple colour, peaking at 540nm, that is found within the Fly Agaric (Amanita muscaria) mushroom, and is a fusion between Betalamic Acid and Muscapurpurinic Acid. As in other betacyanins, the colour is due to the extended chromophore (roughly speaking the separation of charges), which causes a bathochromic shift in the spectrum to a longer wavelength. Note that the Indole is no longer truly Indole, for it has an oxygen atom incorporated into one of the 6-membered rings. In the literature there seems to be some confusion over whether this group is cyclic, or broken open. I have shown it cyclic in the case of Muscapurpurin, and open in the case of Muscapurpurinic Acid, but it could be either. Indeed, the two differ in other ways too - there is one extra oxygen atom in the cyclic Muscapurpurin and four less hydrogen atoms than in the non-cyclic version! One of these is blatantly in-correct. However, the molecule could be unstable and the confusion may simply reflect a breakdown product; an artefact of sample isolation. I have shown the versions which seem to win the vote by way of incidence on the web.
Muscaflavin is also a pigment of the Fly Agaric mushroom, and bears a close relationship to Betalamic Acid, but with a 7-membered ring of
dihydroazepine rather than the 6-membered ring of
dihydropyridine. Without the chromophore structure or separation of charges, its wavelength peak is lower at 336nm resulting in a yellow colour.