A

FIGURE 8.7 (A) Betalain and its resonance structures. The structure below the dashed line is present in all betalain molecules. Betalain would be a betacyanin or a betaxanthin depending on the identity of the Rj and R2 residues. (B) An example of betacyanin and (C) of betaxanthin.

2. Classification

Betalains are commonly classified based on their structural characteristics and consequently divided in two groups: betacyanins and betaxanthins with red-purple and yellow colors, respectively. As can be observed, each group of pigments is characterized by specific RrN-R2 moieties. More than 50 betalains have been described, all with the same basic structure, and their Rj and R2 groups can be hydrogen, an aromatic group, or another substituent. The betalain color is attributable to the resonating double bonds (Figure 8.7A). When the basic structure is substituted with an aromatic nucleus, a change in the absorption maximum from 540 nm (red-purple betacyanins such as betanidine) to 480 nm (yellow betaxanthins such as miraxanthin-II) is observed (Figure 8.7B and C). A large number of betaxanthins can be formed, with the same dihydropyridine moiety, by conjugation with several amine compounds such as amino acids; as an example, the R2 group of vulgaxanthin-I obtained from B. vulgaris is derived from glutamic acid.84 On the other hand, the diversity of betacyanins is associated with the combination of the basic structures (betanidin is the most important followed by isobetanidin, its Cj5 epimer) with different glycosyl and acyl groups attached by the hydroxyl groups at positions 5 and 6 (Figure 8.7B). The most common glycosyl moiety is glucose, although sophorose and rhamnose could also occur, but less frequently. On the other hand, the most common acyl groups are sulfuric, malonic, 3-hydroxy-3-methylglutaric, citric, p-coumaric, ferulic, caffeic, and sinapic acids.85 Table 8.j5 shows examples of betalains.4,j7

TABLE 8.15 Examples of Betalains

Betacyanins Betaxanthins

Name

Substituent group

R, Botanical source

Name

Betanin [3-glucose H

Phyllocactin 6'-0-(malonyl)-P-glucose H

Lamparanthin I 6'-0-/?-coumaroyl-P-glucose H

Amaranthin 2'-0-([3-glucoronic acid)-b-glucose H Celosianin-II 2'-0-[0-(iraras-feruloyl)- H

P-glucuronic acid]-[3-glucose R3 and R4 may be acyl or glycosyl substituent groups.

Sources: Adapted from Jackman and Smith (1996)4 and Delgado-Vargas et at. (2000).17

Beta vulgaris Phyllocactus hybridus Lamparanthus spp. Amaranthus tricolor Celosia cristata L.

Indicaxanthin

Portulacaxanthin-I

Vulgaxanthin-I

Vulgaxanthin-II

Dopaxanthin

Both groups together from proline Both groups together from hydroxyproline H Glutamine

H Glutamic acid

H l-DOPA

Botanical source

Opuntia ficus-indica Portulaca grandiflora Beta vulgaris Beta vulgaris Glottiphyllum longum

Was this article helpful?

0 0

Post a comment