| Popis: |
The betalains, which include the red-violet betacyanins and the yellow betaxanthins (Fig. 10.1), represent a structurally and biosynthetically distinct class of naturally-occurring pigments. These pigments are found in flowering plants only in members of certain families of the order Caryophyllales (Centrospermae) (Table 10.1), though betaxanthins have also been shown to occur in the fly agaric, Amanita muscaria (Dopp et al. 1982). Both betacyanins and betaxanthins possess a dihydropyridine moiety which is attached via a vinyl group to another nitrogenous group. In the betacyanins the latter is a glyeosyl-hydroxylated dihydroindole ring; in the betaxanthins it is any one of several amino acids or amines. The basic chromophore of the betalain pigments is the 1,7-diazaheptamethinium system, as discussed by Mabry et al. (1967) and Mabry and Dreiding (1968). The different spectral properties of the betacyanins and betaxanthins are due to the extension of this conjugated system through the dihydroindole moiety in the betacyanins. The term “betalains” was introduced by Mabry and Dreiding (1968) to emphasize the common biogenetic and structural features of these two groups, which clearly distinguish this class of pigments from the more common group of plant pigments, the anthocyanins. |
