Betalains structure

Muscapurpurin was found relatively recently by Musso [see Ref. 504 in Gill and Steglich (63)] to have the betalaine structure of extended conjugation which causes the purple color of the pigment (Fig. 6). This result modified the earlier structural proposals (61) obtained after several years of investigations. 

The purple pigment [UV/vis. (water) A,max = 303, 540 nm] from A. muscaria has been named muscapurpurin and recently assigned the betalaine structure (179) 504) modifying an earlier, tentative, structural assignment 188, 190, 502). 

Piattelli, M. The betalains, structure, biosynthesis and chemical taxonomy. In The Biochemistry of Plants, Vol. 7, Secondary Plant Products (E. E. Conn, ed.), pp. 557-575. Academic Press, New York 1981... 

In addition, dehydrogenated structures were found to cause a hypsochromic shift. Since, with the exception of neobetanin, these betacyanin degradation products were found not to be native to betalain crops, it is suggested that these peculiar structures may be used as heat indicators to assess the thermal loads of the respective products have undergone during production. 

Besides their interesting color application values, betalainic plants are also worthwhile from a nutritional standpoint. Research on this topic has recently been resumed with great scientific vigor in both in vitro and in vivo studies on red beets, amaranth, red-colored Swiss chard, red-violet pitahayas, and especially cactus pears. ° In the future, betalainic color crops will be benchmarked because of their pigment structure and quantity and also because of the individual and synergistic activities of their components such as colorless phenolics, amino compounds, peptides, proteins, and hydrocolloids. 

FIGURE 13.1 Typical structures for main pigment classes zeaxanthin (carotenoid), chlorophyll a (chlorophyll), quercetin (flavonoid), cyanidin (anthocyanidin), betanin (betalain), and alizarin (anthraquinone). 

Betalains consist of two groups of pigments of similar structure, the yellow compounds classed as betaxanthins and the purple as betacyanins [14]. Both groups contain a betalamic acid residue coupled to an amine or an amino acid to form an iminium salt. Whereas the amino components of the betaxanthins vary considerably. 

FIGURE 13.7 Structures for the representative betalains vulgaxanthin I (betaxanthin) and betanidin (betacyanin). 

The special pigments of Centrospermae have attracted considerable interest during the past 40 years the pigment composition of red beet has especially been the topic of many investigations. Initially the betalains of red beet were separated and isolated using paper electrophoresis (244) and column chromatography (216) their chemical structure was elucidated by means of chemical tests and spectroscopic methods (214). The traditional methods for the quantitative determination of betacyanins and betaxanthins in beet root were spectrophotometry, mainly the Nilsson method (216). More recently, HPLC has become the method of choice for the separation and quantification of beet pigments (247). 

Figure 6-31 Structure of Naturally Occurring Betalains in Red Beets. Source-. From J.H. Von Elbe and I.-Y. Maing, Betalains as Possible Food Colorants of Meat Substitutes, Cereal Sci. Today, Vol. 18, pp. 263-264, 316-317, 1973.

Figure 2.21 Structures of betanldln and a typical anthocyanidin, cyanidin, accumulating as various glycosylated structures and their acylated forms. Both compounds exhibit similar max values. Occurrence of these two classes of pigments mutually excludes each other. The betalains are exclusively found in most families of the plant order Caryophyllales, whereas the anthocyanins are ubiquitously distributed in the other families of the Angiosperms.

Research on betalains has received a significant impetus from recent developments in chromatography, spectroscopy, biochemistry and techniques of molecular biology. This has led to a rapid increase in our knowledge about new structures as well as key steps in their bios)mthesis. Some new structural features of betalains from plants are reviewed below, resulting primarily from advances in work on their bios)mthesis, whilst still being aware of the validity of earlier h3q)otheses. 

Progress in the chemistry of betacyanins depended on the development of efficient methods for their isolation and separation. Betalains often occur as complex mixtures and are easily decomposed during the purification steps, which render the isolation of larger amounts of material for structural studies difficult. It is therefore understandable that some of the compounds described during the 1960s need reinvestigation by modem techniques. 

Most of the betalain pigments described in the 1960s have not been characterized by mass or NMR spectra. With FAB MS now at hand, the molecular ions of underivatized betalains can be easily determined, and the sensitivity of high-field NMR spectrometers allows the complete structural assignment, even of small samples. For the structural determination of oligosaccharide moieties, modem two-dimensional (2D) NMR techniques are now the method of choice. 

In the 1950s betalains were shown to be useful as chemotaxonomic markers for members of the Caryophyllales (Caryophyllidae), even before their actual structures were known ( W-anthocyanins ) (35,38). These pigments have since... 

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