Multi-colored shades of betalains: recent advances in betacyanin chemistry

Covering: 2001 to 2021Betacyanins cover a class of remarkable natural red-violet plant pigments with prospective chemical and biological properties for wide-ranging applications in food, pharmaceuticals, and the cosmetic industry. Betacyanins, forming the betalain pigment group together with yellow betaxanthins, have gained much attention due to the increasing social awareness of the positive impact of natural products on human health. Betalains are commercially recognized as natural food colorants with preliminarily ascertained, but to be further investigated, health-promoting properties. In addition, they exhibit a remarkable structural diversity based on glycosylated and acylated varieties. The main research directions for natural plant pigments are focused on their structure elucidation, methods of their separation and analysis, biological activities, bioavailability, factors affecting their stability, industrial applications as a plant-based food, natural colorants, drugs, and cosmetics as well as methods for high-yield production and stabilization. This review covers period of the last two decades of betacyanin research. In the first part of the review, we present an updated classification of all known betacyanins and their derivatives identified by chemical means as well as by mass spectrometric and NMR techniques. In the second part, we review the current research reports focused on the chemical properties of the pigments (decarboxylation, oxidation, conjugation, and chlorination reactions as well as the acyl group migration phenomenon) and describe the semi-synthesis of natural and artificial fluorescent betalamic acid conjugates, showing various prospective research directions.

Biological Properties and Applications of Betalains

Betalains are water-soluble pigments present in vacuoles of plants of the order Caryophyllales and in mushrooms of the genera Amanita, Hygrocybe and Hygrophorus. Betalamic acid is a constituent of all betalains. The type of betalamic acid substituent determines the class of betalains. The betacyanins (reddish to violet) contain a cyclo-3,4-dihydroxyphenylalanine (cyclo-DOPA) residue while the betaxanthins (yellow to orange) contain different amino acid or amine residues. The most common betacyanin is betanin (Beetroot Red), present in red beets Beta vulgaris, which is a glucoside of betanidin. The structure of this comprehensive review is as follows: Occurrence of Betalains; Structure of Betalains; Spectroscopic and Fluorescent Properties; Stability; Antioxidant Activity; Bioavailability, Health Benefits; Betalains as Food Colorants; Food Safety of Betalains; Other Applications of Betalains; and Environmental Role and Fate of Betalains.

ENVIRONMENTAL AND GEOGRAPHIC STUDY OF BETANIN ACCUMULATION IN PROMISING RED BEET ACCESSIONS FROM THE VIR COLLECTION

Background. Table beet (Beta vulgaris L.) is a source of ‘betanin’, a natural food dye known as food additive E-162. The VIR collection holds a large diversity of red beet accessions. It is the base for obtaining source material to breed cultivars with high betanin content.

Materials and methods. In the framework of this study, which lasted from 2015 to 2018, a group of 29 accessions identified during the screening was analyzed. The pigment was assessed at the laboratory of the ECO RESOURCE Joint-Stock Company using Spectrophotometer SF-2000. Field experiments were performed according to VIR’s guidelines in 2015–2017 at Pushkin and Pavlovsk Laboratories of VIR, Volgograd Experiment Station of VIR, and Maikop Experiment Station of VIR.

Results. Environmental and geographic tests demonstrated that the ability to accumulate the pigment was variable among the red beet accessions, and its concentrations were notably different. The research showed that accumulation of betanin is primarily associated with agroclimatic growing conditions. The dynamics of pigment accumulation can be both positive and negative. The optimal zone for growing the studied accession is Leningrad Province, where the highest content of betanin was observed. Mid-ripening and cold-resistant old landraces were a reference point in the search for the desired genotypes.

Conclusion. The process of pigment accumulation is extremely responsive to weather conditions, soil water balance, and watering uniformity. The negative impact of these factors would smooth down the crop’s varietal polymorphism. Diseases and insect pests have led to slowing and stopping the accumulation of betanin. As a result of the comprehensive agroecological study of prototypes, promising accessions were identified and recommended for cultivation in Leningrad Province with the aim of betanin production.

Advances in the MYB-bHLH-WD Repeat (MBW) Pigment Regulatory Model: Addition of a WRKY Factor and Co-option of an Anthocyanin MYB for Betalain Regulation

Flavonoids are secondary metabolites derived from the general phenylpropanoid pathway and are widespread throughout the plant kingdom. The functions of flavonoids are diverse, including defense against phytopathogens, protection against UV light damage and oxidative stress, regulation of auxin transport and allelopathy. One of the most conspicuous functions of flavonoids has long attracted the attention of pollinators and scientist alike: the vivid shades of red, pink, orange, blue and purple on display in the flowers of angiosperms. Thus, flavonoid pigments have perhaps been the most intensely studied phenylpropanoids. From Mendel to McClintock and up to the present, studies centered on flavonoid pigments have resulted in some of the most important scientific discoveries of the last 150 years, including the first examples of transcriptional regulation in plants. Here we focus on the highly conserved MYB-bHLH-WD repeat (MBW) transcriptional complex model for the regulation of the flavonoid pigment pathway. We will survey the history of the MBW model spanning the last three decades, highlighting the major findings that have contributed to our current understanding. In particular, recent discoveries regarding WRKY protein control of the flavonoid pigment pathway and its relationship to the MBW complex will be emphasized. In addition, we will discuss recent findings about the regulation of the beet betalain pigment pathway, and how a MYB member of the MBW complex was co-opted to regulate this chemically unrelated but functionally equivalent pathway.

Plant betalains: Chemistry and biochemistry

Betalains are vacuolar pigments composed of a nitrogenous core structure, betalamic acid [4-(2-oxoethylidene)-1,2,3,4-tetrahydropyridine-2,6-dicarboxylic acid]. Betalamic acid condenses with imino compounds (cyclo-l-3,4-dihydroxy-phenylalanine/its glucosyl derivatives), or amino acids/derivatives to form variety of betacyanins (violet) and betaxanthins (yellow), respectively. About 75 betalains have been structurally unambiguously identified from plants of about 17 families (known till date) out of 34 families under the order Caryophyllales, wherein they serve as chemosystematic markers. In this review, all the identified betalain structures are presented with relevant discussion. Also, an estimated annual production potential of betalains has been computed for the first time. In addition, mutual exclusiveness of anthocyanins and betalains has been discussed in the wake of new evidences. An inclusive list of betalain-accumulating plants reported so far has been presented here to highlight pigment occurrence and accumulation pattern. Betalain synthesis starts with hydroxylation of tyrosine to DOPA, and subsequent cleavage of aromatic ring of DOPA resulting to betalamic acid formation. This pathway consists of two key enzymes namely, bifunctional tyrosinase (hydroxylation and oxidation) and DOPA dioxygenase (O2-dependent aromatic ring cleavage). Various spontaneous cyclisation, condensation and glucosylation steps complement the extended pathway, which has been presented here comprehensively. The biosynthesis is affected by various ecophysiological factors including biotic and abiotic elicitors that can be manipulated to increase pigment production for commercial scale extraction. Betalains are completely safe to consume, and contribute to health.

Flavonoid Properties in Plant Families Synthesizing Betalain Pigments (Review)

The anthocyanin pigments are contained in the flowers, fruits, leaves and roots of almost plant species. On the other hand, distribution of the betacyanins are limited in eight families of the order Caryophyllales, i.e. Aizoaceae, Amaranthaceae, Basellaceae, Cactaceae, Didiereaceae, Nyctaginaceae, Phytolaccaceae and Portulacaceae. However, other flavonoids, i.e. flavones, C-glycosylflavones, flavonols, flavanones, dihydroflavonols, chalcones, aurones, and flavan and proanthocyanidins, are synthesized in betalain-containing families. In this review, distribution and properties of the flavonoids in eight betalain-containing families are described.

Tom J. Mabry's Natural Products Chemistry Program: 1960–2007

This paper presents an overview of Dr. Mabry's accomplishments in his career as a natural product chemist, first at the University of Zürich as a post-doctoral fellow, and from 1962, as a faculty member at the University of Texas at Austin in the Department of Botany until the late 1990s, when the Biological Sciences programs at UT-Austin were completely reorganized. From then until his retirement in 2006, he was a member of the Molecular Cell and Developmental Biology faculty.

Selected topics from forty years of natural products research: betalains to flavonoids, antiviral proteins, and neurotoxic nonprotein amino acids

The elucidation by NMR and chemical methods of the unique structure of betanidin, the aglycon of the red-violet beet pigment betanin, forty years ago at the University of Zürich, Switzerland, was the beginning of my plant chemistry research program. Many of the same chemical and spectral techniques developed in Zürich have been used at The University of Texas at Austin for the structure analysis of members of many other classes of natural products including especially flavonoids, terpenoids, and alkaloids. Investigations at UT-Austin have concerned many topics such as biochemical and molecular systematics, biosynthetic pathways, structure-activity relationships, and the medicinal importance of natural products and included studies of antiviral proteins in the genus Phytolacca and neurotoxic nonprotein amino acids from cycads and other sources. Following the betalain story and an account of the early development of my UT-Austin biochemical systematic program, the Phytolacca and neurotoxin investigations are discussed herein.

THE CHEMISTRY OF THE AMINOCHROMES. PART VIII: THE PREPARATION AND PROPERTIES OF 4- AND 7-METHYLADRENOCHROME

Two new catecholamines, 2- and 5-methyladrenaline, have been prepared from 3-methylcatechol and they gave 4- and 7-methyladrenochrome, respectively, on oxidation. 7-Iodo-4-methyladrenochrome was obtained on oxidation of 2-methyladrenaline with potassium iodate; however, an iodoaminochrome could not be obtained in a similar manner from 5-methyladrenaline. The expected 5,6-dihydroxyindole derivatives were obtained on reduction of these aminochromes.