Anthocyanidin chemical structure

The basic chemical structure of anthocyanidins (aglycone) is shown in Figure 5.1. Over 600 naturally occurring anthocyanins have been... 

As it can be deduced from the description of the chemical structures of anthocyans and anthocyanidins, polyphenolic pigments are usually water soluble (Figure 1). Noticeable exceptions are the curcumin oils (yellow),... 

The most important members of the flavonoid family include anthocyanidins (e.g., cyanidin, delphinidin, malvidin), flavonols (e.g., quercetin, kaempferol), flavones (e.g., luteolin, apigenin), flavanones (e.g., myricetin, naringin, hesperetin, naringenin), flavan-3-ols (e.g., catechin, epicatechin, gallocatechin) and, although sometimes classified separately, the isoflavones (e.g., genistein, daidzein). For chemical structures see Figure 1. All these phytochemical are frequently referred to as bioflavonoids due to well established effects in human health maintenance. 

It should always be borne in mind that sample absorption refers to a sum value for all constituents absorbing light at the test assay wave length (520-540 nm), irrespectively of their chemical structure. In a berry extract it is for example impossible to differentiate between anthocyanidins present per-se (indicator for raw material quality) and anthocyanidins liberated from anthocyanins (indicator for destructive extraction/concentration techniques). 

Anthocyanins are glycosides of glucose, galactose, rhamnose, arabinose or other monosaccharides of flavylinm cation derivatives. Up to now, more than 500 different anthocyanins were reported [Castaneda-Ovando et al., 2009]. Their stmctnre mainly differs by the number and the position of hydroxyl groups and the nature and number of bonded saccharides. The aglycons of anthoeyanins, the anthocyanidins, are however limited to a few chemical structures. So far, only 23 anthocyanidins were identified of which only 6 are... 

Fig. 18.4. Chemical structures of flavonoids Flavanols (Faol), Anthocyanidines (Acn), Flavanones (Faon), Flavones (Fon), Flavonols (Fool), Isoflavones (Ifon, cf. 16.2.9). R H, OH or OCH3...

FIGURE 17.4 Chemical structure of main types of phenolic compounds associated with cereal grains, (a) Cinnamic acid, (b) Benzoic acid, (c) Ferulic acid, (d) Anthocyanidin. (e) Condensed tannins. 

Among the plant phenols, the flavonoids and the anthocyanidins, belonging to the 1,3-diphenylpropans, have been studied in most detail, mainly because of their potential health benefits. With more than 4,000 different flavonoids known, systematic studies of the effects of variation in molecular structure on physico-chemical properties of importance for antioxidative effects have also been possible (Jovanovic et al, 1994 Seeram and Nair, 2002). Flavonoids were originally found not to behave as efficiently as the classic phenolic antioxidants like a-tocopherol and synthetic phenolic antioxidants in donating... 

Chemically, anthocyanins are glycosides of anthocyanidins and are based on a 2-phenylbenzopyrilium structure. The properties of the anthocyanins depend on the anthocyanidins from which they originate. 

Introduction to 2-D NMR experiments The purpose of the standard 1-D H NMR experiment is to achieve structure-related information about sample protons (i.e., chemical shifts, spin-spin couplings, and integration data) describing the relative number of protons. Applied to anthocyanins, this information may help to identify the aglycone (anthocyanidin), number of monosaccharides present, and anomeric configuration of the monosaccharides. However, for most anthocyanins, the information gained by a standard 1 -D H NMR experiment is insufficient for complete structure elucidation. In recent years, various 2-D NMR experiments have evolved as the most powerful tools for complete structure elucidation of anthocyanins. 

Condensed tannins, on the other hand, occur in the bark of all conifers and hardwoods examined to date, and they are frequently present in the wood. They are primarily responsible for the tan to brown color of wood after it is exposed to air. In their purest form, condensed tannins are colorless, but they become colored very readily once isolated because of their propensity to oxidize to quinones. The primary characteristic of the water-soluble condensed tannins (4) is dehydration/oxidation to intensely colored anthocyanidin pigments (5) when refluxed in butanol and hydrochloric acid (Figure 2). For this reason, there has been a tendency to refer to these compounds as proanthocyanidins in the last few years. Prior to that, they were referred to as leucoanthocyanidins (i.e., the colorless chemical form of anthocyanidins). All references earlier than the late 1950 s, when the structure of these substances was just beginning to be understood, used the term condensed tannin. 

It has been noted that the chemical diversity of plant phenolics is as vast as the plant diversity itself. Most plant phenolics are derived directly from the shikimic acid (simple benzoic acids), shikimate (phenylpropanoid) pathway, or a combination of shikimate and acetate (phenylpropanoid-acetate) pathways. Products of each of these pathways undergo additional structural elaborations that result in a vast array of plant phenolics such as simple benzoic acid and ciimamic acid derivatives, monolig-nols, lignans and lignin, phenylpropenes, coumarins, stilbenes, flavonoids, anthocyanidins, and isollavonoids. 

On Figure 8, the most commonly occurring anthocyanidin structures are shown together with the basic chemical properties. On Figure 9 the most commonly occurring mono-saccharides bound to the anthocyanidin skeleton at position 3 and/or 5 are presented together with their basic chemical properties. 

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