Tannins

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The tannins are common to vascular plants existing primarily within woody tissues. Tannins consist of various phenolic compounds that react with proteins to form water-insoluble copolymers. This reaction with proteins has been used industrially for the conversion of animal skins into leather. Plant tissues that are high in tannin content have a highly bitter taste and are avoided by most feeders. Tannins may be either condensed or hydrolyzable. Condensed tannins are formed biosynthetically by the condensation of catechins to form polymeric networks. Hydrolyzable tannins are derived from gallic acid.

Arabic Letter Work Sheet
FIGURE 1.21 Common classes of flavonoids. 1.3.2.6 Quinones

The quinones typically form strongly colored pigments covering the entire visible spectrum. Typically, however, they are found in the internal regions of the plant and thus do not impart a color to the exterior of the plant. Generally, quinones are derived from benzoquinone, naphthoquinone, or anthroquinone structures (Figure 1.22).

FIGURE 1.22 Quinones.

A wide variety of phenolic natural products exist in plants many of which are of medicinal interest. A selection of these compounds is shown in Figure 1.23.

FIGURE 1.23 Phenols.

FIGURE 1.23 Phenols.

Many of these phenols come from familiar sources. Santalins A and B are the major pigments of red sandalwood (Pterocarpus santalinus). The flowers of the hawthorne tree provide hyperoside, one of the principle flavonoids from this source (Crataegus laevigata). Neohesperidin is responsible for the bitter taste of orange peels (Citrus aurantium), while quercitin is the active ingredient of birch (Betula pendula). Silybin, one of the silymarins, is a mixture of various flavanone derivatives (flavonolignans) and present in the fruit of the milk thistle (Silybum marianum). Silymarin is the active antihepatotoxic complex used for treatment

FIGURE 1.23 (continued)

of liver damage and increases the rate of synthesis of ribosomal ribonucleic acids. Centapicrin is an ultrabitter (bitterness value ca. 4,000,000) secoiridoid glycoside from the century plant (Centaurea erythraea). The circuminoids are responsible for the yellow pigment and cholagogic properties of turmeric (Curcuma domestica). Hypericin, a flavonoid from St. John's wort (Hypericum per-foratum), is a monoamine oxidase inhibitor. Emetine and cephaeline are the active ingredients of syrup of ipecac, powerful emetics from ipecacuanha (Ceph-aelis ipecacuanha). The isoflavones genistein and daidzein are found in high concentrations in soybeans (Glycine max) as well as several other legumes. Both genistein and daidzein have been found to have anticancer activity.

1.4 CARBOHYDRATES

Sugars or carbohydrates are the primary products of photosynthesis and are essential as a source of energy to plants. They are stored as starch or fructans, used as sucrose, and polymerized to form cellulose, the main cell wall structural material of plants. Finally, they combine to form glycosides of many fundamen tal groups of natural products as we have already seen, including terpenes (to form saponins), phenols, and alkaloids.

Sugars are optically active aliphatic polyhydroxlyated compounds which are readily water soluble. This is due to the hydrophilic nature of the hydroxyl functionality, and does not involve the salt formation that we have observed for the phenolics and alkaloids. The common monosaccharides are shown in Figure 1.24. The sugars are classified into three groups depending on their size: monosaccharides, such as glucose; oligosaccharides, including sucrose; and polysaccharides, which include cellulose.

FIGURE 1.24 Monosaccharide types.

1.4.1 Monosaccharides

The most common monosaccharides are glucose and fructose. Less common are xylose, mannose, rhamnose, and galactose, as well as numerous others. In plants these sugars are normally bound as oligo- and polysaccharides or as various glycosides. The individual sugars may be physically separated either enzymat-ically or by treatment with acid.

1.4.2 Oligosaccharides

The oligosaccharides normally include from two to five saccharide (or sugar) units. These are joined by any of three possible ether linkages that can complicate structure elucidation. Common oligosaccharides include sucrose, trehalose, stachyose, and raffinose.

1.4.3 Polysaccharides

Most of the carbohydrates found in plants occur as polysaccharides of high molecular weight. The polysaccharides (or glycans) fulfill a wide variety of functions in plants. Cellulose serves as a structural material, whereas in animals keratin and collagen serve similar structural roles. Cellulose is the most abundant organic compound in plants and the most abundant single polymer in the biosphere. A simple straight-chain polymer without branching is formed using P-(1,4) ether linkages and forms the main structural polysaccharides of the cell wall. Amylose, which is used as a storage rather than a structural glucan, uses a-(1,4) linkages. Amylopectin uses a-(1,4) and a-(1,6) linkages. The linkages of cellulose form straight ribbons that line up side by side forming polymers of high mechanical strength and limited extensibility. Other structural polysaccha-rides include the polygalacturonans (pectic polysaccharides), xylans, glucoman-nans, chitins, and the glycosaminoglycans.

More complex cell-wall polysaccharides act as recognition signals where the saccharide sequence of these heteropolysaccharides is informational, not unlike the nucleic acids. The structures of several of these polysaccharides are shown in Figure 1.25.

FIGURE 1.25 Common polysaccharide linkages.

FIGURE 1.25 Common polysaccharide linkages.

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Good Carb Diet

Good Carb Diet

WHAT IT IS A three-phase plan that has been likened to the low-carbohydrate Atkins program because during the first two weeks, South Beach eliminates most carbs, including bread, pasta, potatoes, fruit and most dairy products. In PHASE 2, healthy carbs, including most fruits, whole grains and dairy products are gradually reintroduced, but processed carbs such as bagels, cookies, cornflakes, regular pasta and rice cakes remain on the list of foods to avoid or eat rarely.

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