Arabinogalactans

Arabinogalactans form a family of polysaccharides with a branched framework made up predominately of (l- 3)-linked jff-D-galactopyranosyl units with varying amounts of (1 6)-linked )ff-D-galactopyranosyl units. Their structures are often complex, with an apparent random distribution of side chains in at least some. They often occur as a mixture of components that differ widely in both molecular weight and chemical structure. 

All parts of higher plants (trunks, stems, roots, leaves, seeds, flowers) contain arabinogalactans and arabinogalactan-proteins, in some cases in considerable amounts. Their role in construction of the primary cell wall has been discussed (3,4,111). 

R = one of jSGalp(1-, LAraf(1-, iSLArap(1- 3)LAraf(1-, iSGIcpA(1-Fig. 4.1. The main structural features of larchwood arabinogalactan 

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Larch Gum. Larch gum [37320-79-9] (larch arabinogalactan) is obtained by water extraction of the western larch tree, iLarix occidentalism the heartwood of which contains 5—35% on a dry wood basis. In the early 1960s, a countercurrent hot water extraction system was developed, and the gum was produced commercially by the St. Regis Paper Co. under the trade name Stractan. The potential production capacity of this gum is 10,000 t/yr based on the wood residues from the lumber industry. However, the product could not compete with gum arabic, and commercial production is now limited to small batches for a specific medical appHcation. 

StmcturaHy, arabinogalactan is a complex, highly branched polymer of arabinose and galactose in a 1 6 ratio (67). It is composed of one fraction with an average molecular weight of 16,000, and one of 100,000 (68). 

Larch arabinogalactan is approved in 21 CFR 172.610 as a food additive for use as an emulsifier, stabilizer, binder or bodying agent for essential oils and noimutritive sweeteners, flavor bases, nonstandardized dressings, and pudding mixes. It has also been used in the preparation of cosmetic and pharmaceutical dispersions and as an emulsifier in oil—water emulsions (69). Industrially, the main use has been in Hthography as a gum arabic substitute. 

The common hemiceUulose components of arborescent plants are listed in Table 3. Xylans, arabinogalactans, and pectic substances are common to all while only traces (if at all) of glucomaimans are found in the cell walls of bamboo. Other polysaccharides are found in trace amounts in wood as well as in bark, growing tissues, and other specialized parts of trees. 

Within the scope of this review, the contributions of the last decade concerning cell-wall polysaccharides isolated from woody and other plant tissues will be reviewed according to the above-proposed classification of hemicelluloses including larch arabinogalactans. The present review article updates and extends previous reviews [3-5] and will focus in particular on new investigated plant sources, isolation methods, structural features, physicochemical and various functional properties of hemicelluloses. Attention will also be paid to the modification of isolated hemicelluloses or hemicellulosic materials and the appHcation possibiUties of hemicelluloses and their derivatives, including their use for the production of composite materials and other biomaterials. 

Arabinogalactans (AGs) are widely spread throughout the plant kingdom. Many edible and inedible plants are rich sources of these polysaccharides. AGs occur in two structurally different forms described as type I and type II, associated with the pectin cell-wall component by physical bonds and some of them are covalently linked to the complex pectin molecule as neutral side chains. Commercial pectins always contain AG 10-15%). AG of type I has a linear (1 4)-y0-o-Galp backbone, bearing 20-40% of of-L-Ara/ residues (1 5)-linked in short chains, in general at position 3. It is commonly found in pectins from citrus, apple and potato [6]. Recently, this AG type has been isolated from the skin of Opuntia ficus indica pear fruits [372]. 

Keywords Arabinogalactans Bioactivity Medicinal plants Rhamnogalacturonans Pectins Structure-activity relations... 

The arabinogalactans have more frequently been reported for activity in various biological systems. Arabinogalactans are often classified in three groups arabino-4-galactans (Type 1), arabino-3,6-galactans (Type II) and polysaccharides with arabinogalactan side chains (Type III) [14]. The latter type are also called the real pectins [10,11]. Only types I and II will be dealt with in this chapter, as Type III are equal to the pectins discussed below. 

Fig. 2 Average structure of the hairy or ramified region of an apectic substance, with a rhamnogalacturonan I backbone substituted at position 4 of the rhamnose units with arabinan and arabinogalactan type II side chains...

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