Polysaccharide primary structure

The polysaccharide primary structure affords numerous opportunities for hydroxyl- and carboxyl-group interactions, leading to polymorphism (al-... 

The plant cell wall contains different types of polysaccharides, proteins (structural glycoproteins and enzymes), lignin and water, as well as some inorganic components (1, 14-16). The plant cell suspensions, however, grow as a population of cells with a primary cell wall(17). The main components of these walls are cellulose-free polysaccharides and pectic polysaccharides in particular, which constitute 1/3 of their dry weight. (18). Some fragments, e g. methanol, acetic, ferulic and p-cumaric acids, are connected with the pectic polysaccharides by ester bonds with the carboxylic and hydroxylic groups. 

Cellulose is the primary structural component of the cell wall and, after removal of lignin and various other extractives, it is also the primary structural component of paper. Chemically, it is a semicrystalline microfibrillar linear polysaccharide of /M,4-linked d-glucopyranose (Figure 2.6). 

The primary structural component of paper is cellulose but non-structural polysaccharides (hemicelluloses) and sometimes lignin may also be present in paper. The physical and mechanical properties of a sheet are, however, in large measure due to the cellulosic fibres. 

We generally describe the structure of both synthetic and natural polymers in terms of four levels of structure primary, secondary, tertiary, and quaternary. The primary structure describes the precise sequence of the individual atoms that compose the polymer chain. For polymers that have only an average structure, such as proteins, polysaccharides, and nucleic acids, a representative chain structure is often given. 

The distinctions between these homopolymers arise from the different ways in which the monomer units are hooked together in polyacetal chains. Starch (qv), plant nutrient material, is composed of two polysaccharides a-amylose and amylopectin. a-Amylose is linear because of exclusive a (1 — 4) linkages, whereas amylopectin is branched because of the presence of a (1 — 6) as well as a (1 — 4) links. The terms linear and branched refer only to primary structure. 

Fig. 1. Primary structures of some common polysaccharides, (a) Alpha-glycoside linkages characterize amylose, amylopectin, and glycogen (b) cellulose has...

All of these techniques have greatly advanced the elucidation of the primary structures of extracted, purified polysaccharides, and the major features of wall structure in higher plants are now recognized. Nonetheless, the overall complexity of these structures, and the technical problems associated with their study, suggest that a complete picture of the cell wall will not be available in the near future. 

From glucose alone various organisms synthesize a whole series of polymeric glucans with quite different properties. Of these, cellulose, an unbranched (1-1,4-linked polyglucose (Fig. 4-5A), is probably the most abundant. It is the primary structural polysaccharide of the cell walls of most green plants.65 For the whole Earth, plants produce 1014 kg of cellulose per year. 

Unlike polysaccharides, proteins do not have branched chains, but several chains may be linked together via disulphide bridges rather than peptide bonds. The primary structure of ox insulin is shown in Fig. S.A2. The protein consists of two peptide chains which are linked via the formation of the disulphide bridges. Disulphide bridges are formed by the condensation of the thiol groups of two cysteine residues. 

It is a remarkable fact that the main energy-storage polysaccharides and the main structural polysaccharides found in nature both have a primary structure of (l,4)-linked polyglucose. Why should two such closely related compounds be used in totally different roles A closer look at the stereochemistry of the a and /3 glycosidic linkage for polyglucose indicates why this is so. 

The primary structure of the extracellular polysaccharide from X. aampestris was recently reinvestigated by Jansson et al. (2), and by Melton et al. (8). The chemical repeat unit is a pentasaccharide. The backbone is a 8-1,4 glucan like cellulose and chitin (9,10) with a trisaccharide side-chain on each cellobiose unit (I). 

The primary structure of the Klebsiella K8 capsular polysaccharide was established by Sutherland (3). It is a poly(tetrasaccharide) with three sugar residues in the backbone and a monosaccharide side-chain (II). 

Linearity of the polysaccharide primary chains facilitates parallel ordering of the tertiary structures in two (e.g., 0 sheets of cellulose) and three... 

Polysaccharides are differentially stable to cold dilute alkali, which facilitates classification as alkali-stable and alkali-sensitive (Reeves and Blouin, 1957). High concentrations of alkali swell fibrils, shift conformation, and depolymerize the primary structure. The 1,4-(3 bonds are more alkali-resistant than the 1,4-a bonds. Nonreducing glycosides are more stable than reducing glycosides. Ester and sulfate groups are easily hydrolyzed. 

Polysaccharide functionality is a variant of the nature and distribution of the substituents. A polar substituent makes a polysaccharide less hydro-phobic, i.e., more hydrophilic, and vice versa. A uniform distribution of -CH2COO- on the cellulose primary structure results in smooth, nongrainy nonthixotropic sols. The probability of nonuniform substitution increases at a low level of substitution of cellulose-containing crystallites, because the reagent is unable to penetrate the crystallite, regions (DuPont). 

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