Polysaccharides uniformity

Sephadex. Other carbohydrate matrices such as Sephadex (based on dextran) have more uniform particle sizes. Their advantages over the celluloses include faster and more reproducible flow rates and they can be used directly without removal of fines . Sephadex, which can also be obtained in a variety of ion-exchange forms (see Table 15) consists of beads of a cross-linked dextran gel which swells in water and aqueous salt solutions. The smaller the bead size, the higher the resolution that is possible but the slower the flow rate. Typical applications of Sephadex gels are the fractionation of mixtures of polypeptides, proteins, nucleic acids, polysaccharides and for desalting solutions. 

The main function of the foam stabilising agent is to reinforce the intercellular film wall by contributing rheological characteristics of viscoelasticity. The increased viscosity may also assist handling. The aim, as so often with auxiliaries, is to achieve an optimum balance. If the bubbles are too thin and wet too quickly they will collapse prematurely, whilst too stable a film could hinder uniform application. Examples of products used as foam stabilisers include thickening agents such as the polysaccharides, hydroxyethylcellulose, methylcellulose,... 

On treatment with periodatd ion, straw and beechwood xylan oxidize rapidly.104 As with other polysaccharides, the oxidation comes to a more definite end point in solutions which are buffered to pH 4-5. While somewhat more than the theoretical amount of periodate ion is consumed, the reaction apparently proceeds uniformly with oxidative cleavage of the 2,3 carbon bond to produce the structure indicated in Figure 4. In the course of the reaction the xylan passes into solution. The optical rotation of the oxidized product is surprisingly high (co. 100°). 

The important feature of many polymers is simultaneous presence of distributions in two and several molecular characteristics. Polymers exhibiting multiple distributions are called the complex polymers or complex polymer systems. A detailed discussion of molecular characteristics of polymers and their average values and distributions can be found in numerous monographs and reviews, for example [34,35]. For the present purpose, it is important to repeat that all synthetic polymers and also polysaccharides are polydisperse in their nature. Only mother nature is able to produce macromolecules, for example many proteins, with uniform molar mass. The latter are often improperly called monodisperse(d) polymers. 

Finally, collagen can form a variety of collagen composites with other water-soluble materials. Ions, peptides, proteins, and polysaccharides can all be uniformly incorporated into a collagen matrix. The methods of composite formation include ionic and covalent bonding, entrapment, entanglement, and co-precipitation. A two-phase composite can be formed between collagen, ceramics, and synthetic polymers for specific biomedical applications. 

Semisolid systems fulfill a special topical need by being able to cling to the surface of application. Such systems are plastic in behavior, which allows semisolids to be mechanically spread uniformly over a surface as an immobile film. For the production of lipid-free ointments, pastes, and creams, several gel-forming polysaccharides are being used. As an emulsifier they can provide a three-dimensional matrix which... 

Thus, polysaccharide molecules with sugar units uniformly 1 — 4 or 1 3 linked seem to produce structures which can align to form strong... 

In reverse to solubilization, soluble polysaccharides are made less soluble by removing branches or substituents to produce a more uniformly linear polysaccharide with improved possibility of intermolecular fit, by removing formal charges, by lessening the number of strong hydration sites, or by completely overcoming hydration effects by introducing hydrophobic substituents. 

The distribution of substituents and branches in polysaccharides may be random or uniform. Distributions are isotactic when the substituents are all on the same side of the main axis, syndiotactic when they alternate on either side, and atactic when they are located at random. Regularity in the first two tacticities is conducive to crystallization (Sperling, 1986). Positional isomerism can lead to dissimilar properties, e.g., sulfate in carrageenans influencing their ability to gel (Anderson et al., 1968 Guiseley et al., 1980). 

The molecular weight (M) of a polysaccharide is the gram mole or molar mass of 6.023 X 1023 molecules (Avogadro s number) that ideally are of a single size. Extracted, isolated, and purified polysaccharides in the same class, from the same sampling source, are seldom uniform in shape and size and are therefore preferably characterized by an average molecular weight M. 

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