Polysaccharides oxidative depolymerization

A breaker an enzyme (at T<140°F), strong oxidizing agent, or an acid, is used to depolymerize polysaccharides and break crosslinks such that viscosity declines at a controlled rate so that the proppant may be deposited in the fracture. Too rapid proppant dropout would cause a premature "sand-out" which prevents future extension of the fracture. Peroxydisulfates are the most frequently used breakers. Less reactive organic peroxides may be preferred for high temperature formations (85). 

It is essential that the oxidation of the glycol groupings in the polysaccharide be complete, and that no overoxidation occurs. The latter is excluded by performing the reaction with the exclusion of light, at low temperature, and at a pH of 3-3.5, as described by Bobbitt.73 The use of oxygen-free water and the addition of propyl alcohol, as a radical scavenger, also lessen radical-induced depolymerization during oxidation.75... 

The methylation of a polysaccharide is usually accompanied by some depolymerization. In alkaline medium, polysaccharides are easily oxidized by atmospheric oxygen, and, even when the reactions are carried out under... 

Partial oxidations [104,105] of polysaccharides is commonly done, both during stmctural analysis and to modify their properties. Oxidation introduces both carbonyl and carboxylate functions at different positions, and especially in alkaline systems, can result in chain cleavage, i. e., depolymerization [108]. Oxidation using sodium hypochlorite by itself [109] or in combination with sodium bromide [110] is practiced in the starch industry, both to introduce specific properties into the product and for depol)merization. This oxidation is non-specific. 

For some polysaccharides as for example Nm group C [99] and H. Influenzae type b [100] the periodic oxidation occurs selectively at a position leading to fragmentation of the CPS. Depolymerized CPSs are further modified for activation. The following examples of Haemophilus influenzae type b CPS modification illustrate very well the broad spectrum of present technology [101] (see Scheme 2). 

Under certain conditions, referred to collectively as oxidative stress, antioxidant mechanisms are overwhelmed and some damage may occur. Damage results primarily from enzyme inactivation, polysaccharide depolymerization, DNA breakage, and membrane destruction. Examples of circumstances that may cause serious oxidative damage include certain metabolic abnormalities, the overconsumption of certain drugs or exposure to intense radiation, or repeated contact with certain environmental contaminants (e.g., tobacco smoke). 

However, when applied to polysaccharides, periodate oxidation seems in many cases to be accompanied by depolymerization, such as the case of alginates and chitosans [18, 19]. In turn, the exposed reducing ends consume additional periodate, thereby leading to over oxidation [20, 21]. 

There are multiple mechanisms of depolymerization, since many of the dialdehyde derivatives are highly susceptible to alkah-catalyzed -elimination, even at moderate temperatures and pH values. Moreover, the degradation of polysaccharide chains caused by free radicals may occur [22, 23]. Periodate itself is unstable and decomposes over time to form radicals, especially in the presence of light. Oxidations are therefore preferentially carried out in the dark and in the absence of oxygen, using freshly prepared periodate solutions. Also, small amounts of trace metals should be removed as they can also catalyse the formation of free radicals. It has been demonstrated that presence of free radical scavenges such as 1-propanol reduces the extent of depolymerization, and it has therefore been routinely added during oxidation, typically at concentrations around 10 % (v/v) [16]. 

A novel polysaccharide gel, which has been isolated from the fruit of Thauma-tococcus danielli, contains residues of L-arabinose, D-xylose, D-glucuronic acid and its 4-0-methyl ether in the ratio 1.0 7.2 1.91 0.66. All of the uronic acid residues were oxidized by periodate and, from the results of methyla-tion analysis, the structure was found to be highly branched. Many of the uronic acid residues were removed by the action of j3-D-glucuronidase but the gel was not completely depolymerized by this treatment. 

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