Starch, complexes alkalis

It will be noted that this type of complex-formation is entirely different from that in which complexes are formed between amylose and certain polar, organic compounds. In contrast to the precipitates of the latter complexes (which are of a distinct, crystalline appearance), the starch-alkaline-earth hydroxide complexes are amorphous, curdlike flocculates. These complexes di,s.sociate on diluting them with water, and the starch redissolves. According to the patent description, the amylose complexes dissolve much more easily than the amylopectin complexes hence, fractionation must occur if water is added stepwise. Likewise, fractionation will take place if the starch complexes are partially neutralized, by the gradual addition of an acid. For obvious reasons, such acids as carbonic acid and sulfuric acid (which give insoluble calcium salts) are preferred. Furthermore, it is claimed that gradual addition of caustic alkali to a starch solu-... 

The limited solubility of starch and its modified products may affect the reversibility of many reactions. This may explain several, apparently unusual, reactions reported in starch chemistry. There are, for example, reports of starch esterification with sodium hydrogenphosphates, acylation of starch with acyl amides (which is equivalent to the transformation of an amide into an ester), and the formation of alkali-metal starchates upon treatment of starch with alkali (a reaction which fails for simple alcohols). A specific property of starch is its ability to form surface sorption and helical inclusion-complexes with many inorganic and organic guest molecules.4... 

Heating starch with sulfur and alkali for 12 h resulted in a product described as thiostarch, but its structure remains unknown.2667 The structure is probably a starch complex of low molecular weight, for example, a thiaheterocyclic compound, as suggested by the other authors2668,2669 who observed formation of these compounds... 

Polycrystalline and well-oriented specimens of pure amylose have been trapped both in the A- and B-forms of starch, and their diffraction patterns84-85 are suitable for detailed structure analysis. Further, amylose can be regenerated in the presence of solvents or complexed with such molecules as alcohols, fatty acids, and iodine the molecular structures and crystalline arrangements in these materials are classified under V-amylose. When amylose complexes with alkali or such salts as KBr, the resulting structures86 are surprisingly far from those of V-amyloses. 

It should be noted that the different structures of amylose and amylopectin confer distinctive properties to these polysaccharides (Table II). The linear nature of amylose is responsible for its ability to form complexes with fatty acids, low-molecular-weight alcohols, and iodine these complexes are called clathrates or helical inclusion compounds. This property is the basis for the separation of amylose from amylopectin when starch is solubilized with alkali or with dimethylsulfoxide, amylose can be precipitated by adding 1-butanol and amylopectin remains in solution. 

A general method for synthesizing starch-metal complexes with Ba, Ca, Sr, Be, Mg, Zn, Al, Fe, and Cu is given by a 1926 British patent,522 wherein alkali starches were treated with one or more metal salts. The cupric complex was reported to have disinfecting properties. 

Whereas all of the methods proposed for large-scale fractionation of starch that have been discussed depend directly on the ability of amylose to form itLsoluble complexes with polar organic compounds. Cantor and Wimmer s process is based on a totally different principle. If a molecularly disperse solution of starch contains a sufficient amount of calcium chloride and caustic alkali is added, a rapid and quantitative precipitation of the starch occurs, because of the formation of complexes (of calcium hydroxide with the starch polysaccharides) which are insoluble in an aqueous, saturated solution of calcium hydroxide. The same phenomenon is observed with the hydroxides of barium and strontium. 

Other starch varieties chemically bind metal atoms only to the hydroxyl groups, in the same manner as alcoholates. Alkali metal starchates were obtained for the first time by the use of either metal amides in liquid ammonia641 or metals in liquid amines.642 These methods were subsequently used by others.643 Sodium 2-starchate could be prepared by refluxing an alcohol solution of NaOH and dry starch.644 The reaction of dry starch with controlled amount of NaOH in 1-butanol provided either monosodium 2-starchate or disodium 2,3-starchate.645,646 However, complexes of NaOH with starch were also characterized.37,38 Thallation of starch could be performed with thallous hydroxide.647... 

Weaver et al. [34] disclosed absorbent polymers prepared in a multi step process. First, a graft copolymer was formed from acrylonitrile and starch. The starch was gelatinized in hot water, followed by a complexation step with Ce + Then acrylonitrile was added to form the graft copolymer. The graft copolymer was isolated, and then saponified with a certain concentration of aqueous alkali such that a soluble, hydrolyzed graft copolymer was obtained. No alcohol was used in the hydrolysis step. Drying this polymer yielded a water Insoluble polymer that absorbed hundreds of times its dry weight in water. 

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