Carbohydrates aqueous solutions

Glycoproteins or mucoproieins. Compounds of proteins with carbohydrates. All glycoproteins contain a hexosamine and usually sulphate, ethanoate and glucuronic acid. The carbohydrate-protein linkages are, in some cases covalent and in others of the salt type. Aqueous solutions of glycoproteins are extremely viscous. 

Because six membered rings are normally less strained than five membered ones pyranose forms are usually present m greater amounts than furanose forms at equilib rium and the concentration of the open chain form is quite small The distribution of carbohydrates among their various hemiacetal forms has been examined by using H and NMR spectroscopy In aqueous solution for example d ribose is found to contain the various a and p furanose and pyranose forms m the amounts shown m Figure 25 5 The concentration of the open chain form at equilibrium is too small to measure directly Nevertheless it occupies a central position m that mterconversions of a and p anomers and furanose and pyranose forms take place by way of the open chain form as an inter mediate As will be seen later certain chemical reactions also proceed by way of the open chain form... 

It IS not possible to tell by inspection whether the a or p pyranose form of a par ticular carbohydrate predominates at equilibrium As just described the p pyranose form IS the major species present m an aqueous solution of d glucose whereas the a pyranose form predominates m a solution of d mannose (Problem 25 8) The relative abundance of a and p pyranose forms m solution depends on two factors The first is solvation of the anomeric hydroxyl group An equatorial OH is less crowded and better solvated by water than an axial one This effect stabilizes the p pyranose form m aqueous solution The other factor called the anomeric effect, involves an electronic interaction between the nng oxygen and the anomeric substituent and preferentially stabilizes the axial OH of the a pyranose form Because the two effects operate m different directions but are com parable m magnitude m aqueous solution the a pyranose form is more abundant for some carbohydrates and the p pyranose form for others... 

Although carbohydrates exist almost entirely as cyclic hemiacetals m aqueous solution they are m rapid equilibrium with their open chain forms and most of the reagents that react with simple aldehydes and ketones react m an analogous way with the carbonyl functional groups of carbohydrates... 

Characteristics due to chemical functionalities (e.g., carboxyl groups) of sample components that control solubility of the sample in aqueous media, viscosity of carbohydrate/polysaccharide solutions, and stability of obtained solutions. 

If the carbonyl and the hydroxyl group are in the same molecule, an intramolecular nucleophilic addition can take place, leading to the formation of a cyclic hemiacetal. Five- and six-membered cyclic hemiacetals are relatively strain-free and particularly stable, and many carbohydrates therefore exist in an equilibrium between open-chain and cyclic forms. Glucose, for instance, exists in aqueous solution primarily in the six-membered, pyranose form resulting from intramolecular nucleophilic addition of the -OH group at C5 to the Cl carbonyl group (Figure 25.4). The name pyranose is derived from pyran, the name of the unsaturated six-membered cyclic ether. 

Effects of Gamma-radiation on Some Carbohydrates, Hydroxy-acids, and Amino-acids in Aqueous Solutions, S. A. Barker, P. M. Grant, M. Stacey, and R. B. Ward, Nature, 183 (1959) 376-377. 

Kravtchenko T.P., Amould I., Voragen A.G.J. Pilnik W. (1992) Improvement of the selective depolymerization of pectic substances by chemical P-elimination in aqueous solution. Carbohydrate Polymers 19,237-242. 

Phillips, G.O. and Moody, G.J. (1960a). Radiation chemistry of carbohydrates. Part IV. The effect of gamma-radiation on aqueous solutions of sucrose. J. Chem. Soc. 155, 762-768. 

TMSIH Does not react with Mino groups. Can be used to form TMS derivatives of carbohydrates in aqueous solution. Does not promote enol-ether formation with unprotected ketone groups. Most generally useful reagent, preferred for most applications. Exceptions are the formation of N-TMS derivatives and the separation of low molecular weight TMS derivatives... 

A long-wavelength probe 29 signaling carbohydrates in aqueous solutions by increasing of fluorescence was developed by Akkaya and Kukre on the basis of a symmetrical squaraine dye containing two phenylboronic acid functions [89]. The emission maximum of this probe is at 645 nm. A maximal response of about 25% was found for fructose. 

Fig. 3.5 Representation of a scheme of an experiment (upper set of drawings) and the obtained experimental results presented as AFM images (middle part) and cross-sectional profiles (bottom) that provides evidence of silica nucleation and shell formation on biopolymer macromolecules. Scheme of experiment. This includes the following main steps. 1. Protection of the mica surface against silica precipitation. It was covered with a fatty (ara-chidic) acid monolayer transferred from a water substrate with the Langmuir-Blodgett technique. This made the mica surface hydrophobic because of the orientation of the acid molecules with their hydrocarbon chains pointing outwards. 2. Adsorption of carbohydrate macromolecules. Hydrophobically modified cationic hydroxyethylcellulose was adsorbed from an aqueous solution. Hydrocarbon chains of polysaccharide served as anchors to fix the biomacromolecules firmly onto the acid monolayer. 3. Surface treatment by silica precursor. The mica covered with an acid mono-...

Mucopolysaccharides (See section IV). In general these compounds have a low but significant protein content and give reactions which are predominantly carbohydrate. Their aqueous solutions show a high viscosity and they tend to become insoluble in water after intensive dehydration. 

Mucoproteins (See Section V).—Protein-carbohydrate compounds with relatively high protein or peptide content, the chemical reactions of which are predominantly protein, are known as mucoproteins. In general they do not coagulate on being heated in aqueous solution. All mucoproteins contain a hexosamine constituent. 

Addition of an aqueous solution of PEG to a saturated aqueous solution of a-CD at room temperature did not lead to complex formation unless the average molecular weight of PEG exceeded 200 [46]. Moreover, carbohydrate polymers such as dextran and pullulan failed to precipitate complexes with PEG, and the same was true for amylose, glucose, methyl glucose, maltose, maltotriose, cyclodextrin derivatives, such as glucosyl-a-CD and maltosyl-a-CD, and water-soluble polymers of a-CD crosslinked by epichlorohydrin. These facts suggested to Harada et al. the direction for further research. 

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