Disaccharide occurrence

Among the disaccharides with more limited occurrence in anthocyanins, lathy-rose was found in dark red berries of Fatsia japonica and linked to one of the major anthocyanins in black carrots. " Until now, laminariobiose was only detected in red onions. A very rare disaccharide 2"- 3-glucopyranosyl-P-galactopyranose was found in anthocyanins from dwarf dogwood. ... 

Carbohydrates are diverse with respect to occurrence and size. Familiar mono and disaccharides include glucose, fructose, sucrose (table sugar), cellobiose, and mannose. Familiar polysaccharides are listed in Table 9.1 along with their source, purity, and molecular weight range. 

As regards the occurrence of /3-D-linked disaccharides in the absence of a /3-D-linked substrate, one is tempted to conclude that these oligosaccharides are synthesized by the enzymic reversion of D-glucose by a /3-D-glucosidase. As White and Maher100 have found that their honey-invertase preparation had no /3-D-glucosidase activity, it would appear that these sugars are carried into the hive as constituents of nectar. 

Keratan sulfate consists of the disaccharide, iV-acetyl-lactosamine, linked /3-(1 3) to the D-galactopyranose residue of the next iV-acetyl-lactosamine unit. Keratan sulfate is the most heterogeneous of the glycosaminoglycans, with variable sulfate content linked to C-4 or C-6 of the D-galactop)Tanose residue in lactosamine, and small amounts of L-fucose (6-deoxy-L-galactose), D-mannose, and iV-acetyl-neuraminic acid residues [93] (also see Sects. 10.2 and O 14 on the occurrence of iV-acetyl-D-neuraminic acid and L-fucose in other systems). Keratan sulfate is found in the cornea, on the surfaces of erythrocytes, in cartilage, and in bone. 

This chapter is exclusively devoted to the occurrence, significance and physical and randomly selected chemical properties of various group of disaccharides, trisaccharides, tetrasaccharides, pentasaccharides and hexasaccharides. 

In disaccharides one can also observe a slower secondary P-relaxation. Recently, it was shown that twisting rotation of the monosugar units around glycosidic bond is responsible for occurrence of this relaxation. This supposition was confirmed by theoretical conformational analysis. Additionally, it was proven that the activation energy provides direct information about structural rigidity of the examined disaccharides. 

A disaccharide unit with an a-galactose moiety, GaI(al-3)GalNAc has been discovered as an O-linked unit in the brains of the rat, rabbit, hen, frog and fish [31], It seems to be concentrated in nervous tissue, because it was not found in five other tissues of the rat. Other sources where the disaccharide has been discovered are the muscle and the adrenal medulla [32] and human teratocarcinoma cells [33]. Its enrichment in brain tissue together with its occurrence in roughly similar amounts in different animal species may suggest a role important for nervous tissue function. However, no information of its possible role or occurrence in specified glycoprotein molecules is yet available. 

New glycopeptide antibiotics has been isolated which contain a 4-oxovancos-amine (dehydrovancosamine) unit, which is largely hydrated (see Vol. 28, p. 257 for previous occurrence of this sugar). Reductive alkylation of the A 82846 family of glycopeptide antibiotics, which occurred selectively on the amino function of the disaccharide, gives increased antibiotic activity. ... 

Polymers of fructose are of widespread occurrence in plants, but are common only in a few orders, particularly the Compositae ind Graminae, They differ in several respects from prokaryotic fructans, especially in that they are intracellular rather than extracellular, that they are far smaller than their bacterial counterparts and that they contain relatively more glucose. Their function in plants is as storage polysaccharides and they are synthesised, ultimately from photosynthetic products. In bacteria such polymers are often assembled from exogenous, rather than endogenous disaccharides. 

Disaccharides having 2,6-dideoxyhexose units at the non-reducing termini are important because of their occurrence in antibiotics. The dlsaccharide unit (22) of mlthramycln has been shown by Koenigs Knorr synthesis and 2D-n.m.r. analysis to be the 3-1,3-linked compound. During this work the 1,4-linked isomers were also produced and the analogue (23)> the terminal disaccharide of the COjMt I---0 COjMc I---0 Me M<-... 

The kinetics of the thermal reactions of some disaccharides have been determined thermogravimetrically. The rate-determining step is the cleavage of the glycosidic bond, which is followed by the formation of anhydrohexopyranoses by elimination of water. The heats of crystallization of sucrose in the 30-60 C range vary between 11.6 and 19.5 kj mol k A recent study of the previously reported enhanced solubility of sucrose in water when excess solid phase is present has failed to confirm the claim. It was concluded that the previous measurement was of a transitory occurrence. ... 

Examination of the occurrence of neuraminic acid moieties in glycoproteins from pig submandibular and rat sublingual glands indicates that one in four neuraminic acid residues in the pig glycoprotein occurs as an a-iV-acylneur-aminyl-(2 8)-7V-acylneuraminyl unit. The disaccharide is absent in the corresponding rat glycoprotein. 

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