Pentoses arabinose

When pyranoses bear a substituent at position 5 (methyl or hydrox3miethyl), they are locked in a single conformation with the C-5 substituent equatorial and the anomeric configuration can be determined from H1-H2 coupling constants. The pentoses (arabinose and xylose), which do not bear a C-5 substituent, do not behave so well and they may exist as interconvertible chairs in the IC4 and Cj conformation depending on the substitution. 

In addition to proteins, polyphenols, and metallic ions, hazes contain 2-4% of glucose and traces of the pentoses, arabinose and xylose [162]. Occasionally, however, hazes are encountered which are essentially different from the protein-polyphenol hazes discussed so far. These include hazes due to microcrystals of calcium oxalate (the solubility of which is 6 07 mg/1 at 13°C) and hazes which are composed largely of carbohydrates. a-Glucans (dextrins) [170], p-glucans [171] and pentosans [172] have been found in beer sediments. 

Insulin induces a rapid drop of the blood concentration of glucose and galactose, but whereas the former is actively metabolized, the latter is not because muscle exhibits no appreciable galactokinase activity. When galactose is administered to eviscerated and nephrectomized animals, all the galactose can be recovered from the carcass in a free form. In contrast to galactose and glucose, other metabolizable hexoses or pentoses, such as fructose, mannose, and rhamnose, do not respond to the action of insulin in eviscerated and nephrectomized animals. Two nonmetabolizable pentoses—arabinose and xylose—are readily eliminated from the extracellular space under the influence of insulin. 

After completion of alcoholic fermentation, low concentrations of hexose sugars may remain in the wine. These include glucose and fructose with lesser amounts of mannose and galactose. Among the five-carbon sugars (pentoses), arabinose, ribose, and xylose are the most common. Further, there may be sufficient quantities of sugar to support the growth of lactic acid bacteria in dry wines. 

They are found in a great variety of plants and are strong fish poisons. On hydrolysis they yield a variety of sugars, frequently several molecules to each aglucone. Glucose, galactose, arabinose are the more common pentoses, methylpentoses and glucuronic acid are also obtained. 

Most free pentoses, hexoses, and heptoses occur primarily in less strained pyranose rings, but the furanose ring is also quite important. The furanose ring is formed in the same way as the pyranose ring and also occurs in a and P forms. This is demonstrated with L-arabinose, which is commonly found in polysaccharides in the form of a-L-arabinofuranosyl units (see Fig. 2). 

Detection and result The chromatogram was freed from mobile phase and immersed in the reagent solution for 1 s and then heated to 130°C for 10 min. Rhamnose (JiRf 35—40) and fructose hRf 70—75) yielded brown and xylose (JhRf 45 — 50) and arabinose (hRf 60—65) red-brown chromatogram zones on a pale background. The detection limit for the pentoses was 0.1 pg and for fructose it was 0.5 pg substance per chromatogram zone. 

Five pentoses, namely, D-ribose, d- and L-arabinose, and D- and L-xylose, have been found in hydrolyzates of bacterial polysaccharides. D-Riboseisthe most common of these, and is a component of different LPS, capsular polysaccharides, and teichoic acid type of polymers. In all these polymers, it occurs as the /I-furanosyl group or residue. 

Neher and Lewis177 obtained 2-furaldehyde from 2,3,4-tri-O-methyl-L-arabinose by heating with dilute acid after preliminary enolization with alkali. Isbell83 proposed a mechanism for this conversion similar to that for the conversion of tetra-0-methyl-(2-hydroxy-D-glucal) into 5-(hydroxymethyl)-2-furaldehyde XLIV was suggested as an intermediate. In Hurd and Isenhour s178 scheme for the formation of 2-furaldehyde from free pentose, the enol (XLV) of a 3-deoxypentosone was regarded as an inter-... 

Challenger and coworkers36 had hoped to obtain pyromeconic acid (XXVI) from pentoses (XXV) with A. oryzae, by analogy with the formation of kojic acid from hexoses. Instead, kojic acid was produced from L-arabinose and D-xylose. Corbellini and Gregorini,31 too, observed the... 

In pentosuria L-xylulose is excreted in urine, the daily amounts varying from 1 to 4 g (H8). The urinary excretion has to be distinguished from alimentary pentosuria in which other pentoses such as arabinose, xylose, or—to a lesser extent—D-ribose are found in the urine. However, traces of L-xylulose have been found in the urine of healthy individuals (cf. Section 5.4). 

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