Ketose test

Fig. 2.2.6.1 Ketoses tested as acceptor substrates of recombinant sucrose synthase 1 expressed in yeast.

The presence of D-fructose does not interfere with the isolation of aldo-benzimidazoles by the above method. It has no effect on the potassium salt fraction but it will precipitate with the barium salts, and later, by degradation, may give rise to small amounts of D-arabo-ben-zimidazole. Therefore, say Moore and Link,16 in the examination of natural products the isolation of D-arabo-benzimidazole from the barium salt fraction, rather than from the potassium salt fraction as expected, is to be regarded with suspicion, for it may have been formed from D-fructose instead of D-arabinose. They suggest in questionable cases that pentose and ketose tests then be tried on the original sample as an aid in solving the problem. 

Distinguish between glucose and fructose by (1) the Ketose Test, (2) the Furfural Test, and (3) the polarimeter. 

Aldoses are reducing sugars because they possess an aldehyde function m then-open chain form Ketoses are also reducing sugars Under the conditions of the test ketoses equilibrate with aldoses by way of enediol intermediates and the aldoses are oxidized by the reagent... 

Although the Tollens reaction is a useful test for reducing sugars, it doesn t give good yields of aldonic acid products because the alkaline conditions cause decomposition of the carbohydrate. For preparative purposes, a buffered solution of aqueous Br2 is a better oxidant. The reaction is specific for aldoses ketoses are not oxidized by aqueous Br2. 

Seliwanoff s test analy chem A color test helpful in the identification of ketoses, which develop a red color with resorcinol in hydrochloric acid. s3 liv3,nofs, test sellite liNORG chem A solution of sodium sulfite (Na2S03) used in the purification of 2,4,6-trinitrotoluene to remove unsymmetrical isomers. se,lTt ... 

The oxidation of an aldose or ketose by silver ions results in the reduction of the silver ion to silver metal. The silver is normally present as Ag (NH3)20H . A positive test has a coating of silver metal precipitating on the sides of the container as a mirror. 

The ketoses D-tagatose 6 (43% relative activity) and D-ribulose 28 (24% relative activity) were identified as new acceptor substrates they are not accepted by recombinant SuSyl from yeast. However the acceptance for D-xylulose 5 is lost. The most significant changes were observed for the aldoses tested L-arabinose 14, D-xylose 12, and D-lyxose 11 are better substrates than D-fructose, with relative activities of 490%, 300%, and 151%, respectively. In the hexose series L-glucose 29 and L-rhamnose 30 were identified as new acceptor substrates, whereas the acceptance for D-and L-mannose, 16 and 21, was improved (Fig. 2.2.6.5) (Sauerzapfe and Elling, unpublished results). 

Problem 22.7 (a) Explain how in basic solution an equilibrium is established between an aldose, its C epimer (a diastereomer with a different configuration at one chiral C) and a 2-ketose. (b) Will fructose give a positive Fehling s test which is done in a basic solution <4... 

Therefore, glucose (open chain is an aldose) and fructose (open chain is a ketose) give positive test and are reducing sugars. 

Anhydrides of ketoses have not been tested for polymerizability, but two monomeric anhydrides of D-fructose have been reported, namely, 2,3-anhydro-l,4,6-tri-0-nitro-D-fructofuranose71 and 2,6-anhy-dro-l-O-methyl-D-fructofuranose.72 The structures of both suggest that they have sufficient strain to be of interest. The first is the product of a... 

There are a number of well-known tests for carbohydrates which depend on color formation. Thus in the Molisch test the substance is treated with mineral acid in the presence of a-naphthol and a characteristic purple color is formed. In Seliwanoff s test only ketoses give an immediate red color when warmed with mineral acid and resorcinol, and similarly ketoses give a blue coloration when warmed with sulfuric acid and diphenylamine in the Ihl-Pechmann reaction. 

Tollens test cannot distinguish between aldoses and ketoses because the basic Tollens reagent promotes enediol rearrangements. Under basic conditions, the open-chain form of a ketose can isomerize to an aldose, which reacts to give a positive Tollens test. 

Any sugar that gives a positive Tollens test. Both ketoses and aldoses (in their hemiacetal forms) give positive Tollens tests, (p. 1119)... 

A test for reducing sugars, employing the same silver-ammonia complex used as a test for aldehydes. A positive test gives a silver precipitate, often in the form of a silver mirror. Tollens reagent is basic, and it promotes enediol rearrangements that interconvert ketoses and aldoses. Therefore, both aldoses and ketoses give positive Tollens tests if they are in their hemiacetal forms, in equilibrium with open-chain carbonyl structures, (p. 1118)... 

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