Aldose sugars open-chain forms

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... 

In the case of the oximes of the aldose sugars, the situation is more complicated because of the possibility of both open-chain and cyclic structures. That aldose oximes can react in the open-chain form follows from the formation of the nitriles and from the isolation of acylated open-chain aldose oximes as secondary products in preparation of nitriles. For example, Wolfrom and Thompson, by the action of sodium acetate-acetic anhydride on n-glucose oxime, not only obtained pentaacetyl-D-glucononitrile, in 40% yield, but also isolated a small amount of hexaacetyl-oWeAydo-D-glucose oxime (V) identical with that prepared by mild acetylation of pentaacetyl-aWe%do-D-glucose oxime (IV) whose structure was assured by its formation from pentaacetyl-aldehydo-D-glucoae (III). 

The two most common functional groups found in monosaccharides (in open chain form) are aldehyde and ketone. When a monosaccharide contains an aldehyde, it is known as an aldose, e.g. glucose, and in the case of ketone, it is called a ketose or keto sugar, e.g. fructose. 

The Cyclic Hemiacetal Form of Glucose Aldoses contain an aldehyde group and several hydroxyl groups. The solid, crystalline form of an aldose is normally a cyclic hemiacetal. In solution, the aldose exists as an equilibrium mixture of the cyclic hemiacetal and the open-chain form. For most sugars, the equilibrium favors the cyclic hemiacetal. 

No. The reaction catalyzed by phosphoglucose isomerase is a simple isomerization between an aldose and a ketose and involves the open-chain structures of both sugars. Since glucose 6-phosphate and fructose 6-phosphate are both reducing sugars, their Haworth ring structures are in equilibrium with their open-chain forms. This equilibration is very rapid and does not require an additional enzyme. Note that this isomerization reaction is of the same type as that catalyzed by triosephosphate isomerase. 

There is no direct proof for the existence of the open-chain forms. However, small quantities of the acetylated open-chain forms are obtained along with the ring forms when some sugars are acetylated (see under Acetyl sugars). Sugar solutions contain isomers which are reducible at the dropping-mercury electrode of the polarograph (S2). The amounts of the reducible form present in 0.25 M solutions of several aldoses at pH 7.0 and 25°C. are shown in Table IV. As may be seen from the table, the amount of the reducible... 

COAL 7 Given a Lewis diagram of a monosaccharide in its open-chain form, determine whether the sugar is an aldose or a ketose. COAL 8 Distinguish among monosaccharides, disaccharides, and polysaccharides. 

The open-chain forms of the benzoyl and acetyl derivatives of aldose and ketose sugars are well-characterised crystalline substances which can be... 

The reduction of aldose sugars with sodium borohydride gives, as we would expect, a primary alcohol in place of the aldehyde. These diols are called alditols. Thus, glucose is reduced to glucitol (Figure 16.29), more commonly referred to as sorbitol. Sorbitol is less sweet than sucrose and is metabolized slowly (to fructose), so it is often used in sugar-free products, especially for diabetics. It occurs naturally in peaches, plums, apples, and pears. Since alditols do not contain a carbonyl group, they cannot cyclize and exist in the open-chain form. 

OZTs from aldoses and ketoses. The second and the oldest-as well as the less studied—method is based on the condensation of O-unprotected sugars with thiocyanic acid, generated in situ from potassium thiocyanate and a protic acid. The reaction involves the free anomeric position and a y- or (5-hydroxyl group able to promote intramolecular cyclization of a transient open-chain isothiocyanate, to form the thermodynamically most stable OZT. The first results obtained by Zemplen in d-gluco and D-Fru series reported the formation of OZTs fused to pyran backbones (Scheme 20).42... 

Figure 20-1 Structure and configuration of the D-aldoses from C3 to C6l showing the configurational relationship to D-glyceraldehyde. Open-chain and cyclic forms are shown. The oxacyclohexane (pyranose) form is more stable than the oxacyclopentane (furanose) form for the free sugar. The oxacyclopentane structure is shown for ribose because this is the form in which it occurs in many important substances, such as the nucleic acids. Only the a anomers are shown (see Section 20-2B).

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)... 

Ribose-5-phosphate isomerase, which catalyses the interconversion of ribose-5-phosphate and ribulose-5-phosphate, activates the carbonyl differently. The aldose substrate can form a furanose (but not a pyranose) ring, but the furanose forms are so disfavoured (see Table 1.1) that there are likely to be appreciable amounts of open-chain sugar present at equilibrium. Moreover, this equilibrium, involving only furanose forms whose opening is possibly assisted by general acid catalysis from the 5-phosphate, is likely to be achieved rapidly. Although the enzyme from other sources crystallise with ring forms of the substrate bound, that from Thermus thermophilus crystallises with both ribose-... 

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