Cyclic and Conformational Structures

Aldoses and ketoses are more stable in their five- or six-membered cyclic hemiacetal form than in acyclic form. Cyclization of an aldose

tablf. 1-1 Abbreviated Names for Some Common Carbohydrates *

Name

Abbreviation

Arabinose

Ara

Fructose

Fru

Fucose

Fuc

Galactose

Gal

Galacturonic acid

GalA

^-Acetylgalactosamine

GalNAc

Glucose

Glc

Glucuronic acid

GlcA

/V-Acetylglucosamine

GlcNAc

Iduronic acid

IdoA

Mannose

Man

MAcetylneuraminic acid

Neu5Ac

Rhamnose

Rha

Xylose

Xyl

  • Abbreviations for di-, oligo-, and polysaccharides often add d- or L- to indicate the enantiomeric form, p or f to indicate the pyranose or furanose ring form, a- or p- to indicate the stereochemistry of the glycosidic linkage, and carbon numbers to indicate the carbon atoms that are O-linked by the glycosidic bond. The designation is often omitted for the more common o-enantiomers and p-ring form.
  • Abbreviations for di-, oligo-, and polysaccharides often add d- or L- to indicate the enantiomeric form, p or f to indicate the pyranose or furanose ring form, a- or p- to indicate the stereochemistry of the glycosidic linkage, and carbon numbers to indicate the carbon atoms that are O-linked by the glycosidic bond. The designation is often omitted for the more common o-enantiomers and p-ring form.

occurs via an intramolecular reaction of the nucleophilic hydroxyl group on C-4 or C-5 (C—0—) with the C-l aldehyde (Fig. 1-3). This spontaneous reaction transforms the car-bonyl carbon (C-l of an aldose) into a chiral carbon (the anomeric carbon), giving new isomers: the a- and p-anomers of both the furanose and pyranose ring forms. The same type of cyclization reaction occurs with the ketoses in which the C-5 or C-6 hydroxyl group (C—0—) reacts with the C-2 ketose carbonyl. In the Fischer formula, the hemiacetal (anomeric) —OH of the a-anomer is on the same side of the carbon chain as the D designator oxygen (C5—0 of a hexose). In aqueous solution, the acyclic and cyclic isomers are in equilibrium, with the most energetically stable isomers predominating. For most aldoses, the six-membered pyranose ring is more stable than the five-membered furanose ring. However, some sugars, including arabinose, ribose, and fructose, frequently occur in the furanose ring form in disaccharides, oligosaccharides, and polymers. The equilibrium nature of the hemiacetal reaction dictates that all hydroxyls and the carbonyl group can undergo reactions, i.e., the sugar can react in acyclic or in ring form.

Sugar ring structures are depicted in several ways, as shown for glucose and fructose (Fig. 1-4). The Fischer projection formula is based on the convention that the carbons are either in the plane of the paper or in back of the plane (never in front). The Haworth formula was introduced as a more realistic depiction of the bond lengths in the cyclic sugars. Hydroxyl groups on the right of the ho—c—h

ch2oh

ß-D-Glucopyranose

ch2oh aldehydo- D-Glucose

Haworth Formula
CH2OH a-D-G I ucopy ranose
Ch2oh Ch2oh

CH2OH CH2OH

(3-D-Glucofuranose a-D-G lucof u ranose

Figure 1-3. The equilibrium mixture of cyclic anomeric and acyclic forms of d-glucose in aqueous solution.

ß-D-Glucopyranose:

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Responses

  • abramo
    How are cyclic isomers form?
    7 years ago

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