sugars

sugars (shug´erz)

Those carbohydrates (saccharides) having the general composition (CH2O)n and simple derivatives thereof. Although the simple monomeric s. (glycoses) are often written as polyhydroxy aldehydes or ketones, e.g., HOCH2-(CHOH)4-CHO for aldohexoses (e.g., glucose) or HOCH2-(CHOH)3-CO-CH2OH for 2-ketoses (e.g., fructose), cyclization can give rise to varied structures as described below. S. are generally identifiable by the ending -ose or, if in combination with a nonsugar (aglycon), -oside or -osyl. S. especially d-glucose, are the chief source of energy, by oxidation, in nature, and they and their derivatives (e.g., d-glucosamine, d-glucuronic acid), in polymeric form, are major constituents of mucoproteins, bacterial cell walls, and plant structural material (e.g., cellulose). S. are often found in combination with steroids (steroid glycosides) and other aglycons.
Fischer projection formulas of s.representations, by projection, of cyclic s., or derivatives thereof, in which the carbon chain is depicted vertically. The lowest-numbered asymmetric carbon atom (C-1 in aldoses; C-2 in 2-ketoses, e.g., fructose) is drawn at the top, and the rest of the carbon atoms of the chain are drawn in sequence below the top carbon atom. For each carbon atom, depicted in projection as lying in the plane of the paper, the carbon-to-carbon bond(s), which actually point away from the viewer, are drawn as vertical lines. The left-hand and right-hand bonds of each carbon atom, which actually point toward the viewer, are, in projection, depicted as horizontal lines.
The conventions for the Fischer formulas of cyclic s. are as follows: 1) If the highest-numbered asymmetric carbon atom has its OH (or its replacement) lying to the right, as is the 2-OH of d-glyceraldehyde, the sugar has the d configuration; if the OH is to the left, the sugar has the l configuration. 2) On the anomeric carbon atom (C-1 in the aldoses; C-2 in the 2-ketoses), an OH or substituted OH that lies to the right, with the OH of the highest-numbered asymmetric carbon atom also to the right is defined to be a; if it is to the left, with the OH of the highest-numbered carbon atom still to the right, it is beta; the reverse applies if the latter OH is to the left. 3) The orientation of a terminal CH2OH group in the aldoses carries no configurational significance, as it contains no asymmetric carbon atom.
Haworth conformational formulas of cyclic s.for the pyranoses, these depict those shapes (conformations) on which none, one, or two ring-atoms lie outside the plane of the ring. If there are two such atoms para to each other, they can lie 1) on opposite sides of the plane (trans), giving chair forms, or 2) on the same side of the plane (cis), giving boat forms. For beta-d-ribopyranose, the two chair forms (4C1 and 1C4) are depicted.
Similarly, there are six boat conformations. If the two (trans) exoplanar atoms are meta to each other, the conformation is a skew form; if the two atoms are ortho to each other, the conformation is a half-chair form.
For the furanoses, the envelope conformations have one ring-atom exoplanar. If there are three adjacent, coplanar ring-atoms (the two exoplanar ring-atoms on opposite sides of the plane), the conformations are twist forms.
Haworth perspective formulas of cyclic s.perspective representations of furanose or pyranose structures as pentagons or hexagons, respectively, with the connecting bonds so shaded as to make them appear as though the plane of the ring is at an angle of 30° to the plane of the paper, and the bonds to H and OH are at right angles to the plane of the ring. These formulas depict the planar conformation, a situation not usually met. Other conformational formulas, e.g., Haworth conformational formulas of cyclic s., attempt to depict the many deviations from planarity.
The basic conventions in Haworth formulas of cyclic s. (cyclic glycoses) are as follows: 1) The lowest-numbered asymmetric ring-carbon atom is depicted at the right. 2) If the highest-numbered asymmetric carbon atom is d, the sugar is d; the formula of an l-glycose may be derived from that of its d-isomer by reversing the up or down direction of all groups attached to the ring-carbon atoms. 3) If the hydroxyl group attached to the anomeric carbon (C-1 in aldoses; C-2 in 2-ketoses) is below the plane of the ring of a d-glycose, it is a; if above, it is beta; the reverse applies if the sugar is l. See also Fischer projection formulas of s.

 

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