From: Polyphenols: Mechanisms of Action in Human Health and Disease (Second Edition), 2018

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Simple sugars are single molecules, whereas more complex sugars are more than one molecule joined together by chemical bonds. Sugars are made up of carbon, oxygen and hydrogen; no nitrogen, except for amino sugars. The simplest sugars are monosaccharides. Six-carbon monosaccharides are prevalent but monosaccharides can have from 3 to 7 carbons in their structures (3 carbons, triose; 4 carbons, tetrose; 5 carbons, pentose, 6 carbons, hexose and 7 carbons, heptose). The hexoses are common and important. Glucose is the most representative member of the hexoses. The structure of glucose is shown in Fig. 6.17 in a simple stick model (Fischer projection) and how it is closed into a ring structure (Haworth projection). Sugars contain chiral carbons which are those carbons having 4 different substituents. If 2 of the substituents of a carbon atom are the same (e.g., double bond to an oxygen or single bonds to 2 hydroxyls or to 2 hydrogens) that carbon is achiral. Inspecting the open stick model of glucose, for example, there are 4 chiral centers at C2, C3, C4, and C5. The number of chiral carbons determines the number of stereoisomers, thus, for an aldotetrose with 2 chiral centers, the number of stereoisomers is 22 or 4 stereoisomers; for glucose with 4 chiral centers, it has 24 or 16 stereoisomers.

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Figure 6.17. Stick model of glucose. Carbon 1 is the top carbon and the numbering proceeds towards the bottom; the lowest is carbon 6. The ring is closed through a nucleophilic addition between the C1 aldehyde and the C5 hydroxyl. The proton on C5 migrates to the oxygen of C1.

There are a number of simple sugars ranging from three carbons (triose) to seven carbons (heptose), although sugars of six-carbon length (hexose) are considered here. A five-carbon sugar is a pentose and a four-carbon sugar is a tetrose. Sugars are either aldoses or ketoses; an aldose sugar has an aldehyde group (e.g., the C1 of d-glucose seen clearly in the open stick model); a ketose has a ketone group (e.g., the C2 of d-fructose clearly seen in the open stick model). The simple stick structures (Fischer projections) are shown in Fig. 6.18.

Figure 6.18. Monosaccharides, either natural or synthetic showing aldose sugars (top) and ketose sugars (bottom).

The number of chiral carbons determines the number of stereoisomers. The 4 stereoisomers of a tetrose are shown in Fig. 6.19.

In the process of cyclization of glucose, the carbon 1 carbonyl can be attacked from two sides generating the possibility of α- or β-forms, with the rearrangement of a proton in the process as shown in Fig. 6.20.

Figure 6.20. d-Glucose, in the Fischer projection, is converted to the Haworth projection with glucose as a five-member ring structure (pyranose). The thick lines at the bottom of the ring structures indicate extension of that part of the structure outward from the page toward the reader.

The α- and β-forms in the ringed form of glucose are named based on the position of the hydroxyl on carbon-1 as shown in Fig. 6.21.

β-d-glucose has the C-1 hydroxyl on the left and α-d-glucose has the C-1 hydroxyl on the right. “D” refers to dextro, or right (as with the amino acids). When the sugar is in the form of a ring, the carbon bonds can bend into either one of two forms: a “chair” configuration or a “boat” configuration. One form may be favored over the other, depending on hydroxyl substituents in the ring. The general forms of these structures for a six-carbon sugar skeleton are shown in Fig. 6.22.

Figure 6.22. (A) Permitted configurations for six-member sugars. (B) Alternate chair conformations for 6-membered sugars: An “up” carbon (e.g., axial) shown on right (circled in red) and a rotated version showing the axial carbon as “down” on left (circled in red). α- and β-Forms are determined by the position of the hydroxyl group attached to the anomeric carbon (asterisk) and the CH2OH attached to the other carbon next to the ether. α-Carbohydrates have a cis configuration between the OH group attached to the anomeric carbon and the CH2OH group (circled in blue). The OH group and the CH2OH group are on opposite sides of the ring. β-Carbohydrates have a trans configuration between the OH group attached to the anomeric carbon and the CH2OH group (circled in blue). The OH group and the CH2OH group are on the same side of the ring.

Sugar rings do not form a flat ring like benzene; the benzene ring contains three double bonds that are shorter (1.34 Å) than carbon-to-carbon single bonds (1.40 Å). Simple sugars can be represented in four ways (not including the boat structure) as shown in Fig. 6.23.

Figure 6.23. Four different ways of writing the structures for the hexose, glucose, or for the pentose, ribose, including the chair configuration.

Disaccharides are formed using the α- (axial bond down) or the β-(equatorial bond up) hydroxyl on the ring as shown in Fig. 6.24. The location of a specific substituent (in this case, hydroxyl group) as up or down stems from the stereoisomeric center (anomeric carbon) of the sugar. In the ring forms (chair, e.g.), the anomeric carbon can be located as the carbon next to the ring oxygen not attached to CH2OH. In the chair configuration, one hydrogen on each carbon is equatorial and one hydrogen is axial. The equatorial hydrogens radiate from around the ring while the axial hydrogens point along an axis or parallel to an axis; axial bonds are upwards or downwards along an axis through the center of the ring. Also the hydroxyl substituent of the anomeric carbon can be either up or down. If it is down (axial), the sugar is in the α form: if it is up (equatorial), the sugar is in the β form (e.g., α-d-glucopyranose or β-d-glucopyranose).

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Figure 6.24. Monosaccharides interact using α-hydroxyls or β-hydroxyls on the ring structure to produce some naturally occurring disaccharides. In lactose, the glucose moiety can open and function as a reducing sugar, whereas, in sucrose both rings are locked making it a nonreducing sugar. The glucose moiety on the left of d-maltose cannot open, whereas the glucose moiety on the right is able to open making maltose a reducing sugar. Maltose is the repeating sugar unit in starch.