885d_c07-238-27211/21/037:38 AM Page240Mac113mac113:1aEDL 240 Part I Structure and Catalysis of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D CHO CHO isomers, and those with the same configuration as L- glyceraldehyde are L isomers. When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers CH2OH Figure 7-3 shows the structures of the D stereoiso- mers of all the aldoses and ketoses having three to six carbon atoms. The carbons of a sugar are numbered be- ginning at the end of the chain nearest the carbonyl Ball-and-stick models group. Each of the eight D-aldohexoses, which differ in the stereochemistry at C-2, C-3, or C-4, has its own name: D-glucose, D-galactose, D-mannose, and so forth CHO CHO (Fig. 7-3a). The four- and five-carbon ketoses are des H-C-OH HO--C-H ignated by inserting"ul"into the name of a correspond ing aldose; for example, D-ribulose is the ketopentose CH,OH CH,OH corresponding to the aldopentose D-ribose. The keto- hexoses are named otherwise: for example, fructose (from the Latin fructus, fruit"; fruits are rich in this Fischer projection formulas sugar) and sorbose (from Sorbus, the genus of moun- tain ash, which has berries rich in the related sugar al- cohol sorbitol). Two sugars that differ only in the con figuration around one carbon atom are called epimers; H-C-OH HO-C-H D-glucose and D-mannose, which differ only in the stere- ochemistry at C-2, are epimers, as are D-glucose and D- CHOH CH。OH galactose(which differ at C-4)(Fig. 7-4) L-Glyceraldehyde Some sugars occur naturally in their L form; exam ples are L-arabinose and the L isomers of some sugar de- rivatives that are common components of glycoconju- FIGURE 7-2 Three ways to represent the two stereoisomers of glyc- gates(Section 7.3) eraldehyde. The stereoisomers are mirror images of each other. Ball- H O and-stick models show the actual configuration of molecules By con- vention, in Fischer projection formulas, horizontal bonds project out H-C-OHl of the plane of the paper, toward the reader; vertical bonds project ehind the plane of the paper, away from the reader. Recall (see Fig HO-C-H 1-17)that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away CHOH By convention, one of these two forms is designated the The Common Monosaccharides D isomer the other the l isomer. as for other biomole cules with chiral centers, the absolute configurations of Have Cyclic Structures sugars are known from x-ray crystallography. To repre- For simplicity, we have thus far represented the struc sent three-dimensional sugar structures on paper, we tures of aldoses and ketoses as straight-chain molecules often use Fischer projection formulas (Fig. 7-2) (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet- In general, a molecule with n chiral centers can roses and all monosaccharides with five or more carbon have 2" stereoisomers. Glyceraldehyde has 2=2; the atoms in the backbone occur predominantly as cyclic aldohexoses, with four chiral centers, have 2=16 (ring) structures in which the carbonyl group has stereoisomers. The stereoisomers of monosaccharides formed a covalent bond with the oxygen of a hydroxylBy convention, one of these two forms is designated the D isomer, the other the L isomer. As for other biomole￾cules with chiral centers, the absolute configurations of sugars are known from x-ray crystallography. To repre￾sent three-dimensional sugar structures on paper, we often use Fischer projection formulas (Fig. 7–2). In general, a molecule with n chiral centers can have 2n stereoisomers. Glyceraldehyde has 21
2; the aldohexoses, with four chiral centers, have 24
16 stereoisomers. The stereoisomers of monosaccharides of each carbon-chain length can be divided into two groups that differ in the configuration about the chiral center most distant from the carbonyl carbon. Those in which the configuration at this reference carbon is the same as that of D-glyceraldehyde are designated D isomers, and those with the same configuration as L￾glyceraldehyde are L isomers. When the hydroxyl group on the reference carbon is on the right in the projection formula, the sugar is the D isomer; when on the left, it is the L isomer. Of the 16 possible aldohexoses, eight are D forms and eight are L. Most of the hexoses of living organisms are D isomers. Figure 7–3 shows the structures of the D stereoiso￾mers of all the aldoses and ketoses having three to six carbon atoms. The carbons of a sugar are numbered be￾ginning at the end of the chain nearest the carbonyl group. Each of the eight D-aldohexoses, which differ in the stereochemistry at C-2, C-3, or C-4, has its own name: D-glucose, D-galactose, D-mannose, and so forth (Fig. 7–3a). The four- and five-carbon ketoses are des￾ignated by inserting “ul” into the name of a correspond￾ing aldose; for example, D-ribulose is the ketopentose corresponding to the aldopentose D-ribose. The keto￾hexoses are named otherwise: for example, fructose (from the Latin fructus, “fruit”; fruits are rich in this sugar) and sorbose (from Sorbus, the genus of moun￾tain ash, which has berries rich in the related sugar al￾cohol sorbitol). Two sugars that differ only in the con￾figuration around one carbon atom are called epimers; D-glucose and D-mannose, which differ only in the stere￾ochemistry at C-2, are epimers, as are D-glucose and D￾galactose (which differ at C-4) (Fig. 7–4). Some sugars occur naturally in their L form; exam￾ples are L-arabinose and the L isomers of some sugar de￾rivatives that are common components of glycoconju￾gates (Section 7.3). The Common Monosaccharides Have Cyclic Structures For simplicity, we have thus far represented the struc￾tures of aldoses and ketoses as straight-chain molecules (Figs 7–3, 7–4). In fact, in aqueous solution, aldotet￾roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl L-Arabinose C O A A A A O O O C OH H OCO H HO H CH2OH HO O C H G J 240 Part I Structure and Catalysis FIGURE 7–2 Three ways to represent the two stereoisomers of glyc￾eraldehyde. The stereoisomers are mirror images of each other. Ball￾and-stick models show the actual configuration of molecules. By con￾vention, in Fischer projection formulas, horizontal bonds project out of the plane of the paper, toward the reader; vertical bonds project behind the plane of the paper, away from the reader. Recall (see Fig. 1–17) that in perspective formulas, solid wedge-shaped bonds point toward the reader, dashed wedges point away. Mirror CH2OH Ball-and-stick models CH2OH CHO CHO OH H H OH CHO C H CH2OH HO L-Glyceraldehyde Perspective formulas L-Glyceraldehyde C CH2OH H CHO CHO CHO H C OH CH2OH D-Glyceraldehyde OH D-Glyceraldehyde C CH2OH H HO Fischer projection formulas 8885d_c07_238-272 11/21/03 7:38 AM Page 240 Mac113 mac113:122_EDL: