16.2 Monosacharides 497

Vibrational pattern of a single lightwave in a beam ofordinarylight viewed from the side

\dhen you look at the vibration head on, it looks like this.

Vibrations in a beam ofordinarv A polarizing filter lets through light in light are in all directions. which vibrations are in only one direction.

Polarizing filter Polarizing filter Sample tube sl2t/,1\= Figure16.5 o-(+) glyceraldehyde Oneof a pairof opticalisomers rotatesplane-polarized light to the right.lts mirrorimage rotates the lightto the left.The instrument usedto measureoptical rotationis r-(-) glyceraldehyde calfeda polorimeter.

directions. Plane-polarized light is light in which uibrations are in only one direction. stereoisomers ire often called optical isomers.optical isomers are stereoisomersthat rotate plane-polarized light in oppositedirections.For example, n-glyceraldehyde rotates plane-polarized light to the right, and r-glyceraldehyde rotates plane-polarized light to the left. Sometimes you may seethe name o-(+)-glyceraldehyde.The o refersto the handednessof the particular glyceraldehyde;the plus sign refers to the direction of rota- tion of plane-polarLed light (Fig.16.3). There is no connection betweenthe o and the (+); other D sugars may rotate plane-polarized light to the left. Then the sugarwould be designatedo-(-).

16,2Monosacchorides

AIMS: To drow open-choinFischer proiections for the common simple sugars.To identify o sugor os D or t by looking ot its Fischerproiection formulo.

The simplest carbohydratemolecules, Focus notbonded to any other carbohydrate, are called simple sugars or monosaccharides. Many of the monosaccha- Most monosaccharides contain rides in nature contain four, five, or six carbons. Sugarscontaining an alde- four, five, or six carbons. hyde functional group and consisting of four, five, and six carbons are aldotetroses,aldopentoses, and aldohexoses,respectively. In this section we will examine some of them. 498 CHAPTERl6 Carbohydrates

The four-carbon sugarsl n-Threose,with a chain of four carbons, is a naturally occurring tetrose; the aldehyde functional group makes threose an aldotetrose. TWo of the car- bons of threose, carbon 2 and carbon 3, have four different groups attached, so threose has two asymmetric carbons:

(a) (b) Fischer projection fbrmulas tcno tcno CHO CHO zl nofJ-H Ho-l-u H-l-oH H-C-OH I l nji-oH HToH HoTH HojJ-H nJ",o" tl-CH,OH cH2oH cH2oH o-Threose l-Threose

The Fischer projections of the stereoisomer have an -OH group on each of their two asl.rnmetric centers,one pointing to the left and one to the right. If they were not labeled for handedness,how could you tell which is the o and which is the r isomer?First, draw a Fischer projection of the sugar being considered.Then look at the hydroxyl group attached to the last 1 aqrnmetric carbon in the chain. If the hydroxyl group points to the lght, '). the sugar belongs to the n family; if it points to the left, the sugar belongs to the r family. In the caseof threose, the last asymmetric carbon in the chain is at carbon 3, so structure (a) belongs to the o family of sugars.Structure (b) belongs to the r family. The structures of o- and r-threose are mirror images. However, since there are two asymmetric carbons in a four-carbon sugar molecule, another pair of stereoisomerscan exist. These stereoisomersare o- and r-erythrose: 'cHo tcHo ,l ,l H-:C-OH HO-:C-H rl '41 H_:c_oH Ho_:g_H nt",on nC*r,o"

o-Erythrose l-Erythrose It is easy to calculate the number of possible stereoisomers of a sugar that contains multiple asymmetric carbons. This number is 2', where n is the number of asyrnmetric carbons. Thus glyceraldehyde,with one asym- metric carbon, has 2', or 2, stereoisomers;these are the I and r isomers. Four-carbon sugarswith two asymmetric carbons can exist as22, or 4, stereoisomers.Wecan calculate the number of mirror-image pairs by divid- ing the number of stereoisomersby 2. The four aldotetrosel fbrm two pairs of mirror images.

.. PRACTICEEXERCISE I6.4 Draw Fischer projections for the two aldotetroses that belong to the o ' family. 16.2 Monosacharides 499

Ihe frve-carbon sugars several aldopentoses are found in nature. n-Arabinose and o-xylose are five-carbon sug.us produced by plants. o-Arabinose is sorrr-etimescalled pectin sugar Pectin, the polysaccharide from which it is obtained, forms gels that are usefrrl in making jelly. Because it is isolated from wood, o- xylose is sometimes called wood sugar. tcHo tcHo tl .)| HO--:C-H H-:C-OH njJ-oH HojJ-H al -," H-c+on'' H-1i-oH: 'cHroH-l_l "cHrort o-Arabinose o-Xylose

other important aldopentoses are o:ribose and l-2-deoxyribose, a related compound that lacks an -OH group at carbon 2: tcHo tcno nlJ-on uZt-u njl-on ujC-on n-43-oH H-11-oH uJ",o" uCnroH

o-Ribos-e o-2-Deoxyribose

Thesetwo sugarsare an integral part of the hereditarymaterials ribonucleic acid (RNA) and deoxyribonucleic acid (DNA). The structures of RNA and DNA are discussedin Chapter 20.

Ihe slx-carbon sugafsl Only three aldohexosesappear in nature:o-glucose,o-galactose, and D- mannose: The relative sweetness of sugars tctto tcHo ?u' ib and sugar substitutes varies over a tc'o wide range. Lactoseis about one- sixth as sweet and glucose about njJ-on ujJ-on noll-n three-quarters as sweet as sucrose nojJ-n nojl-H uoji-n (table sugar). Fructose is not quite twice as sweet as sucrose.The nll-oH no-lJ-H n-1C-on artifi cial sweetenersaspartame (NutraSweet)and saccharin are Hjl-oH Hjl-on Hjl-on about 150 and 500 times as sweet uJ",o" uJ",o" uJ",o" as sucrose. o-Glucose o-Galactose o-Mannose

The o form of glucose has a central role in the nutrition of virtually all species, including plants and humans. The biochemistry of glucose is so 500 CHAPTERl6 Carbohydrates

important that Chapter 24 is devoted to it. l-Glucose is abundant in all life forms. Depending on the source, it has been called grape sugati corn sugai and blood sugar.Urine usually contains a trace of o-glucose, but the con- centration is greatly increased in the urine of patients with untreated dia- betes mellitus. r-Galactose is a constituent of lactose,also called milk sugar (seeSec. 16.7). o-Mannose is a major constituent of polymeric molecules called mannans,which are found in severalplants. I Monosaccharidesthat contain a ketonefunctional group -i- are called ketoses. Ketosescontaining three, four, five, and six carbons ate ketotrioses, ketotetroses,ketopentoses, and ketohexoses,respectively. No discussion of hexoseswould be complete without including o-fructose, a ketohexose becauseof the presenceof a ketone carbonyl group in the molecule at car- bon2. o-Fructose and o-glucose differ in structure only at carbons I and2. The identical stereochemistryat carbons 3, 4, and 5 exists because the breakdornmof o-glucose in living systemsinvolves conversion of o-glucose to n-fructose.

1 CH,OH -c:o^l ^t Ho-:C-H t1 H_]C-OH _l u-lc-on ^l cH2oH

D-FruCtOSe o-Fructose occurs in a large number of fruits and in honey. It is also the only sugar found in human semen. o-Fructose is one of those sugarswhich belongs to the o family but rotates plane-polarized light in a left-handed direction.

PRACTICEEXERCISE I6.5 ,: Identifu each structure as D or L. (a) CHo (b) cHo (c) cH2oH (d) cHo I H-l-oH HToH Fo Ho-fH CH2OH HOl" HO I CH2OH H HO t) To" H To" cH2oH cH2oH 0 PRACTICEEXERCISE I6.6 Draw the Fischer projection formula for o-glucose. Number the car- bons, and identiff each asymmetric carbonwith an asterisk.