Page 51 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioChemistry

P1: GTQ Final
Encyclopedia of Physical Science and Technology EN006P-81 June 29, 2001 21:48

Glycoconjugates and Carbohydrates 835

H hydo function), especially in alkaline solution. This char-
H O
acteristic was used as an analytical tool for many years,
C H C OH
and the term reducing sugar was employed to designate
H C OH C O saccharides with a free (or potentially free, in aqueous
solution) carbonyl function. Prior to the advent of spe-
HO C H HO C H
cific enzymatic methods, this was the standard procedure
H C OH H C OH for measurement of blood glucose levels (i.e., reducing
sugar).
H C OH H C OH
Monosaccharidesthatdifferfromoneanotheratasingle
CH 2 OH CH 2 OH asymmetric center other than the one derived from the
D-Glucose, D-Fructose, carbonyl carbon are termed epimers (D-galactose is the
an aldohexose a ketohexose 4-epimer of D-glucose, and D-mannose is the 2-epimer,
FIGURE 2 Projection formulas of D-glucose and D-fructose. for example; Fig. 7). Mirror-image structures are termed
enantiomers.
An important and characteristic feature of saccharides
are introduced. Accordingly, there are four aldotetroses, is their ability to interact with plane-polarized light. Thus,
eight aldopentoses, four ketopentoses, eight ketohexoses, each sugar has a characteristic optical rotation that is a
etc. (Fig. 3). complex function of the asymmetric centers, their polar-
Fischer recognized that sugars such as glucose existed izability, solvent interactions, etc. The sign (+ or dextro-
in a ring form (i.e., a hemiacetal or hemiketal formed rotatory, − or levorotatory) and magnitude cannot be de-
by addition of a hydroxyl group to the carbonyl carbon). termined a priori. Nor is it true that all sugars of the same
Based on his knowledge of lactones, he assigned the par- configuration (D- or L-) will have the same sign of op-
ticipating hydroxyl to C-4 in the case of glucose, thus tical rotation. Thus, D-glucose is dextrorotatory, whereas
forming a five-membered (furanose) ring. The thermo- D-fructose is levorotatory. Since monosaccharides tend to
dynamics, however, show that the six-membered (pyra- crystallize in a single anomeric form rather than as mix-
nose) ring is the more stable form and that is the one tures, dissolving them in water results in a change in the
predominating in solution. Note that the internal addition optical rotation from that of the pure anomer (either alpha
reaction at C-1 causes that carbon to become asymmetric or beta) to that of the thermodynamically defined equilib-
and, therefore, two new isomeric structures are formed. To rium mixture. This phenomenon is termed mutarotation
distinguish this asymmetry from that present at the other (Fig. 8).
asymmetric centers, the term anomer is employed with As the principles of conformational analysis became
the designation α or β reflecting projection of the newly known, it was rapidly realized that the pyranose form of
formed hydroxyl group on the same or opposite side of sugars adopted a chair structure analogous to that of cyclo-
the ring, respectively (Fig. 4). hexane (Fig. 9). In this spatial arrangement, equatorial hy-
The ring structures (aldopyranose for the six-membered droxyl groups are thermodynamically more stable (energy
rings and aldofuranose for the five-membered rings) are, differences of about 1.5 kcal) than axial ones with the exo-
in aqueous solution, in ready equilibrium with the open- cyclic hydroxymethyl group having a much larger effect.
chain, free aldehyde form. Thus, a solution of glucose It is not surprising, therefore, that β-D-glucose, the all-
in water contains five species: free aldehyde, two fura- equatorialstructure,isthemostprevalentnaturalsugarand
noses, and two pyranoses (Fig. 5). The aldehyde form the most prevalent organic compound on earth (Fig. 10).
represents about 0.025% of the total, with the bulk made As a philosophical aside, it may be inferred that the choice
up of the two pyranoses (the beta form predominates; between D-and L-glucosewasmadeveryearlyandonaba-
see below). Although the six-membered ring is the en- sis we do not understand since they are conformationally
ergetically preferred form, the predominant ketohexose, equivalent. It is also expected that other naturally occur-
D-fructose, is found in the furanose form in combined ring sugars such as D-mannose or D-galactose would have
structures (sucrose, for example) although it too prefers only a single axial hydroxyl. Of the 16 possible aldohex-
the six-membered ring when free in aqueous solution. oses, D-glucose, D-mannose, and D-galactose are widely
Other commonly occurring furanosides include ribose and distributed in nature while the remainder are of laboratory
deoxyribose as components of RNA and DNA (Fig. 6); interest only. Parenthetically, idose (three axial hydroxyls
galactose is also found as a furanoside in several plant and in the classic conformer) has never been obtained in crys-
microbial polysaccharides. talline form. Additional widely distributed sugars include
A key property of the free monosaccharides is their abil- D-xylose (all-equatorial aldopentose), D-ribose, and 2-
ity to be readily oxidized (initially via the available alde- deoxy-D-ribose (backbone components of RNA and DNA,