Chapter 25: Biomolecules: Carbohydrates
Polyhydroxy aldehydes and ketones, and the vocabulary for sorting them
The word carbohydrate is a nineteenth-century accident. Early elemental analyzes of sugars kept returning formulas of the form Cn(H₂O)n — glucose is C₆H₁₂O₆, which can be rewritten as C₆(H₂O)₆ — and chemists concluded they were looking at hydrates of carbon. They were not. There is no water in glucose; the ratio is a consequence of every carbon carrying exactly one oxygen. The name outlived the idea, and it fails cleanly for several important sugars: 2-deoxy-D-ribose, the sugar of DNA, is C₅H₁₀O₄, and the amino sugars of cartilage and bacterial cell walls carry nitrogen. The definition that…
Reading and manipulating a Fischer projection
An aldohexose has four stereocenters. Drawn with wedges and dashes it is a thicket, and comparing two such drawings by eye is unreliable. Emil Fischer's solution, devised while he was working out the structure of glucose in the 1890s, was to standardize the drawing so completely that comparison becomes mechanical. The carbon chain runs vertically, numbered from the top; the most oxidized carbon — the aldehyde of an aldose, or the CH₂OH nearest the ketone of a ketose — goes at the top; and each stereocenter is drawn as a cross with its two non-chain substituents on the horizontal arms. The…
D and L, epimers, and counting the aldose family
When Fischer was working, no experiment could establish which of two mirror-image structures a given sugar actually was. What he could do was fix an arbitrary reference and report everything else relative to it. He chose glyceraldehyde, the smallest chiral sugar, drew it as a Fischer projection with the aldehyde on top, and named the isomer with the C2 hydroxyl on the right D and its mirror image L. The guess was fifty-fifty, and it was not confirmed until X-ray work in 1951 — but it happened to be correct, so nothing in the older literature had to be redrawn. Extending the reference to…
Why the open chain is a fiction: intramolecular hemiacetal formation
Every textbook draws glucose as an open chain with an aldehyde at one end, and almost none of the glucose in a glass of water looks like that. About three molecules in a hundred thousand are open-chain at any instant. The rest have closed on themselves. The reaction responsible was covered in Chapter 19 as ordinary carbonyl chemistry: an alcohol adds reversibly to an aldehyde to give a hemiacetal, a carbon bearing one OR group and one OH group. For two separate molecules that equilibrium sits badly to the left, because forming the hemiacetal converts two molecules into one and pays a steep…
Two crystalline glucoses, one solution: anomers and mutarotation
Two different crystalline forms of glucose can be isolated, and the fact was known long before anyone could explain it. Crystallizing glucose from water below 50 °C gives a solid melting near 146 °C whose freshly made solution has a specific rotation of +112 degrees. Crystallizing from pyridine or from hot acetic acid gives a different solid, melting near 150 °C, whose fresh solution reads +18.7 degrees. Both analyze as C₆H₁₂O₆; both give the same derivatives; and left standing in water, both solutions drift to the same value of +52.6 degrees and stop there. The two solids are the α and β…
Haworth projections: converting from Fischer without losing a stereocenter
A Fischer projection cannot draw a ring, and it has no way to say which face of a ring a substituent occupies. Once the chapter moves to cyclic sugars a different drawing is needed, and the one in universal use is the Haworth projection, introduced by Norman Haworth in the 1920s while he was establishing the ring sizes of the common sugars. The conventions are strict, and following them is most of the work: The ring is drawn edge-on, as a hexagon for a pyranose or a pentagon for a furanose, with the bottom edge bold to show that it is nearest the viewer. The ring oxygen goes at the back,…
The pyranose chair, the all-equatorial sugar, and the anomeric effect
The Haworth projection is a bookkeeping device, and it stops being useful the moment a question turns on crowding. Six-membered rings pucker, and a pyranose ring — five carbons and one oxygen — puckers into a chair for exactly the reasons cyclohexane does, with the same alternating axial and equatorial positions and the same 1,3-diaxial interactions. Everything learned about cyclohexane conformation in Chapter 4 applies here without amendment; the ring oxygen changes the electronics, as we will see, but not the geometry. The conversion from Haworth to chair follows one principle: a…
Glycosides: converting the hemiacetal to an acetal locks the anomeric carbon
Chapter 19 treated hemiacetals and acetals as consecutive stages of the same reaction: an alcohol adds to a carbonyl to give a hemiacetal, and under acid catalysis a second alcohol displaces the hemiacetal's hydroxyl to give an acetal. A cyclic sugar has already completed the first stage internally. Supplying an external alcohol and an acid catalyst completes the second, and the product is a glycoside. The naming is systematic. The general class is glycosides; a specific one is named by replacing the parent sugar's terminal -e with -ide and prefixing the group contributed by the alcohol.…
Reducing sugars, alditols, and the two oxidation levels above an aldose
Long before spectroscopy, chemists identified sugars by watching them reduce metal ions. Tollens' reagent is silver(I) held in solution as its diammine complex; a sugar that reduces it deposits metallic silver as a mirror on clean glass. Benedict's reagent is copper(II) complexed with citrate in carbonate buffer; reduction throws down brick-red copper(I) oxide, and the color change from clear blue through green and yellow to orange-red is vivid enough that Benedict's test was the standard clinical assay for glucose in urine for decades. A sugar that gives a positive result is a reducing sugar…
Reading a glycosidic linkage, and what one inverted carbon does to a polymer
Everything in this chapter has been building toward one observation: a glycosidic linkage carries more information than "these two sugars are joined," and the extra information decides whether the resulting material is food, fiber, or the sweetener in a soft drink. A linkage is fully specified by three facts, and the standard shorthand packs all three into a few characters. Read α-1,4 as follows. The α is the anomeric configuration at the sugar that donated its anomeric hydroxyl to the bond — the same α and β established four sections ago, now frozen permanently because the anomeric carbon…
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