• Their structures, as the name implies, can be factored into combinations of carbon and water - C_n(H₂O)_m.

• They often are impossible to crystallize.

• They are crucially important,
  • Sources of dietary energy and carbon atoms for many living creatures

  • Components of genetic material

  • Part of cellular recognition systems

  • Possibly an aid to protein folding

For now we look at only three sugars: ribose, deoxyribose, and glucose. The first two are key constituents of genetic material, and the third is the monomer from which is formed the world's most abundant biomolecule: cellulose.

The row below the table heading presents the structures of these three in one of their several incorrect representations: the Haworth structures.

• These pictures were devised by the British chemist W. N. Haworth (Nobel Laureate, 1937) in about 1920, when it was believed that 5- and 6-rings were planar. (D. H. R. Barton didn't teach us otherwise until about 1950.)

• They do carry the correct information about the cis- and trans-placement of substituents - configurational information

• They are useless for conformational information. Hence the structures in the second row of the table.

D-Ribose	D-Deoxyribose	Glucose (b-D-glucose)

Thesecond row of pictures are standard modern organic chemists pictures of the half-chair or twist form of a 5-ring, and the chair of a 6-ring.

• They enable us to see, for example, that b-D-glucose has all substituents equatorial

• They are an attempt to convey the information also presented in the ball-and-stick figures of the third row

Another issue is represented by the "D" in the titles of the table.

• This is an outmoded and confusing stereochemical designator, which represents the configuration of all stereogenic centers in the molecule at once.

• To explain it, we need to introduce another structural representation, which, like the "D", was introduced by the great Emil Fischer (Nobel Laureate, 1902).

• This kind of picture, called a Fischer projection, is shown at left below for D-ribose:

D-Ribose

In a Fischer projection, the longest carbon chain is arranged vertically, with the most oxidized carbon at the top.

(Where's the ring? Well, you did recognize that the sugars in the table were drawn as hemiacetals, didn't you? More later.)

The other two bonds to each carbon then are drawn as a single horizontal line. In effect, each tetrahedral carbon is viewed along an axis passing through the carbon and exactly bisecting the H-C-OH angle, and then pressed flat onto the page.

• Now, look at the highest numbered stereogenic center, in this case, the one next to the CH₂OH.

• If the OH is to the right, the configuration is D-, from the Latin dextro; if to the left, L (laevo).

• The configuration at the other stereogenic centers is specified by changing the parent name.
  • For example, the isomeric structure in which the middle OH is to the left is called D-xylose.

Confused? Good. For more about what's wrong with this system, here's another page.

Fischer projections also have the problem that you are not allowed to pick them up mentally and manipulate them; they must be viewed only in the plane of the paper.

• Turning one over is equivalent to changing the configuration at all stereogenic centers.

The standard organic chemists' representation, which can be manipulated mentally just as if it were a model, is shown at right in the picture.

• The stereogenic centers each are assigned a configuration according to the Cahn-Ingold-Prelog convention

Here is D-glucose, similarly viewed in both Fischer and standard organic notation.

D-Glucose

Where'd the b go? To explain, we'll also address the hemiacetal issue, in the picture below:

This is glucose again. Glucose in the cyclic hemiacetal form has an additional stereogenic center, created from the carbonyl carbon of the aldehyde.

• This center, marked with the blue star, is produced by addition of the CH2OH to the carbonyl, and is called the anomeric position.

• The b refers to the configuration at this center; an equatorial bond is b-, an axial one is a-.

• In aqueous solution the two diastereomers are in equilibrium via the open-chain isomer (a constitutional isomer of the cyclic ones).

• Although acyclic hemiacetals are unstable relative to the carbonyl/alcohol pair, 5- and 6-ring hemiacetals are more stable.

• Only traces of the open form are present in the equilibrium mixture, which consists essentially entirely of 35% a- and 65% b

If one creates a solution starting with pure a- or pure b-, one ultimately develops the equilibrium mixture.

• Since the two structures are diastereomers, they must have different properties (that's why the mixture isn't 50:50), including optical activity.

• One can follow the isomerization by monitoring the optical activity of the solution.

• The equilibration process is called mutarotation (changing rotation) for this reason.

Finally, a bit of systematic numenclature. Generic names for sugars are built of the pieces shown in the table:

Thus, glucose is an aldohexose, ribose is an aldopentose. The corresponding hemiacetals are called pryanose (6-ring) and furanose (5-ring).

Enough for one page. Move on to the next if you want to learn about some intriguing sugars like glycosides, and polymers of sugars.