25.4 Configurations of the Aldoses

Aldoses are monosaccharides that contain an aldehyde functional group. Because they have multiple chiral centers, they exist as families of stereoisomers with distinct biological roles. Telling these stereoisomers apart requires a reliable way to draw and name their configurations.

Configurations of Aldoses

Stereoisomers of Aldoses

Aldoses are classified by the number of carbon atoms they contain:

• Triose (3 C): glyceraldehyde
• Tetrose (4 C): erythrose, threose
• Pentose (5 C): ribose, xylose, arabinose, lyxose
• Hexose (6 C): glucose, galactose, mannose, and others

Every carbon that bears four different substituents is a chiral center (also called a stereocenter). The more chiral centers an aldose has, the more stereoisomers are possible. The formula is $2^n$ stereoisomers for $n$ chiral centers. Glyceraldehyde has one chiral center ($2^1 = 2$ stereoisomers), while an aldohexose like glucose has four ($2^4 = 16$ stereoisomers).

These stereoisomers fall into two categories:

• Enantiomers: non-superimposable mirror images of each other. D-glucose and L-glucose are enantiomers.
• Diastereomers: stereoisomers that are not mirror images. D-glucose and D-galactose are diastereomers. A special subtype is an epimer, which differs at only one chiral center. Glucose and galactose are C-4 epimers; glucose and mannose are C-2 epimers.

Fischer Projections of Monosaccharides

A Fischer projection is a 2D shorthand for showing the 3D arrangement around each chiral center in a chain:

• Horizontal lines represent bonds pointing toward you (out of the page).
• Vertical lines represent bonds pointing away from you (into the page).
• The carbon chain is drawn vertically with the most oxidized carbon (the aldehyde) at the top.

D vs. L configuration is assigned by looking at the chiral center farthest from the aldehyde (the highest-numbered chiral carbon):

• If the $\text{-OH}$ on that carbon points to the right, the sugar is D.
• If the $\text{-OH}$ points to the left, the sugar is L.

Most naturally occurring sugars are D-sugars. Each D-sugar has an L-enantiomer that is its exact mirror image (every chiral center flipped left-to-right in the Fischer projection).

Common examples across each size class:

• Trioses: D-glyceraldehyde / L-glyceraldehyde
• Tetroses: D-erythrose, D-threose (and their L-enantiomers)
• Pentoses: D-ribose, D-arabinose, D-xylose, D-lyxose
• Hexoses: D-allose, D-altrose, D-glucose, D-mannose, D-gulose, D-idose, D-galactose, D-talose

Mnemonics for Aldose Structures

The eight D-aldohexoses and four D-aldopentoses can be hard to keep straight. Mnemonics help you remember their names in order as they appear on the aldose family tree (reading left to right). The R/S or R/L labels below refer to whether each $\text{-OH}$ group (from C-2 down to the penultimate carbon) points right (R) or left (L) in the Fischer projection.

D-Aldohexoses — "All Altruists Gladly Make Gum In Gallon Tanks"

Notice that every hexose in this list ends with R at C-5; that's what makes them all D-sugars. Also note that Talose's pattern is L L L R, not the same as Idose (a common mix-up).

D-Aldopentoses — "Ribs Are Xtra Lean" (Ribose, Arabinose, Xylose, Lyxose)

Again, every entry ends with R at the penultimate carbon because these are all D-sugars.

Stereochemistry and Optical Activity

Chirality means a molecule cannot be superimposed on its mirror image. Any carbon bonded to four different groups is a chiral center (asymmetric carbon). All aldoses except the achiral dihydroxyacetone contain at least one chiral center.

Chiral compounds are optically active: they rotate the plane of plane-polarized light. The direction of rotation is labeled (+) for clockwise (dextrorotatory) or (−) for counterclockwise (levorotatory). This is measured experimentally with a polarimeter and cannot be predicted from the D/L label alone. D-glucose happens to be (+), but D-fructose is (−).

To assign R or S configuration at a specific chiral center, you use the Cahn-Ingold-Prelog (CIP) priority rules:

1. Rank the four substituents on the chiral carbon by atomic number of the atom directly attached (higher atomic number = higher priority).
2. If there's a tie, move outward along the chain until a difference is found.
3. Orient the molecule so the lowest-priority group points away from you.
4. Trace a path from highest to lowest priority among the remaining three groups. Clockwise = R; counterclockwise = S.

R/S is an absolute configuration label for a single chiral center. D/L is a relative configuration label for the whole sugar based on the bottom chiral center in the Fischer projection. Both systems are used in carbohydrate chemistry, so keep them distinct.