Aldoses are sugar molecules with an aldehyde group. They come in different sizes and shapes, creating a variety of stereoisomers. Understanding their configurations is key to grasping how these important biological molecules function.

Fischer projections help visualize aldose structures in 2D. D and L configurations, along with mnemonics for remembering specific aldoses, make it easier to navigate the complex world of sugar stereochemistry.

Configurations of Aldoses

Stereoisomers of aldoses

• Aldoses are monosaccharides contain an aldehyde functional group and are classified by the number of carbon atoms
  • Triose has 3 carbons (glyceraldehyde), tetrose has 4 carbons (erythrose), pentose has 5 carbons (ribose, xylose), and hexose has 6 carbons (glucose, galactose)
• Stereoisomers have the same molecular formula but different spatial arrangements of atoms
  • Aldoses contain chiral centers allowing for the existence of stereoisomers
    • Number of stereoisomers increases with the number of chiral centers
      • $2^n$ possible stereoisomers for n chiral centers
  • Enantiomers are non-superimposable mirror images of each other
    • D and L notation distinguishes between enantiomers
  • Diastereomers are stereoisomers that are not mirror images of each other
    • Epimers are diastereomers that differ at only one chiral center

Fischer projections of monosaccharides

• Fischer projections represent the three-dimensional structure of a molecule in a two-dimensional drawing
  • Horizontal lines represent bonds coming out of the plane towards the viewer
  • Vertical lines represent bonds going behind the plane away from the viewer
• D and L configurations determined by the orientation of the hydroxyl group on the chiral center farthest from the aldehyde group
  • D configuration has the hydroxyl group on the right side
  • L configuration has the hydroxyl group on the left side
• Common monosaccharides in D and L configurations:
  • D-Glyceraldehyde and L-Glyceraldehyde (trioses)
  • D-Erythrose and L-Erythrose (tetroses)
  • D-Ribose, L-Ribose, D-Xylose, and L-Xylose (pentoses)
  • D-Glucose, L-Glucose, D-Galactose, and L-Galactose (hexoses)

Mnemonics for aldose structures

• Mnemonics help remember the names and configurations of aldohexoses and aldopentoses
  • "All Altruists Gladly Make Gum in Gallon Tanks" for aldohexoses:
    1. Allose (all R)
    2. Altrose (RLRR)
    3. Glucose (RLRS)
    4. Mannose (RLSR)
    5. Gulose (RSRR)
    6. Idose (RSSR)
    7. Galactose (RSRS)
    8. Talose (RSSR)
  • "Rarely Let Very Xylophones Ruin" for aldopentoses:
    1. Ribose (all R)
    2. Lyxose (RLRR)
    3. Xylose (RLRS)
    4. Ribulose (RLSR)

Stereochemistry and Optical Activity

• Stereochemistry studies the three-dimensional arrangement of atoms in molecules
• Chirality refers to molecules that are non-superimposable on their mirror images
  • Chiral molecules contain at least one asymmetric carbon (a carbon bonded to four different groups)
• Optical activity is the ability of chiral compounds to rotate plane-polarized light
• Cahn-Ingold-Prelog priority rules are used to assign R or S configuration to chiral centers