Sugars come in two main flavors: D and L. These stereoisomers differ in their spatial arrangement, which we can see using Fischer projections. D sugars, like glucose, have their OH group on the right, while L sugars have it on the left.

Most natural sugars are D sugars, matching D-glyceraldehyde's configuration. This stereochemistry is crucial for how enzymes recognize and interact with sugars in our bodies. Understanding these differences helps us grasp how sugars function in living systems.

Stereochemistry of D and L sugars

Differentiate between D and L sugars based on their stereochemistry and Fischer projections

• D and L sugars are stereoisomers differing in configuration at the chirality center farthest from the carbonyl group
  • D sugars have the hydroxyl group on the right side of the Fischer projection at this chirality center (D-glucose)
  • L sugars have the hydroxyl group on the left side of the Fischer projection at this chirality center (L-glucose)
• Fischer projections represent the three-dimensional structure of a molecule in a two-dimensional format
  • Horizontal lines represent bonds pointing towards the viewer
  • Vertical lines represent bonds pointing away from the viewer
• Carbonyl group (aldehyde or ketone) always placed at the top of the Fischer projection
• Longest carbon chain oriented vertically, with the carbonyl group at the top

D,L system vs R,S configuration

• Both D,L system and R,S configuration system describe stereochemistry of monosaccharides
• R,S system assigns absolute configuration based on Cahn-Ingold-Prelog (CIP) priority rules
  • R (rectus) configuration has lowest priority group pointing away from viewer when highest priority group oriented away (R-glyceraldehyde)
  • S (sinister) configuration has lowest priority group pointing towards viewer when highest priority group oriented away (S-glyceraldehyde)
• D,L system based on configuration of highest-numbered stereocenter (farthest from carbonyl group) compared to D-glyceraldehyde
  • D sugars have same configuration as D-glyceraldehyde at highest-numbered stereocenter (D-glucose)
  • L sugars have opposite configuration as D-glyceraldehyde at highest-numbered stereocenter (L-glucose)
• In most cases, D sugars have R configuration and L sugars have S configuration at highest-numbered stereocenter
  • Exceptions occur when sugar has even number of carbon atoms (D-erythrose has S configuration)

Stereochemistry in natural sugars

• Most naturally occurring sugars are D sugars, having same configuration as D-glyceraldehyde at highest-numbered stereocenter
  • Examples: D-glucose, D-fructose, D-ribose, D-deoxyribose
• D-glyceraldehyde is simplest sugar with single chirality center and serves as reference for determining configuration of other sugars
  • D-glyceraldehyde has hydroxyl group on right side of Fischer projection at chirality center
• L sugars (L-arabinose, L-fucose) less common in nature but still play important biological roles
• Stereochemistry of naturally occurring sugars essential for recognition by enzymes and proper functioning in biological systems

Optical activity and related concepts

• Stereocenter: A carbon atom bonded to four different groups, creating chirality
• Enantiomers: Mirror image molecules that are non-superimposable, differing in configuration at all stereocenters
• Epimers: Stereoisomers that differ in configuration at only one stereocenter
• Optical activity: The ability of chiral molecules to rotate plane-polarized light
• Mutarotation: The change in optical rotation observed when a sugar converts between its α and β anomeric forms in solution

Carbohydrate Nomenclature

Identify the components of monosaccharide names and their meanings

• Monosaccharide names consist of prefix, root, and suffix
• Prefix indicates number of carbon atoms in monosaccharide
  • Tri- (3 carbons), Tetr- (4 carbons), Pent- (5 carbons), Hex- (6 carbons), Hept- (7 carbons)
• Root describes type of carbonyl group present
  • -ose: Aldose (aldehyde group)
  • -ulose: Ketose (ketone group)
• Suffix indicates stereochemistry of monosaccharide
  • -D: Configuration at highest-numbered stereocenter matches D-glyceraldehyde
  • -L: Configuration at highest-numbered stereocenter opposite to D-glyceraldehyde

Identify common monosaccharides based on their structures and names

• Glucose (D-glucose): Aldohexose, most abundant monosaccharide in nature
  • Key component of polysaccharides (starch, cellulose)
  • Main energy source for cells
• Fructose (D-fructose): Ketohexose, commonly found in fruits and honey
  • Sweetest naturally occurring sugar
• Ribose (D-ribose): Aldopentose, component of RNA and ATP
• Deoxyribose (D-deoxyribose): Deoxyaldopentose, component of DNA
  • Lacks hydroxyl group at 2' position compared to ribose
• Galactose (D-galactose): Aldohexose, component of lactose (milk sugar)
  • Differs from glucose in configuration at C-4