Carbohydrate Chemistry and Classification

Monosaccharides: Structure & Isomerism - Sweet Singles

• Simplest sugars, general formula $(CH_2O)_n$, where $n \ge 3$. Classified by:
  • Carbon count (e.g., Triose, Pentose, Hexose).
  • Functional group (Aldose: aldehyde; Ketose: ketone).
• Structure:
  • Linear form: Fischer projection.
  • Cyclic form: Haworth projection (Pyranose: 6-membered ring; Furanose: 5-membered ring).
  • Anomeric Carbon: Carbonyl carbon (C1 in aldoses, C2 in ketoses) that becomes chiral upon cyclization.
• Isomerism:
  • Enantiomers: Non-superimposable mirror images (D- and L-forms). 📌 Most human sugars are D-isomers.
  • Diastereomers: Stereoisomers that are not mirror images.
    • Epimers: Differ in configuration at ONE specific chiral carbon (e.g., Glucose/Galactose at C4).
    • Anomers: Differ in configuration at the anomeric carbon ($\alpha$ and $\beta$ forms); arise from cyclization.

⭐ Glucose and galactose are C4 epimers, while glucose and mannose are C2 epimers. Anomers ($\alpha$ and $\beta$) differ at the anomeric carbon formed during cyclization.



Disaccharides & Oligosaccharides - Double Sugars

• Disaccharides: Two monosaccharides joined by a glycosidic bond.
  • Sucrose: Glucose + Fructose ($\alpha-1,\beta-2$ linkage). Non-reducing.
  • Lactose: Galactose + Glucose ($\beta-1,4$ linkage). Reducing. Milk sugar.
  • Maltose: Glucose + Glucose ($\alpha-1,4$ linkage). Reducing. Malt sugar.
  • 📌 Reducing: Lactose, Maltose (have free anomeric carbon). Non-reducing: Sucrose.
• Oligosaccharides: 3-10 monosaccharide units (e.g., Raffinose, Stachyose).

⭐ Sucrose (table sugar), composed of glucose and fructose, is a non-reducing sugar because its anomeric carbons (C1 of glucose and C2 of fructose) are involved in the $\alpha-1,\beta-2$ glycosidic bond.

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Polysaccharides: Types & Functions - Complex Chains

Monosaccharide polymers; glycosidic bonds.

• Homopolysaccharides (Homoglycans): Single monosaccharide type.
  • Storage:
    • Starch (plants): Amylose ($\\\\alpha$-1,4), Amylopectin ($\\\\alpha$-1,4 & $\\\\alpha$-1,6).
    • Glycogen (animals): $\\\\alpha$-1,4 & $\\\\alpha$-1,6 (highly branched).
  • Structural:
    • Cellulose (plants): $\\\\beta$-1,4 glucose. Indigestible.
    • Chitin (fungi, arthropods): $\\\\beta$-1,4 N-acetylglucosamine.
• Heteropolysaccharides (Heteroglycans): Multiple monosaccharide types/derivatives.
  • Glycosaminoglycans (GAGs): Repeating disaccharides (amino sugar + uronic acid/galactose). E.g., Hyaluronic acid, Heparin.

⭐ Glycogen, the animal storage polysaccharide, has alpha-1,4 glycosidic bonds for linear chains and alpha-1,6 glycosidic bonds at branch points (every 8-12 residues), enabling compact storage and rapid glucose mobilization.

Carbohydrate Reactions & Tests - Sugar Sleuthing

• Key Reactions:
  • Oxidation: Aldonic, uronic, aldaric acids.
  • Reduction: Alditols (sorbitol). $R-CHO \rightarrow R-CH_2OH$.
  • Osazone Formation: Phenylhydrazine; characteristic crystals.
  • Esterification: Phosphate esters (G6P, ATP).
• Qualitative Tests:
  • Molisch's: General (conc. $H_2SO_4$, $\alpha$-naphthol $\rightarrow$ purple ring).
  • Benedict's: Reducing sugars (alkaline $CuSO_4$ $\rightarrow$ brick-red $Cu_2O \downarrow$).
  • Barfoed's: Monosaccharides (acidic $Cu(CH_3COO)_2$ $\rightarrow$ faster red $Cu_2O \downarrow$).
  • Seliwanoff's: Ketoses (resorcinol, $HCl$ $\rightarrow$ cherry-red).
  • Bial's: Pentoses (orcinol, $FeCl_3, HCl$ $\rightarrow$ blue-green).
  • Iodine: Starch (blue); Glycogen (red-brown).

⭐ Benedict's test distinguishes reducing sugars, which possess a free aldehyde or ketone group, from non-reducing sugars. The formation of a colored precipitate indicates a positive result.

High‑Yield Points - ⚡ Biggest Takeaways

• Carbohydrates are polyhydroxy aldehydes or ketones.
• Key monosaccharides: glucose, fructose, galactose. Key disaccharides: sucrose (non-reducing), lactose, maltose.
• Epimers (e.g., glucose/galactose at C4) differ at one chiral center.
• Anomers (α/β) differ at the anomeric carbon.
• Reducing sugars have a free anomeric carbon; sucrose is non-reducing.
• Glycogen (animal storage) has α-1,4 and α-1,6 linkages.
• Cellulose (plant structure) has β-1,4 linkages, indigestible by humans.