Carbohydrates: General Biochemistry (UMBB-FS) — Comprehensive Notes

1. Definition – Role

• Carbohydrates (glucides) derive from Greek glukus meaning "sweet"; the term commonly used is carbohydrates because many do not taste sweet and their general formula is $ext{Cn(H<em>2O)}</em>n.$

• Oses (monosaccharides) are small organic molecules with carbon chains bearing hydroxyl groups and aldehyde or ketone functions, possibly with carboxyl or amino groups. They are water-soluble and reducing sugars.

• Roles in cells:

  • Energy reserve in polymerized form: starch (plants) and glycogen (animals). Amidon is the main form of photosynthetic energy storage in the biosphere.

  • Structural elements: mucopolysaccharides in animals, cellulose in plants, chitin in some invertebrates, etc.

  • Recognition & communication between cells: polysaccharides in blood groups and bacterial antigens.

  • Components of many macromolecules: glycoproteins, nucleic acids (ribose, deoxyribose), coenzymes, antibiotics.

2. Sources of carbohydrates

• Plant products provide the majority: fruits, vegetables, potatoes, sugar cane, beets, cereals, legumes, etc.

• Animal-derived foods contain some carbohydrate (notably milk, liver, seafood).

• Human diet spans simple sugars to complex polysaccharides.

3. Classification

• Based on degree of polymerization:

  • Monosaccharides (os es): the simplest units.

  • Oligosaccharides: 2–10 oses (disaccharides are most common).

  • Polysaccharides: more than 10 units.

• Polysaccharides can be:

  • Holosides (homosaccharides): composed only of sugars.

  • Hétérosides (heterosaccharides): sugars plus non-sugar aglycone parts.

• Note on terminology: Osmides (glycosides) form when a sugar’s anomeric carbon bonds to an alcohol or phenol (glycosidic linkage). See section 8 for osides.

4. Structure of oses

4.1 Linear structure

• General formula: $ext{Cn(H<em>2O)}</em>n$; the old term “hydrates de carbone.”

• Each ose contains a reducing group: an Aldehyde (aldose) or a ketone (ketose) and at least one hydroxyl group.

• Aldoses vs. ketoses:

  • Aldoses: have an aldehyde group (-CHO).

  • Ketoses: have a ketone group (>C=O).

• Nomenclature by number of carbons (generic names):

  • 3 carbons: triose (aldotriose, ketotriose)

  • 4 carbons: tetrose (aldotetroses, ketotetroses)

  • 5 carbons: pentose (aldopentose, ketopentose)

  • 6 carbons: hexose (aldohexose, ketohexose)

  • 7 carbons: heptose (aldoheptose, ketoheptose)

• Carbon numbering: for aldoses, C1 is the aldehydic carbon; for ketoses, the carbonyl carbon is C2.

• Early natural monosaccharides: glyceraldehyde (aldotriose) and dihydroxyacetone (ketotriose).

• Example: glucose is an aldohexose with formula $ext{C}<em>6 ext{H}</em>{12} ext{O}_6$.

• In Fischer projection, for hexoses, D-series derive from D-glyceraldehyde; L-series derive from L-glyceraldehyde.

4.1.2 Chirality - Stereoisomerism

• Chiral objects (non-superposable on mirror image) have stereocenters; a carbon with four different substituents is chiral (often C*).

• Glyceraldehyde is used as reference; enantiomers are non-superimposable mirror images.

• Emil Fischer defined dextro-rotatory (D) vs. levo-rotatory (L) enantiomers; Bijvoet later confirmed the link to absolute configuration.

• Most natural sugars are of the D-series.

• Enantiomers have identical chemical properties except optical rotation and enzyme interactions.

4.1.3 D & L series (Fischer projection)

• In Fischer projections, aldoses with the last-2nd hydroxyl oriented to the right belong to the D-series; relationships between successive sugars involve adding a new chiral center (between terminal carbon and adjacent carbonyl).

• For aldoses with n carbons, there are 2n−2 stereoisomers; for ketoses (related to dihydroxyacetone, which has no chiral carbon) there are 2n−3 stereoisomers.

• Example: glucose is a D-aldose (aldohexose, C6) with formula $ext{C}<em>6 ext{H}</em>{12} ext{O}_6$; natural sugars are typically D-series.

• Epimers and diastereoisomers

  • Epimers differ at only one chiral center (e.g., D-mannose and D-galactose are epimers of D-glucose).

  • Diastereoisomers are stereoisomers that are not enantiomers.

4.2 Structure cyclique

4.2.1 Hemiacetal and mutarotation

• Open-chain (linear) forms do not capture all properties; aldoses can form hemiacetal via intramolecular reaction with a nearby alcohol.

• For hexoses, mutarotation occurs in solution: alpha- and beta- forms interconvert until reaching an equilibrium.

• Specific observations for glucose: mixture of a- and b- forms; mutarotation changes the optical rotation over time.

• Aldoses generally do not color fuchsine in their linear aldose form.

• If methanol reacts with glucose, only one methanol molecule can form a hemiacetal, not two as with an aldehyde.

4.2.2 Mechanism of cyclization and Haworth representation

• Cyclization involves the formation of a hemiacetal by reaction of the aldehyde group with the hydroxyl at C5 (or C4 for furanose formation).

• Tollen’s insight (1884) showed cyclization yields five- or six-membered rings; six-membered pyranose rings are most important in nature; five-membered furanose rings also exist.

• Haworth representation assumes the ring is planar; substituents on the right in Fischer projection point down in Haworth; left ones point up.

• Anomeric carbon: C1 (ring carbonyl carbon) becomes stereogenic upon cyclization; alpha and beta forms differ by the position of the anomeric hydroxyl relative to the ring plane.

• Pyranose vs. furanose naming: six-membered rings are pyranose; five-membered rings are furanose.

4.2.3 Spatial conformation

• Cyclohexane ring conformations: chair is the most stable; boat is less stable; axial and equatorial positions describe substituent orientation.

• For glucopyranose, the chair is dominant; furanose rings adopt envelope-type conformations.

4.2.4 Consequence of mutarotation

• Mutarotation shifts equilibrium toward the most stable cyclic forms.

• Glucose in solid form is mainly α-pyranose with [α]20D ≈ +113°; in aqueous solution at 25°C, equilibrium is approximately:

  • 36.4% α, 0.003% open form, 63.6% β

• This overall shift and the presence of anomeric forms affect properties like optical rotation and reactivity.

• Mutarotation is a general property of most monosaccharides with five or more carbons.

5. Physical properties

5.1 Solubility

• Very soluble in water due to many hydroxyl groups and hemiacetal/hemiketal functionality; forms viscous syrups.

• Extensive hydrogen bonding with water and other molecules.

5.2 Sweetening power

• Sweetness is measured relative to sucrose (set as 1.00 at 30 g/L in water at 20°C).

• Examples (relative to sucrose):

  • Lactose: $0.30$

  • Glucose: $0.70$

  • Maltose: $0.30-0.50$

  • Fructose: $0.80-1.30$

  • Saccharose (sucrose): $1.00$ (reference)

  • Fructo-oligosaccharides: $0.30-0.60$

• Artificial sweeteners: some have much higher sweetness than sucrose (e.g., saccharine, aspartame, others listed in the table).

• Perception of sweetness is subjective and varies among individuals.

5.3 Rotatory power

• Solutions of natural oses are optically active due to chiral centers.

• D-glucose enantiomers rotate light to the right; α-anomer around +112.2° and β-anomer around +17.5° for D-glucose.

• D-fructose rotates light to the left (levorotatory).

• The Specific Rotatory Power is defined as $[ ext{α}]^D_{20}$ and can be measured with a polarimeter using the D-line of sodium (589 nm).

• The Biot law relates rotation to concentration and path length:
  $[ heta] = rac{ heta}{l c} \, ext{or} \, [ ext{α}]^D_{20} = rac{ ext{observed rotation}}{l imes c}$

• A racemic mixture (equal enantiomers) shows no optical rotation.

• The figure shows the instrument and a standard setup.

5.4 Spectral features

• Carbohydrates absorb very little in the visible and UV ranges but have characteristic IR spectra.

6. Chemical properties

6.1 Properties related to the reducing group (C=O)

6.1.1 Oxidation

• Oses are weaker reducing agents than true aldehydes/ketones; oxidation depends on conditions.

• Mild oxidation (Br2 or I2 in basic medium) converts the aldose carbon-1 into carboxylic acid, forming aldonic acids (e.g., glucose → gluconic acid, mannose → mannonic acid, galactose → galactonic acid). Aldoses give aldonic acids; ketoses generally are not oxidized under these conditions.

• Stronger oxidation with hot nitric acid yields aldaric acids (diacids) with carboxyl groups at C-1 and C-6 (e.g., glucose → glucaric acid).

• If the aldehyde function is protected during oxidation, uronic acids result (oxidation only of the primary alcohol at C-6, e.g., glucose → glucuronic acid; galactose → galacturonic acid).

• Reactions often yield acids in biological contexts (e.g., glucuronic acid is a precursor for vitamin C synthesis in some species; galacturonic acid is a component of pectins).
  $ext{R-CHO} + I<em>2 + OH^- ightarrow ext{R-COOH} + 2I^- + Na^+ + H</em>2O$

6.1.2 Reduction

• Reduction by catalytic hydrogenation or by NaBH4/LiBH4 converts the aldehyde/ketone to an alcohol, giving alditols (polyols).

• For aldoses: D-glucose → sorbitol (D-glucitol); D-mannose → mannitol.

• For ketoses: two epimeric alditols arise at C-2 after reduction.

• Aldoses can reduce metals (e.g., CuO in Fehling’s test) under basic and warm conditions, demonstrating their reducing power.

6.1.3 Action of phenylhydrazine (osazone formation)

• Phenylhydrazine reacts with aldose/ketose to form osazones (bis-phenylhydrazones) at the two first carbons, enabling purification and identification by crystallization.

• Hydrolysis of osazone yields a dicarbonyl compound that can be reduced to a ketose by selective reduction of the terminal aldehyde.

6.1.4 Reactions of condensation

• With cyanide and hydroxylamine, condensation reactions extend the carbon skeletons (Kiliani-Fischer and Wohl-degradation concepts).

• Wohl degradation (degradation with hydroxylamine) involves oximes, cyanhydrines, and nitriles, ultimately shortening the sugar chain by removing terminal carbons.

• Cyanohydrin formation (from aldehyde with HCN) extends carbon skeletons; hydrolysis and lactonization can occur, leading to ring-opening and eventual re-layout of the sugar chain.

6.2 Properties related to alcoholic functions (OH)

6.2.1 Boron complexes

• Complexes with boron enable electrophoretic separation of sugars (they are otherwise uncharged), and reveal anomeric relationships (cis vs trans orientation of the C1–OH relative to C2–OH in D-glucose).

• Complexation helps determine the anomeric configuration via mobility differences in electrophoresis.

6.2.2 Methylation (permethylation)

• Reagents like dimethyl sulfate or methyl iodide with base convert all free OH groups to O-CH3, forming methyl ethers; if the reducing end is free, the anomeric OH can also be methylated.

• In acidic medium, the glycosidic bond can be hydrolyzed; permethylation followed by oxidation and hydrolysis is used to identify linkage positions within a sugar or within oligo-/poly-saccharides.

• For oligo-/poly-saccharides, complete methylation followed by hydrolysis and analysis reveals which hydroxyls participated in linkages (e.g., amylose yields a tetra- or tri-O-methyl derivative pattern).

6.2.3 Formation of esters and ethers

• Alcohols form esters with acids; phosphate esters (e.g., sugar phosphates) are important intermediates in metabolism (e.g., fructose-1,6-bisphosphate).

• Hydroxyls also form ethers; glycosidic linkages (osidic bonds) form through condensation with alcohols.

6.2.4 Nitrogen derivatives: osamines

• Replacement of a non-anomeric OH by NH2 yields amino sugars, typically N-acetyl derivatives.

• Examples:

  • D-glucose → D-glucosamine; D-galactose → D-galactosamine.

  • N-acetyl-D-glucosamine is a component of chitin; N-acetyl-mannosamine-6-phosphate is a precursor to sialic acids.

6.3 Other properties

6.3.1 Action of strong acids (concentrated HCl, H2SO4, etc.)

• Strong acid treatment leads to dehydration and formation of furfural or related aldehydes from sugars, which react with phenols to give characteristic color tests (used for detection and quantification):

  • α-naphthol (Molisch test) forms red-brown color with general sugars.

  • Resorcinol (Selivanoff test) gives red color with ketoses.

  • Orcinol (Bial test) gives blue-violet color with pentoses.

6.3.2 Action of dilute bases

• Base-catalyzed isomerization interconverts aldoses and ketoses (and epimerizes at C2 in aldoses). For example, fructose can convert to mannose and glucose under basic conditions; other sugars show similar interconversions.

7. Study of some oses and derivatives

7.1 Monosaccharides (Oses)

7.1.1 Glucose: (aldohexose - pyranose)

• Formula: $ext{C}<em>6 ext{H}</em>{12} ext{O}_6$; D-series; positive rotation ([+]) in solution; major energy source; principal fuel of cells through glycolysis.

• Structure: four stereocenters → 16 stereoisomers; only two occur naturally: D-galactose and D-mannose.

• Important notes:

  • Widespread in plants and animals; stored as glycogen (animals) or starch (plants).

  • In Haworth projection, the C5 hydroxyl position in the ring helps distinguish D vs. L series in pyranose forms; the anomeric carbon (C1) defines α vs β.

7.1.2 Arabinose: (aldopentose - pyranose)

• One of the rare natural sugars in the L-series; abundant in plants; also exists in the D-series.

• Plays a role in structural support; is a precursor to glucose/mannose in metabolism.

• In structural terms, the determination of D/L depends on the configuration at the penultimate carbon (for aldopentoses, C4).

7.1.3 Fructose: (ketohexose - furanose/pyranose)

• A natural ketohexose found in fruits and honey; levorotatory in solution; D-series; important as a metabolic intermediate linking glycolysis and other pathways.

• The linear form is often present at high concentration and equilibrates with furanose/pyranose forms; sugar balance with furanoid rings is notable in nature.

7.1.4 Galactose and Mannose: (aldohexose - pyranose)

• Less abundant than glucose but essential components of glycoproteins and glycolipids; contribute to oligosaccharide and glycan structures.

7.2 Osamines (sugars amines)

• Osamines are formed by replacing a hydroxyl group (often at C2) with an amino group (NH2); the amino group is typically N-acetylated.

• Examples and roles:

  • D-glucosamine

  • D-galactosamine

  • N-acetylglucosamine is a key component of chitin and of bacterial peptidoglycans.

• Synthesis pathways link to amino sugar metabolism and glycan biosynthesis.

7.3 Sialic acids

• Acids derived from neuraminic acid (e.g., N-acetylneuraminic acid); located at terminal positions of glycoprotein and glycolipid chains.

• They impart negative charge (acidic character) to glycoproteins and influence cell recognition and immune interactions.

7.4 Muramic acids

• Muramic acid is a component of peptidoglycans (cell walls of bacteria); N-acetylmuramic acid is a building block for bacterial cell walls in conjunction with peptide chains.

7.5 Sugar phosphates

• Phosphorylated forms (e.g., glucose-6-phosphate, ribose-5-phosphate) are produced by kinases and serve as intermediates in glycolysis and pentose phosphate pathways.

• O- and N- derivatives; essential for energy metabolism and nucleotide synthesis.

8. Study of some osides and derivatives

8.1 Definition – glycosidic linkage

• Osides (glycosides) are formed when the hydroxyl of the anomeric carbon of a sugar condenses with an alcohol (ROH) or phenol (ArOH).

• The glycosidic bond is an O-glycosidic linkage (R-O-R′) with water eliminated: $ext{R-OH} + ext{R′-OH} <br>ightarrow ext{R-O-R′} + ext{H}_2 ext{O}.$

• Osides are either oligosaccharides (2–20 units) or polysaccharides (>20 units).

• Glycosyl donor and acceptance position depend on which sugar’s anomeric carbon is involved; linkages can be α or β depending on anomeric configuration.

8.2 Determination of the structure of an oside

• Determine constituent sugars by hydrolysis (acidic or enzymatic) to identify monosaccharides.

• Determine the linkage pattern by methylation analysis (permethylation), followed by hydrolysis, oxidation, and reduction to reveal which hydroxyls participated in linkages.

• For complex polysaccharides, perform controlled hydrolysis to obtain oligosaccharides and analyze stepwise.

• Determine anomeric configuration by enzymatic or mutarotation-based approaches after hydrolysis.

• Determine whether a sugar is reducing or non-reducing by methods such as borohydride reduction.

• Determine molecular mass and chain length by osmometry, ultracentrifugation, light scattering, viscosity, size-exclusion, electrophoresis with borate complexes, etc.

8.3 Oligosaccharides (Oligoholosides)

• Formed from several sugar units linked by glycosidic bonds; several factors influence naming and structure:

  • Nature of monomer sugar(s)

  • Ring type (pyranose vs. furanose)

  • Anomeric carbon involved in the linkage (α or β)

  • Configuration at the linkage carbons

• Nomenclature examples (conventional condensed forms):

  • Glucose, Galactose, Mannose, Fructose, Glucosamine, etc.

• Representative disaccharides (diholosides):

  • Maltose: α-D-glucopyranosyl-(1→4)-D-glucopyranose; reducing sugar due to free anomeric OH on the second glucose; forms osazone with phenylhydrazine; methylation analysis yields characteristic products.

  • Lactose: β-D-galactopyranosyl-(1→4)-D-glucopyranose; only natural reducing disaccharide; hydrolyzed by β-galactosidase to glucose and galactose.

  • Sucrose (saccharose): α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside; non-reducing due to both anomeric carbons involved in the glycosidic bond.

  • Cellobiose: β-D-glucopyranosyl-(1→4)-D-glucopyranose; epimer of lactose at one residue; reduces to glucose.

  • Trehalose: α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside; non-reducing; found in fungi and insects; accumulates under stress; can be hydrolyzed by trehalase.

8.3.2 Trisaccharides (Triholosides)

• Gentianose: β-D-glucopyranosyl-(1→6)-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside (from Gentiana).

• Raffinose: α-D-galactopyranosyl-(1→6)-α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside (found in beets, raffinates).

• Some natural oligosaccharides contain 4 or 5 sugar units (e.g., stachyose, verbascose).

8.4 Polysaccharides (Polyholosides)

8.4.1 Homopolysaccharides

• Starch (plant energy reserve): a mixture of amylose and amylopectin; both have a single reducing end.

  • Amylose: linear polymer of D-glucose with α-(1→4) linkages; typically 1000–4000 units; forms a helical structure; iodine test yields blue complex; soluble in hot water.

  • Amylopectin: highly branched, α-(1→4) linked chains with α-(1→6) branch points about every 25 residues; branch length typically ~20 residues; amylopectin is a major component of starch and affects digestibility.

• Glycogen: animal storage polysaccharide; highly branched like amylopectin but more densely branched (every ~8–12 residues); very large MW (up to ~10^8); stored in liver and muscle; degraded by glycogen phosphorylase and debranching enzymes.

• Inulin: fructan, polymer of β-D-fructofuranose units; 30–100 units; only known in β configuration; found in some plants (dahlias, artichokes, Jerusalem artichoke).

• Dextrans: glucose polymers with α-(1→6) backbone and occasional α-(1→2/3/4) branches; produced by microbes; used in industry.

• Cellulose: plant structural polymer; long, linear chains of β-(1→4)-linked glucose; 100–200 residues per chain; MW up to many millions; forms crystalline fibers; humans cannot digest due to lack of β-(1→4) glycosidic hydrolases; important in plant cell walls; foundational for cotton, paper, and nitrocellulose production; forms hydrogen-bonded fibers.

• Chitin: similar to cellulose but with N-acetylglucosamine (GlcNAc, β-(1→4) linkage); exoskeletons of arthropods; often mineralized with CaCO3.

• Their physical state and organization (ribbons, fibrils, crystalline vs amorphous) impart mechanical properties to biological tissues.

8.4.2 Heteropolysaccharides (Heteropolysaccharides)

• Glycoconjugates: sugars bound to non-sugar moieties, including:

  • Glycolipids: membrane lipids bearing oligo- or polysaccharide chains.

  • Proteoglycans: long glycosaminoglycan (GAG) chains covalently attached to core proteins; highly negative due to uronic acids and sulfates; major components of extracellular matrix.

  • Glycoproteins: proteins with short to moderate length carbohydrate chains (
    typically 1–20% carbohydrate by weight).

  • Peptidoglycans: bacterial cell wall polymers made of repeating sugar units cross-linked with peptides.

  • Lectins: carbohydrate-binding proteins involved in cell recognition and adhesion; in plants, many are agglutinins; in animals, they participate in cellular targeting and immune functions.

• Glycosylation is a post-translational modification in eukaryotes, key for protein targeting and membrane/secreted protein functions (e.g., blood group antigens).

• Examples: ABO blood group determinants, with core H antigen; A antigen adds N-acetylgalactosamine; B antigen adds galactose.

9. Methods of studying carbohydrates

• Emil Fischer and historical context of carbohydrate chemistry (1852–1919) underpin many analytical approaches.

• Extraction: typically with 80% ethanol at elevated temperature to isolate carbohydrates.

• Chromatography (separation & identification):

  • Paper chromatography or thin-layer chromatography (TLC) using a mobile organic solvent; detection with phenol (Molisch test) or other reagents.

  • After chromatography, visualization using nitrate silver-amine or other reagents; compare against standards to identify sugars.

• Gas chromatography (GC): used for volatile derivatives; sugars are usually derivatized to volatiles (permethylated derivatives) to enable GC analysis.

• Structural determination of oligo-/poly-saccharides:

  • Complete methylation followed by hydrolysis and analysis reveals linkage patterns and branching.

  • For branched polysaccharides, partial hydrolysis yields oligosaccharides that are analyzed individually.

  • Enzymatic hydrolysis using specific glycosidases helps determine linkage and anomericity.

• Common glycosidic linkages and enzymes:

  • Glycoproteins and proteoglycans contain complex glycan structures; their analysis often uses mass spectrometry and glycosidase enzymes.

  • Specific glycosidases include lactase (β-galactosidase) for lactose, maltase for maltose, isomaltase for isomaltose, sucrase (invertase) for saccharose, trehalase for trehalose, cellobiase for cellobiose, etc.

• Polysaccharide hydrolysis and digestion in biological systems: many enzymes act on specific linkages (β- or α-, 1→4, 1→6, etc.).

Appendix: Key relationships and formulas

• General sugar formula: $ext{Cn(H<em>2O)}</em>n$

• Aldoses/ketoses naming by carbon count, e.g., triose, tetrose, pentose, hexose, heptose, etc.

• Common oxidation products:

  • Aldonic acids (e.g., gluconic acid) from mild oxidation of aldoses.

  • Aldaric acids (e.g., glucaric acid) from strong oxidation of aldoses.

  • Uronic acids (e.g., glucuronic acid) from selective oxidation of the primary alcohol.

• Mutarotation observations (glucose at 25°C in water):

  • Approximately 36.4% α-D-glucose, 63.6% β-D-glucose; very small open-chain fraction (~0.003%).

• Optical rotation and Biot’s law (polarimetry):
  $[ ext{α}]^D_{20} = rac{ ext{observed rotation}}{l imes c}$
  where l is path length (dm) and c is concentration (g/mL).

• Anomeric carbon and α/β designations are determined by the orientation of the C1 hydroxyl relative to the ring plane in Haworth projection (α = below plane for D-glucose, β = above plane).

• Important disaccharides: Maltose (α-D-glucopyranosyl-(1→4)-D-glucopyranose), Lactose (β-D-galactopyranosyl-(1→4)-D-glucopyranose), Sucrose (α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside, non-reducing), Cellobiose (β-D-glucopyranosyl-(1→4)-D-glucopyranose), Trehalose (α-D-glucopyranosyl-(1→1)-α-D-glucopyranoside, non-reducing).

• Major polysaccharides: starch (amylose: α-(1→4) linear; amylopectin: α-(1→4) chains with α-(1→6) branching), glycogen (more branched than amylopectin), cellulose (β-(1→4) glucose, no digestibility in humans), chitin (GlcNAc, β-(1→4)).

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