Week 4

Carbohydrates: Key Concepts

  • Carbohydrates contain an aldehyde or a ketone; aldehydes can be oxidized to carboxylic acids.
  • Reducing sugars: aldoses and other sugars that can reduce Ag⁺ in Tollens’ test; ketoses may be reversibly converted to aldoses under basic conditions.
  • Tollens’ test uses Tollens’ reagent: [Ag(NH<em>3)</em>2]+[\mathrm{Ag(NH<em>3)</em>2}]^+; aldehydes reduce Ag⁺ to metallic silver forming a silver mirror.
  • General Tollens’ reaction (aldehyde):
    R−CHO+2Ag++H2O→R−COOH+2Ag(s)+2H+\mathrm{R-CHO} + 2\mathrm{Ag^+} + \mathrm{H_2O} \rightarrow \mathrm{R-COOH} + 2\mathrm{Ag(s)} + 2\mathrm{H^+}
  • Aldoses are thus reducing sugars; ketoses are non-reducing sugars unless isomerized.
  • D-Glucose is the main aldohexose used to illustrate cyclic forms and mutarotation.
  • Fischer vs Haworth projections: aldoses typically form cyclic hemiacetals (pyranose or furanose).
  • Anomeric center: C1 becomes a new stereocentre upon ring closure; α- and β- anomers arise from the orientation of the C1 OH group.
  • Mutarotation: cyclic hemiacetals interconvert with the open-chain form; in solution, equilibrium favors β-anomer for glucose.
    • Initial rotations: [α]ᵈ = +112°, [β]ᵈ = +19°; equilibrium rotation ≈ +53°.
    • Equilibrium composition for glucose in water: ~64% β, 36% α.
  • Aldose vs ketose oxidation: aldehydes can be oxidized to carboxylic acids; Tollens’ test discriminates reducing sugars from non-reducing sugars.
  • Monosaccharides: D- and L- configurations; D-series has hydroxyls on the right in the Fischer projection farthest from carbonyl.
  • Common monosaccharides include aldoses (e.g., glucose, galactose) and ketoses (e.g., fructose).
  • Reducing sugars vs non-reducing sugars in disaccharides:
    • Maltose: glucose–glucose with α(1→4); reducing sugar (one free anomeric carbon).
    • Cellobiose: glucose–glucose with β(1→4); reducing sugar.
    • Sucrose: glucose–fructose with α(1→2); not reducing (both anomeric carbons involved).
    • Trehalose: glucose–glucose with α(1→1); not reducing (no hemiacetal at the ends).
  • Disaccharides and polysaccharides form glycosidic bonds (acetal linkages) via the anomeric carbon of one sugar attaching to an –OH of another.
  • General rule for identifying glycosidic bonds: locate the anomeric center; if it is linked to another carbon via an O-glycosidic bond, that bond is glycosidic.
  • Hydrolysis of polysaccharides yields monosaccharides; disaccharides yield two monosaccharides.
  • Starch and cellulose are polysaccharides of glucose with different linkages: starch (α(1→4)); cellulose (β(1→4)).
  • Starch is digestible by humans (α-linkages); cellulose is not digestible by humans (β-linkages).
  • Ribose and deoxyribose: aldopentoses that are the building blocks of RNA (ribose) and DNA (deoxyribose).

Cyclic Forms and Anomerism in Glucose

  • Carbohydrates form cyclic hemiacetals via internal nucleophilic addition of an alcohol to an aldehyde within the same molecule.
  • Resulting cyclic forms: 6-membered pyranose and 5-membered furanose rings; for glucose, the predominant form is the pyranose.
  • C1 becomes the anomeric center; ring closure creates a new stereocentre.
  • α- vs β- anomers differ inC1–OH orientation relative to CH₂OH at C5; mutarotation equilibrates α and β forms in solution.
  • The cyclic form is generally more stable; the β-anomer is usually more stable than the α-anomer in glucose.

Aldoses, Ketoses, and Reducing Sugars

  • Aldose example: D-glucose (aldohexose); reduces Tollens’ reagent.
  • Ketose example: D-fructose; can tautomerize to aldose under basic conditions to reduce Tollens’ reagent.
  • Reducing sugars are capable of reducing metal ions (e.g., Ag⁺ in Tollens’ test).
  • Non-reducing sugars (e.g., sucrose, trehalose in specific linkages) do not reduce Tollens’ reagent because the anomeric carbons are involved in glycosidic bonds.

Glycosidic Bonds, Disaccharides, and Glycosides

  • Glycosidic bond is an acetal linkage formed between the hemiacetal carbon (anomeric center) of one sugar and an –OH of another.
  • Glycosides are acetals formed from monosaccharides; outside bond to the anomeric carbon is the glycosidic bond.
  • Disaccharides:
    • Maltose: glucose–glucose, α(1→4); reducing sugar.
    • Cellobiose: glucose–glucose, β(1→4); reducing sugar.
    • Sucrose: glucose–fructose, α1→2; not reducing (both anomeric carbons involved).
    • Trehalose: glucose–glucose, α1→1; not reducing (no hemiacetal at ends).
  • Glycosidic bonds can be head-to-tail (1→4) or head-to-head (1→1, 1→2).

Polysaccharides

  • Polysaccharides are polymers of monosaccharides joined by glycosidic bonds.
  • Examples:
    • Starch: primarily α(1→4) linkages; consists of amylose (unbranched) and amylopectin (branched, includes α(1→6) points).
    • Cellulose: β(1→4) linkages; not digestible by humans; forms structural plant cellulose.
  • Hydrolysis of polysaccharides yields monosaccharides.

Mutarotation and Anomeric Stability

  • Open-chain form equilibrates with cyclic hemiacetals; mutarotation leads to mixture of α- and β-forms.
  • For glucose in solution: ~64% β and 36% α at equilibrium; the β form is more stable.
  • Optical rotation reflects the α:β composition; mutarotation equilibrates to a single rotation value for the mixture (~+53° for glucose).

Ribose, Deoxyribose, and Nucleic Acids

  • Ribose and deoxyribose are aldopentoses; ribose is the RNA sugar; deoxyribose lacks the 2'-OH group and is in DNA.
  • Their cyclic forms follow the same hemiacetal chemistry and produce glycosidic linkages in nucleic acids.

Reducing Sugars: Summary Points

  • Reducing sugars contain an aldehyde group (or a potential aldehyde) in open form and can be oxidized to carboxylic acids.
  • Tollens’ test detects reducing sugars via formation of a silver mirror.
  • D-Glucose is an aldohexose; mutarotation yields α- and β- anomers in equilibrium; β is more stable.

Lipids: Key Concepts

  • Lipids are defined by solubility: hydrophobic, nonpolar; insoluble in water, soluble in organic solvents.
  • Major lipid classes: fats & oils (triglycerides), waxes, phospholipids, steroids (cholesterol), and others.
  • Functions: energy storage, membranes, water repellence, signaling.

Fats, Oils, and Triglycerides

  • Structure: glycerol backbone with three fatty acids (triesters).
  • Hydrolysis (saponification) yields glycerol and fatty acids; base-catalyzed hydrolysis yields soap salts.
  • Glycerol: a triol; fatty acids are carboxylic acids with long hydrocarbon chains.

Fatty Acids: Saturated vs Unsaturated

  • Saturated fatty acids: no C=C bonds; chain packs tightly; high melting points.
  • Unsaturated fatty acids: one or more C=C bonds; cis configurations cause kinks; lower melting points.
  • Trans fats: partial hydrogenation converts some cis to trans, producing straighter chains with higher melting points; health concerns.
  • Common saturated fats (high mp): palmitic, stearic acids; cis-unsaturated fats have lower mp.

Hydrogenation and Margarine

  • Hydrogenation adds H₂ across C=C bonds; used to convert vegetable oils (liquid) to fats (solid or semi-solid).
  • Partial hydrogenation yields margarine with a mixture of saturated and cis/trans unsaturations.
  • Total hydrogenation yields fully saturated fats.

Iodine Number (Degree of Unsaturation)

  • Iodine number measures the grams of iodine that react with 100 g of fat/oil.
  • Higher iodine number = more double bonds (more unsaturation).
  • Oils typically have higher iodine numbers than fats.

Waxes and Phospholipids

  • Waxes: esters of long-chain alcohols and long-chain fatty acids; water-repellent.
  • Phospholipids: glycerol backbone with two fatty acids and a phosphate group (often linked to choline in lecithin).
  • Phospholipids are amphipathic and form lipid bilayers in cell membranes.

Cell Membranes and Lecithin

  • Lipid bilayer: hydrophobic tails in the interior; polar heads on the surface.
  • Lecithin (a major phospholipid) is a common component of cell membranes and an emulsifier.

Steroids: Cholesterol and Vitamin D

  • Steroids have a tetracyclic ring structure (three cyclohexane rings and one cyclopentane ring).
  • Cholesterol: essential membrane component, regulates fluidity; precursor to other steroids and vitamin D; can contribute to arterial plaque when in excess.

Antioxidants and Food Additives

  • Unsaturated fats are prone to oxidation; antioxidants like Butylated hydroxyanisole (BHA) scavenge free radicals to prevent rancidity.
  • Oxidation of fats leads to radical chain reactions and rancidity; antioxidants interrupt these processes.

Practical Lipid Concepts

  • The degree of saturation and chain length influence melting point and physical state (solid vs liquid).
  • Olestra: sucrose octaester used as a fat substitute; indigestible due to steric hindrance.
  • Phospholipids and cholesterol play crucial roles in membranes and health (LDL/HDL balance, atherosclerosis risk).

Quick Recap: Core Connections

  • Carbohydrates: aldehyde/ketone functionality → cyclic hemiacetals → α/β anomers; glycosidic bonds form disaccharides and polysaccharides; reducing vs non-reducing sugars determined by anomeric carbon availability.
  • Lipids: structure determines properties (triglycerides, phospholipids, waxes, steroids); hydrogenation controls hardness; iodine number gauges unsaturation; antioxidants protect against oxidation.