Biomolecules
Biomolecules questions check structures of sugars, the peptide and nucleic-acid backbone, and the vocabulary of denaturation, anomers and reducing sugars.
Carbohydrates
- glucose is an aldohexose; fructose is a ketohexose; both exist mainly as cyclic hemiacetals
- anomers differ only at the anomeric carbon (C1 of glucose, C2 of fructose)
- mutarotation is the change in specific rotation as α- and β-forms equilibrate in solution
- reducing sugars have a free anomeric hydroxyl: glucose, fructose, maltose, lactose
- sucrose is non-reducing because its glycosidic bond links C1 of glucose to C2 of fructose; hydrolysis gives glucose and fructose (invert sugar)
Proteins and enzymes
- amino acids are zwitterionic near neutral pH; the isoelectric point is the pH of zero net charge
- the peptide bond is an amide linkage formed by condensation; primary structure is the sequence, secondary structure includes α-helix and β-pleated sheet
- denaturation destroys secondary and tertiary structure but not the primary sequence
- enzymes are specific biocatalysts; a competitive inhibitor resembles the substrate
Nucleic acids
- DNA: deoxyribose, thymine, double helix with A–T and G–C base pairs
- RNA: ribose and uracil, usually single-stranded
- purines are adenine and guanine; pyrimidines are cytosine, thymine and uracil
Worked example
Sucrose does not reduce Tollens' or Fehling's reagent, yet after boiling with dilute acid the solution does. Hydrolysis cleaves the glycosidic bond, freeing the anomeric hydroxyls of glucose and fructose, which are reducing sugars - this is why hydrolysed sucrose behaves as a reducing mixture.
Traps that cost marks
- Calling sucrose a reducing sugar.
- Saying denaturation breaks peptide bonds.
- Mixing up which base occurs in RNA but not DNA (uracil).