§ C-40 Biopolymers: Peptide Bond Planarity, Protein α-Helix & DNA Double Helix (A=T, G≡C) (Grade 10–11 · Biochemistry: Proteins & DNA)
Chapter XL. Pauling–Corey α-Helix (3.6 res/turn) vs. Watson–Crick B-DNA, Chargaff’s Rules & Melting Tₘ. «It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material.», James D. Watson & Francis H. C. Crick, 1953
Theory Summary & Scientific Mechanism
Biopolymers (proteins and nucleic acids) fold into regular 3D secondary structures stabilized by cooperative networks of directional hydrogen bonds.
In the Pauling–Corey protein α-helix (3.6 residues/turn, pitch 0.54 nm), planar trans peptide bonds (–CO–NH–) coil into a right-handed helix where every backbone C=O of residue i hydrogen-bonds to the amide N–H of residue i+4.
In Watson–Crick B-DNA (10 bp/turn, pitch 3.4 nm), two antiparallel sugar-phosphate strands pair via complementary base hydrogen bonding obeying Chargaff’s rules: Adenine–Thymine (A=T) forms 2 H-bonds, whereas Guanine–Cytosine (G≡C) forms 3 H-bonds. Consequently, the thermal melting temperature Tₘ increases linearly with %GC content: Tₘ ≈ 69.3 + 0.41(%GC).
Everyday Analogy
The DNA double helix is a twisted ladder whose rungs are held together by hydrogen-bond "magnets": A=T pairs have 2 H-bonds while G≡C pairs have 3 H-bonds! That extra bond makes GC-rich DNA much harder to melt apart by heat.
Interactive Question & Derivation
Why does a DNA double helix with a higher guanine–cytosine (G≡C) content have a significantly higher thermal denaturation (melting) temperature Tₘ than an A=T-rich duplex?
- A: Watson–Crick G≡C base pairs are held together by three complementary hydrogen bonds and stronger π–π base stacking, whereas A=T pairs form only two hydrogen bonds (Tₘ = 69.3 + 0.41(%GC))
- B: Adenine pairs with cytosine via four covalent bonds, while guanine pairs with thymine via one bond
- C: Heating DNA strengthens hydrogen bonds so the two antiparallel strands can never separate
Study Hint: Watson–Crick A=T pairs form 2 H-bonds whereas G≡C pairs form 3 H-bonds, raising the melting temperature Tₘ with %GC.
Proof & Explanation: Biological macromolecules rely on cooperative hydrogen bonding for their 3D secondary structure: 1) In the Pauling–Corey protein α-helix (3.6 amino acid residues per turn, pitch 0.54 nm), each backbone carbonyl C=O of residue i forms an intramolecular H-bond with the amide N–H of residue i+4, while the peptide bond –CO–NH– remains rigid and planar due to sp² resonance. 2) In Watson–Crick B-DNA (10 bp/turn, pitch 3.4 nm), two antiparallel polynucleotide strands pair via complementary purine–pyrimidine H-bonds: A=T (2 H-bonds) and G≡C (3 H-bonds), obeying Chargaff’s rule ([A]=[T], [G]=[C]).
Key Equations & 3D Simulation Parameters
Chargaff: [A]=[T], [G]=[C], N_H-bonds = 2 N_AT + 3 N_GC, Tₘ ≈ 69.3 + 0.41 (%GC) [^°C]
- Biopolymer (1=Protein α-Helix, 2=B-DNA Double Helix): 1 … 2 (default: 2)
- G≡C base-pair content (% G≡C, 3 H-bonds) / stability: 10 … 90 % (default: 50 %)
- Number of biopolymer helix turns Nₜurns: 2 … 6 turns (default: 4 turns)
- Solution temperature t (thermal denaturation above Tₘ): 20 … 95 °C (default: 37 °C)