Chemistry Intermolecular Forces Solution: The DNA double helix (Figure25.40) at the atomic l...

Solution: The DNA double helix (Figure25.40) at the atomic level looks like a twisted ladder, where the "rungs" of the ladder consist of molecules that are hydrogen-bonded together. Sugar and phosphate groups make up the sides of the ladder. Shown are the structures of the adenine-thymine (AT) "base pair" and the guanine-cytosine (GC) base pair.You can see that AT base pairs are held together by two hydrogen bonds, and the GC base pairs are held together by three hydrogen bonds.Which base pair is more stable to heating?

Problem

The DNA double helix (Figure25.40) at the atomic level looks like a twisted ladder, where the "rungs" of the ladder consist of molecules that are hydrogen-bonded together. Sugar and phosphate groups make up the sides of the ladder. Shown are the structures of the adenine-thymine (AT) "base pair" and the guanine-cytosine (GC) base pair.
You can see that AT base pairs are held together by two hydrogen bonds, and the GC base pairs are held together by three hydrogen bonds.
Thymine and adenine are joined by 2 H-bonds. Thymine is a 6-membered ring with N at points 1 and 3. There is a double bond between C5 and C6. N1 is single bonded to the sugar; C5 is bonded to CH3; C2 and C4 are each double bonded to an O. N3 and the O double bonded to C4 are H-bonded to H atoms in adenine. Adenine is bicyclic, consisting of a 6-membered ring joined to a 5-membered ring. The 6-membered ring has N at points 1 and 3, and shares C4 and C5 with the 5 membered ring. C6 is bound to an NH2 group. There are double bonds between C2 and N3, C4 and C5, and N1 and C6 of the 6-membered ring (total 3). C5 is bound to N7 and C4 is bound to N9 of the 5-membered ring. N9 is single-bonded to the sugar. There is a double bond in the 5-member ring between N7 and C8. N1 H-bonds to thymines H bonded to N3, and an H of NH2 is bonded to the O attached to adenines C6.
Cytosine and guanine are joined by 3 H-bonds. Cytosine is a 6-membered ring with N at positions 1 and 3 and an NH2 group bonded to C4. C2 is double bonded to an O. There are double bonds in the ring, 1 between N3 and C4 and another between C5 and C6. N1 is bound to the sugar. The NH2 group, N3 and the O all engage in H-bonding with guanine. Guanine is bicyclic and structurally similar to adenine. C6 is double-bonded to an O (instead of an NH2 group). C2 is bonded to an NH2 group. Guanine is lacking a double-bond between N1 and C6. N9 is bound to the sugar. The NH2 connected to C2 bonds with cytosines O. Guanines H bonded to N1 binds with cytosines N3. And guanines O binds with an H of NH2 group bonded to C4 of cytosine.

Which base pair is more stable to heating?

Solution

  • Intermolecular forces make structure more durable and resistant to harsh conditions. The stronger IMFs present, the stronger/stable the structure is

  • One of the strong IMF present are H-bonds

  • Analyzing the base pairs, AT base pair has 2 H-bondings present on it (illustrated by blue bonds)

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