The formation of a double helix from two short DNA molecules CGCGATAT and ATATCG

Question

The formation of a double helix from two short DNA molecules CGCGATAT and ATATCGCG is spontaneous at 25 o C . The DG for the reaction is -54 kJ/mol and 256 kJ/mol of heat is released from the reaction. Please calculate the entropy change of this reaction and explain why it is negative. What kind of non-covalent interactions are involved in the formation of the double helix? As the temperature increases the double stranded DNA become less stable. At what temperature does the dissociation of double stranded DNA becomes favorable?

Explanation / Answer

delta G = -54 KJ/mol = -54000 J/mol

delta H = -266 KJ/mol = -266000 J/mol

T = 25 oC = (25+273) K =298 K

use:

delta G = delta H - T* delta S

-54000 = -256000 - 298*delta S

delta S =677.9 J/K/mol

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delta S is negative because there is less entropy on product side than reactant side. On product there are 1 molecules on product side as compared to 2 on reactant side . So product has lessentropy.

delta S =S(Product) - S (reactant).

Hence it is negative.

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It forms Hydrogen bonds.

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Temperature at which it becomes favourable is when delta G=0

delta G = delta H - T* delta S

0= -256000 + T*677.9

T= 377.6 K

Answer: 377.6 K

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