Question

Assume that majority of the heat supplied went into melting the DNA molecules. Provide a method (including equations for calculation) for the researcher to determine the number of A:T pairs in one DNA molecule. (Hint: the researcher knows that the energy for breakingg a hydrogen bond is Delta_ε.)

Answer 1

Basically, bond energy is a measure of the amount of energy needed to break apart one mole of covalently bonded gases. The SI units used to describe bond energy is kiloJoules per mole of bonds (kJ/mol).

Melting DNA molecules involves breaking the hydrogen bonds to make them single starnded from double stranded.

In this case we assume that X kJ/mol of energy has been supplied to a system which contains the DNA molecules. Now, we already know that most of the energy has been utilized to break the hydrogen bonds.

There are 2 H-bonds between A and T ( 1 H-N bond and 1 H-O bond) and 3 H-bonds between G and C ( 1 H-N bond and 2 H-O bond).

The average bond energy of H-N is 391 kJ/mol and of H-O is 467 kJ/mol. But for easier calculation we can assume that the bond energies for both bonds is 420 kJ/mol.

Thus the equation becoms:

X kJ/mol = 2(420 kJ/mol)n + 3(420 kJ/mol)m

Where solving for 'n' will give the number of A-T pairing.