A thermodynamic treatment allows predictions of the stability of DNA. The table below lists the standard Gibbs energies, enthalpies, and entropies of formation at 298 K of short sequences of base pairs as two polynucleotide chains come together:
To estimate the standard Gibbs energy of formation of a double stranded piece of DNA, ∆DNAG, we sum the contributions from the formation of the sequences and add to that quantity the standard Gibbs energy of initiation of the process, which in the case treated in this exercise may be set equal to ∆initG = 14.2 kJ/mol: ∆DNAG = ∆initG + ∆seqG(sequences)
Similar procedures lead to ∆DNAH and ∆DNAS.
a) Briefly explain why ∆Ginit > 0 J/mol b) Estimate the standard Gibbs energy, enthalpy, and entropy changes for the following reaction (formation of a double strand): 5′-A-G-C-T-G-3′ + 5′-C-A-G-C-T- 3′ → 5′-A-G-C-T-G-3′ 3′-T-C-G-A-C-5′ Use ∆initH = 2.5 kJ/mol ݈and ∆initS = -37.7 J/K/mol .݈ c) Estimate the ‘melting’ temperature of the piece of DNA shown in part (b)