Question

a. Use bond energies to estimate ΔH for the reaction of two molecules of glycine to form a peptide linkage.

b. Would you predict ΔS to favor the formation of peptide linkages between two molecules of glycine?

c. Would you predict the formation of proteins to be a spontaneous process?

Short answer

(a) ΔH = -23 kJ/mol

(b) We would not expect to this reaction to be favourite

(c) We would not predict the formation of peptide bond to be spontaneous

Step-by-step solution

To estimate ΔH

Formation of peptide linkage involves breaking one C-O, and one N-H bond and forming one C-N and one H-O bond.

For bonds to be broken, energy must be added to the system – an endothermic process. Therefore, the energy terms associated with bond breaking have positive signs.

The formation of a bond releases energy – an exothermic process. Therefore, the energy terms associated with bond making have a negative sign.

The enthalpy change for a reaction is equal to the sum of the energies required to break old bonds plus the sum of energies released in the formation of new bonds.

ΔH = ∑ D(bonds broken) - ∑D (bonds formed)

ΔH = (D C-O+D N-H)- (D C-N+ D O-H)

ΔH = (358 kJ/mol +391 kJ/mol) – (305 kJ/mol +467 kJ/mol)

ΔH = - 23 kJ/mol

Hence ΔH = - $23$kJ/mol

To predict ΔS

We would not expect the reaction of peptide bond creation to be favourable. If we take a look into the reaction process, the same number of molecules exist before and after the reaction. Before the reaction, there were two peptides. Since peptide bond-forming is a hydrolysis reaction, one dipeptide and molecule of water is created. Therefore, a number of molecules do not influence the entropy. Enthalpy of the reaction is positive because we have to bring some energy to form peptide bond and Gibbs free energy is positive as well, the reaction should have lower entropy.

Hence, we would expect to this reaction to be unfavourable as entropy decrease.

To predict the formation of proteins

We would not expect for this reaction to be spontaneous because Gibbs free energy is positive. Since $∆\mathrm{H}$is positive and $∆\mathrm{S}$is negative, $∆\mathrm{G}$ is positive as well. Therefore, $∆\mathrm{G}$is positive, which indicates a non-spontaneous reaction. Hence, we would not predict the formation of peptide bonds to be spontaneous.