Question

The dimerization of two alanine residues to form an alanine-alanine dimer can be used as a simple model for the protein folding process (see figure below). There are actually two processes involved-the formation of the dimer and reorganization of the surrounding water. 2. a) Estimate the ΔH and ΔS for each of the processes (<0, 0, >0). b) Estimate the AH and ΔS for the overall process (<0,0, >0).

Answer 1

(a)

When two alanine molecules combine to form a dimer, there is both a decrease in enthalpy as well as decrease in entropy.

This is because, when the dimer forms, the intermolecule forces dominate and hence the stability of dimer increase, which means that the reaction proceeds with release of heat, hence dH is negative.

Since dimer is a more structured form as compared to two monomers, so the entropy of the system also decreases because of decrease in randomness.

Thus, both dH<0 and dS<0.

For reorganization of water, dS > 0 because the H-bonds between water molecules are broken, and this also needs energy input, so dH > 0.

(b)

Overall, the entropy of system increases and the enthalpy decreases, because the increase in entropy caused by the reorganization of water molecules is far greater than the decrease caused due to dimer formation.

Hence, for overall process, dH < 0 and dS > 0, making it a spontaneous process at all temperatures.

Hope this helps !