Question

8. Let us consider the transitions of ordered, helical conformations of polypeptides and polynu- cleotides to disordered states. In the most of the cases the enthalpy changes and entropy changes are positive as expected; heat is absorbed and entropy is gained. However, when the synthetic polypeptide polybenzyl-L-glutamate undergoes a transition from an ordered helix to a disordered coil in an ethylene dichloride-dichloroacetic acid solvent at 39°C and 1 atm, AH° = -4.0 kJ (per mole of amide) and AS = -12.0 J/K (per mole of amide). Page II (a) Give a possible explanation for the experimental result that heat is released and entropy is decreased for this transition. (b) Is the reaction spontaneous at 39°C? What thermodynamic criterion did you use to reach this conclusion? (c) At what temperature (°C) will the helix-coil reaction be reversible? This temperature is of- ten called the melting temperature of the helix. Assume that AHº and AS™ are independent of temperature.

Answer 1

Answer :- Given that the synthetic polypeptide poly benzyl- -L-glutamate undergo a transition from a ordera helix to a disordered coil The temps is, T = 39 c = (273 +39) « = 312K AHO = -4.0kJ (pen more of amide) = -4000 J (per more of amide). 45 = -12 Jik' (pero more of amide). Heat is relased :- During the transformation from highly ondered helix structure to a disordered round - om coil form, there are large number of strong Hydrogen bond are broken, which is present in highly ordered helix form. That's why heart is released ie AH <0 to Entropy decreased :- It is expected that on going from highly ordered state to a disordered state, entropy should increase. But here reverse occured. Because in the disordered form large number of Water molecentes are in assouated form, so there is decreemse in

randomness and hence entropy is negative 6 from thermodynamics we know that, 49° = AH – tasº =(40 5) – 6312,48 12 7.X') (poemalo = 64000 + 3744) 3 (per mole of 3-856 I (pero mole of amide) .:19° = -256 5 (per more of amride) 'amide) thence the reaction is spontaneous at 39°c. A Thermodynamic criterion :- According to the thermodynamics, a reaction is spontaneous when the Gibbs free energy change of the reaction is negative ie AG <0

According to the thermodynamic, a reaction is reversible or at equilibrium, when the AG=0. Given that Aho & Aso are independent on temperature. from thermodynamic, 46° = AHO - TASO => 0 = $-4000 s (per mole of amide)? ki/peso mole 0003 (pare Mall = - of amide) amide) ► 7 (-1234. Com cas ) = - 4003 ( → To 7 4000 g. (pore more of comide) + 12 J.K' (per mole of amid) > T = 333.334 >> T = (333-33 -273)°C => [T = (60.330°C Hence the reaction is reversible at (60.33) C temperature is called meeting tempe - rature of the helix.