The solubility of two amino acids in two solvents at 25 degree C are given below

Question

The solubility of two amino acids in two solvents at 25 degree C are given below; the numbers represent the concentrations present in saturated solutions. a) Calculate the standard free energy change Delta mu degree for transfer of 1 mol of glycine from the solid to aqueous solution at 25 degree C. [Recall the standard state of solid is the pure solid and the standard state for solution is 1 M, assuming ideal behavior and using the molarity scale]. b) Calculate the standard free energy of transfer Delta mu degree of 1 mol of glycine from ethanol to the aqueous solution at 25 degree C. c) Assuming that the effects of the amino acid backbone and side chain are additive (glycine essentially has no side chain) calculate the standard free energy of transfer of the (CH_3)_2CH- side chain of valine from water to ethanol at 25 degree C. d) Ethanol can be considered to mimic the interior of a protein. Will the mutation of a glycine to a valine in the interior of a protein favor the folding of the protein in water if interaction with the solvent is the dominant effect?

Explanation / Answer

From the solubility data of the amino acids

a) standard free energy change

= -RTlnK

= -8.314 x 298 ln(3.09)

= -2.795 kJ/mol

b) standard free energy change

= -RTlnK

= -8.314 x 298 ln(4.04 x 10^-4)

= 19.360 kJ/mol

c) For valine,

dG = -8.314 x 298 ln(1.32 x 10^-3/0.6)

      = 15.161 kJ/mol

d) With ethanol mimicking the interior of a protein, the mutarotation would faor folding of protein in water as the solvent due to strong hydrogen bonding.

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