Justify your answer 6 qjk2 continued part c 6 points

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Unformatted text preview: of the histidine in the mutant protein? Note that under chemical standard conditions the standard states for all reactants and products, including H+, is assumed to be 1 M. (iii) (3 points) The protein undergoes a mutation so that an aspartic acid sidechain is placed next Q2 A (10 points) Consider a histidine sidechain on the surface of a protein. The pKA value of the histidine is 6.5, and the temperature is 300 K. (i) (3 points) What fraction of the protein molecules in a solution would have this histidine residue protonated at pH 7.0? (ii) (4 points) The acid/base reaction governing histidine protonation is: His + His + H + Assume that the pH is held fixed at 7.0 ([H+]=10-7) due the presence of a buffer. Under these conditions, the equilibrium constant KA (acid dissociation constant) is given by: KA = [ His ] ⎡ H + ⎤ ⎣⎦ ⎡ His + ⎤ ⎣ ⎦ . What is the value of the standard free energy change for the reaction (chemical standard states) ( ΔG 0 )? (iii) (3 points) The protein undergoes a mutation so that an aspartic acid sidechain is placed next to the histidine. The aspartic acid stabilizes the protonated form of the histidine, so that the value of ΔG 0 for histidine deprotonation is +60 kJ.mol-1 for the mutant protein. What is the pKA value of the histidine in the mutant protein? Note that under chemical standard conditions the standard states for all reactants and products, including H+, is assumed to be 1 M. Final Exam MCB C100A / Chem C130 Fall 2012 Question 8. (15 points total) A student in a biochem lab made a buffer solution using imidazole for studies of a protein. She looked up the thermodynamics for the ionization reaction: ImH+ → Im + H+ and found ΔG° = +34 kJ mol-1 (chemist’s standard state), and ΔH° = + 30 kJ mol-1, both at 25° C. Q8. A. (8 points) She dissolved 0.001 moles of ImH+ and 0.001 moles of Im in 100 ml of water. What will the pH of the buffer solution be? Q8. B. (7 points) Before she put her protein into the buffer she realized she needed to add some salt. She found a concentrated stock solution of sodium chloride that she could add, but it was at a pH of 3 (but no buffer in it). If sh...
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