Below is the structure of a three amino acid portion of a protein A. Fill in the

Question

Below is the structure of a three amino acid portion of a protein A. Fill in the pKa values for the amino acid sidechains in the boxes with structure A. If this protein was in a pH 11 buffer, what would be the protonation state of these amino acids indicate this in the circles on structure B (either a H or a charge). Using structures and words, briefly rationalize the difference in acidity for the two far left amino acids in structure A above, using one of the five criteria discussed in lecture. Indicate which is the more acidic sidechain and why.

Explanation / Answer

for -CH2SH : pKa = 8

for -CH2OH pka = 13

for C6H5-OH pKa = 10.07

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when the pH of the medium is greater than pka then side groups will be deprotonated.

for -CH2SH : pKa = 8   it will exist as -CH2S- (-1 charge)

for -CH2OH pka = 13    No change

for C6H5-OH pKa = 10.07 it will exist as C6H5O-    (-1 charge)

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