Below is the structure of a three amino acid portion of a protein A. A) Fill in
ID: 487654 • Letter: B
Question
Below is the structure of a three amino acid portion of a protein A. A) Fill in the pKa values for the amino acid sidechains in the boxes with structure A. B) 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). C) 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
The pKa value of phenolic side chain is 10.07 (first circle from the left) , the value of hydroxyl group is (-OH) 10.5 and that of -SH group is 8.00. The values are indicating that -SH group is strongly acidic than others. This is due to difference size of atoms between S and H which makes the bond to break easily. In comparing with phenol and alcohol, the phenol is more acidic because the phenoxide ion formed is stable by resonance. So the phenoxide ion will form easily in the solution giving H+ ions.
At pH=11 the solution is strongly basic and the phenol group and -SH group will be deprotanated and -OH will remain as it is.
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