All amino acids have two ionizable groups (an alpha-amino group with pKa~9.3, and an alpha-carboxyl group with pKa~2.2). Lysine also has an ionizable side-chain (R) with a pKa of 10.5.
What percentage of the Lysine side chain is deprotonated at pH 11.5?
0.099%
9.1%
0.99%
90.9%
I got 0.99% ...am I right?
I don't think so. I'll look at your work if you post it.
To determine the percentage of the Lysine side chain that is deprotonated at pH 11.5, we need to compare the pH of the solution with the pKa of the side chain group.
The pKa of the Lysine side chain group is given as 10.5. This means that at a pH equal to the pKa, half of the side chain group will be deprotonated, and the other half will be protonated.
At pH 11.5, which is higher than the pKa of 10.5, the solution is more basic. In a basic solution, the tendency is for acidic groups to donate protons and become deprotonated.
Since the pKa of the side chain group is lower than the pH of the solution, a very small portion of the side chain group will be deprotonated. We can calculate the percentage of deprotonated Lysine side chain by using the Henderson-Hasselbalch equation:
Deprotonated fraction = 10^(-(pH - pKa))
Using the values given:
Deprotonated fraction = 10^(-(11.5 - 10.5))
= 10^(-1)
= 0.1
Therefore, the percentage of the Lysine side chain that is deprotonated at pH 11.5 is 0.1%, which is equivalent to 0.1/100 = 0.001 or 0.1%.
So, the correct answer is 0.1% or 0.099%, which is close to the option you selected, 0.99%. However, 0.99% is not the exact value, so the most accurate choice is 0.099%.