EasyChemistryHigh School (11–12)Organic Compounds with Functional Groups › BiomoleculesNumerical

Three amino acids have no ionisable side chain…

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Three amino acids have no ionisable side chain. Serine has $\text{p}K_1 = 2.21$ and $\text{p}K_2 = 9.15$ . Leucine has $\text{p}K_1 = 2.36$ and $\text{p}K_2 = 9.60$ . Threonine has $\text{p}K_1 = 2.09$ and $\text{p}K_2 = 9.10$ . Find the isoelectric point of each.

Need a reminder? The key ideaShow

The isoelectric point is the pH at which the average charge is zero. For a neutral side chain, $\text{pI} = (\text{p}K_1 + \text{p}K_2)/2$ . For other amino acids, average the two pK values on either side of the neutral form. The fraction of a group that is still protonated is $\dfrac{1}{1 + 10^{\text{pH} - \text{p}K}}$ for a group that is positive when protonated.

The worked solution

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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.

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