Valine (Val) and lysine (Lys) are amino acids. Their structures are given in the Data Booklet. The isoelectric point of an amino acid is the pH at which it exists as a zwitterion. Valine has an isoelectric point of $6.0$. Lysine has an isoelectric point of $9.7$.
(a)[1]
Draw the structure of valine at pH $6.0$.
(b)[3]
A lysine solution is prepared with pH $9.7$. Dilute sulfuric acid is then added slowly until the pH of the solution reaches $1.0$. The sulfuric acid reacts with lysine to form different organic ions that are not present in significant amounts at pH $9.7$. In the boxes, draw the structures of three organic ions formed during the addition of sulfuric acid. Put the organic ion present at pH $1.0$ in box C.
(c)[2]
Draw the structure of the dipeptide Val-Lys. The peptide bond should be displayed in full.
Worked solution & mark scheme
This 6-mark question has a full step-by-step worked solution and mark scheme. One marking point: “Accurate zwitterionic amino acid structure” …