Question 20.3: Draw the structural formula for the tripeptide Ala–Gly–Val....

Step 1: The N-terminal end of the peptide involves alanine. Its structure is written first.
\begin{matrix} H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ \ \ CH_{3} \end{matrix}

Step 2: The structure of glycine is written to the right of the alanine structure, and a peptide bond is formed between the two amino acids by removing the elements of H_{2}O and bonding the N of glycine to the carboxyl C of alanine.

\begin{matrix} H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ \ \ CH_{3} \end{matrix} + \begin{matrix} H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ H \ \ \ \end{matrix} → \begin{matrix} H \quad \ O \quad \ H \quad \ H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-\overset{||}{C}-\overset{|}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ CH_{3} \quad \quad \quad\quad H \quad \end{matrix} + H_{2}O

Step 3: To the right of the just-formed dipeptide, draw the structure of valine. Then repeat Step 2 to form the desired tripeptide.

\begin{matrix} H \quad \ O \quad \ H \quad \ H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-\overset{||}{C}-\overset{|}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ CH_{3} \quad \quad \quad\quad H \quad \end{matrix} + \begin{matrix} H \ \ \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-COO^{-} \\ \quad\quad\quad \ \underset{|}{C} H-CH_{3} \\ \ CH_{3} \end{matrix} → \begin{matrix} \ \ H \quad \ O \quad \ H \quad \ H \quad \ O \quad \ H \quad \ H \quad \\ H_{3}\overset{+}{N}-\overset{|}{\underset{|}{C}}-\overset{||}{C}-\overset{|}{N}-\overset{|}{\underset{|}{C}}-\overset{||}{C}-\overset{|}{N}-\overset{|}{\underset{|}{C}} -COO^{-} \\ \quad\quad\quad CH_{3} \ \ \ \ \ \quad\quad\quad H \ \ \ \ \ \quad\quad\quad\quad \underset{|}{C}H – CH_{3} \\ \quad\quad\quad\quad\quad\quad\quad\quad \ \ \quad\quad\quad\quad CH_{3}\end{matrix} + H_{2}O