Question 21.5: Peptide Formation Show how to form the dipeptide aspartylser...
Peptide Formation
Show how to form the dipeptide aspartylserine (Asp—Ser).
Strategy
Start by drawing the two amino acids. Orient them so that both read (from left to right): amino group, alpha carbon, carboxyl group. Then draw the reaction between the first amino acid’s carboxyl group and the second amino acid’s amino group to give the peptide bond.
The name implies that this dipeptide is made of two amino acids, aspartic acid (Asp) and serine (Ser), with the amide bond being formed between the α-carboxyl group of aspartic acid and the α-amino group of serine. Therefore, we write the formula of aspartic acid with its amino group on the left side. Next, we place the formula of serine to the right, with its amino group facing the α-carboxyl group of aspartic acid. Finally, we eliminate a water molecule between the —\text{COO}^{-} and —{\text{NH}_{3}^{+}} groups that are next to each other, forming the peptide bond:
\begin{matrix}\underset{||}{\text{O}}\hspace{97 pt}\underset{||}{\text{O}}\hspace{106 pt}\underset{||}{\text{O}}\hspace{59 pt}\underset{||}{\text{O}}\\ \text{H}_{3}\overset{+}{\text{N}}-\underset{|}{\text{C}}\text{H}-\text{C}-\text{O}^{-} + \text{H}_{3}\overset{+}{\text{N}}-\underset{|}{\text{C}}\text{H}-\text{C}-\text{O}^{-}\longrightarrow \text{H}_{3}\overset{+}{\text{N}}-\underset{|}{\text{C}}\text{H}-\text{C}-\underset{|}{\text{N}}-\underset{|}{\text{C}}\text{H}-\text{C}-\text{O}^{-} + \text{H}_{2}\text{O}\\ \hspace{-27 pt}\underset{|}{\text{C}}\text{H}_{2}\hspace{87 pt}\text{CH}_{2}\text{OH}\hspace{79 pt} \underset{|}{\text{C}}\text{H}_{2}\hspace{29 pt}\text{H}\hspace{11 pt}\text{CH}_{2}\text{OH}\\ \hspace{-97 pt} \text{COO}^{-}\hspace{191 pt}\text{COO}^{-}\\ \hspace{-51 pt}\substack{\text{Asp}}\hspace{97 pt}\substack{\text{Ser}}\hspace{139 pt}\substack{\text{Asp—Ser}} \end{matrix}■ Quick Check 21.4
Show how to form the dipeptide valylphenylalanine (Val—Phe).