What is the net charge on the dipeptide Arg-Pro at pH 9.0? The table below gives the pKa?s of the ionizable groups on the free amino acids. $\begin{array}{|l|c|c|c|}
\hline \text { Free amino acid } & \begin{array}{c}
\text { pKa of } \
\alpha \text {-Carboxyl group }
\end{array} & \begin{array}{c}
\text { pKa of } \
\alpha \text {-amino group }
\end{array} & \begin{array}{c}
p \text { Ka of side } \
\text { chain }
\end{array} \
\hline \text { Arginine } & 1.8 & 9.0 & 12.5 \
\hline \text { Proline } & 2.0 & 10.6 & \cdots \
\hline
\end{array}$
A) -1
B) -0.5
C) 0
D) +0.5
E) +1

Question

Quizplus · Accepted Answer

The pKa of the α-amino group of Arg is 9.0, which means that at pH 9.0, half of the α-amino groups will be protonated and half will be unprotonated. The pKa of the side chain of Arg is 12.5, which means that at pH 9.0, almost all of the side chains will be protonated. The pKa of the α-carboxyl group of Pro is 2.0, which means that at pH 9.0, almost all of the α-carboxyl groups will be deprotonated. Therefore, the net charge on the dipeptide Arg-Pro at pH 9.0 will be +0.5.

What Is the Net Charge on the Dipeptide Arg-Pro at PH