$$\begin{array}{l}
\text { Exhibit } \mathbf { 3 A } \text { (for this chapter) }\
\begin{array} { l l l l }
\text { Amino acid } & \boldsymbol { \alpha } \text {-carboxyl group pK a } & \boldsymbol { \alpha } \text {-amino group pK a } & \text { R-group pK } \
\hline \text { Ala } & 2.3 & 9.7 & \
\text { Arg } & 2.2 & 9.0 & 12.5 \
\text { Asn or Gln } & 2.1 & 9.0 & 4.0 \
\text { Asp or Glu } & 2.2 & 9.8 & 8.3 \
\text { Cys } & 1.7 & 10.8 & 6.0 \
\text { His } & 1.8 & 9.2 & \
\text { Ser } & 2.2 & 9.2 & 10.1
\end{array}
\end{array}$$
-Refer to Exhibit 3A. At pH 7, what percent of histidine has a neutral charge?

Question

$$\begin{array}{l}
\text { Exhibit } \mathbf { 3 A } \text { (for this chapter) }\
\begin{array} { l l l l } 
\text { Amino acid } & \boldsymbol { \alpha } \text {-carboxyl group pK a } & \boldsymbol { \alpha } \text {-amino group pK a } & \text { R-group pK } \
\hline \text { Ala } & 2.3 & 9.7 & \
\text { Arg } & 2.2 & 9.0 & 12.5 \
\text { Asn or Gln } & 2.1 & 9.0 & 4.0 \
\text { Asp or Glu } & 2.2 & 9.8 & 8.3 \
\text { Cys } & 1.7 & 10.8 & 6.0 \
\text { His } & 1.8 & 9.2 & \
\text { Ser } & 2.2 & 9.2 & 10.1
\end{array}
\end{array}$$
-Refer to Exhibit 3A. At pH 7, what percent of histidine has a neutral charge?

Quizplus · Accepted Answer

At pH 7, the histidine side chain has a neutral charge because the pKa of the imidazole group is 6.0. At pH 7, the imidazole group is 50% protonated and 50% deprotonated, resulting in a neutral charge.