Chapter 17, Problem 127AP

Amino acids are building blocks of proteins. These compounds contain at least one amino group $\left({\text{-NH}}_{\text{2}}\right)$ and one carboxyl group $\left(\text{-COOH}\right)\text{. Consider glycine }\left({\text{NH}}_{\text{2}}{\text{CH}}_{\text{2}}\text{COOH}\right)$. Depending on the pH of the solution, glycine can exist in one of three possible forms:

Fully protonated: ^- ${\text{NH}}_{\text{5}}{\text{-CH}}_{\text{3}}\text{-COOH}$

Dipolar ion: ${}^{\text{-}}{\text{NH}}_{\text{3}}{\text{-CH}}_{\text{2}}{\text{-COO}}^{\text{-}}$

Fully ionized: ${\text{NH}}_{\text{2}}{\text{-CH}}_{\text{2}}{\text{-COO}}^{\text{-}}$

Predict the predominant form of glycine at pH 1.0, 7.0. and 12.0. The $\text{p}{K}_a$ of the carboxyl group is 2.3 and that of the ammonium group ${\text{(NH}}_3^{+}\text{)}$ is 9.6.