Question

The increasing order of $p{k}_a$ for the following amino acids (Gly, Asp, Lys, Arg) in aqueous solution is:

• A. Asp<Gly<Arg<Lys
• B. Arg<Lys<Gly<Asp
• C. Gly<Asp<Arg<Lys
• D. Asp<Gly<Lys<Arg

Answer 1

The correct option is D Asp<Gly<Lys<Arg

The structures of the above given amino acids are:$\mathrm{HOOC}-{\mathrm{CH}}_2-\mathrm{CH}\left({\mathrm{NH}}_2\right)-\mathrm{COOH}\mathrm{Aspartic}\mathrm{acid}{\mathrm{NH}}_2-{\mathrm{CH}}_2-\mathrm{COOH}\mathrm{Glycine}{\mathrm{NH}}_2-\mathrm{CH}{\left(\mathrm{NH}\right)}_2-{\left({\mathrm{CH}}_2\right)}_3-\mathrm{CH}\left({\mathrm{NH}}_2\right)-\mathrm{COOH}\mathrm{Arginine}{\mathrm{NH}}_2-{\left({\mathrm{CH}}_2\right)}_4-\mathrm{CH}\left({\mathrm{NH}}_2\right)-\mathrm{COOH}\mathrm{Lycine}$
Based on the acidic strength,
Aspartic acid > glycine >arginine>lycine
So, the increasing order of $p{k}_a$ is
Asp<Gly<Arg<Lys