Shown below is the titration curve for the amino acid glycine (side chain -H). What is the composition of glycine at pH 10.0?
100% H3N-CH2-COO
50% H3N-CH2-COO: 50% H2N-CH2-COO
50% H2N-CH2-COOH; 50% H2N-CH2-COO
100% H2N-CH2-COOH
The Correct Answer and Explanation is :
At pH 10.0, glycine predominantly exists in the form H₂N-CH₂-COO⁻. This corresponds to the fully deprotonated state, where the amino group (-NH₂) is neutral, and the carboxylate group (-COO⁻) carries a negative charge.
Glycine, the simplest amino acid, has two ionizable groups: a carboxyl group (-COOH) and an amino group (-NH₂). The carboxyl group has a pKa₁ around 2.34, and the amino group has a pKa₂ near 9.60. The pKa value represents the pH at which half of the molecules of that group are deprotonated.
The titration curve of glycine reflects these ionization events. At low pH, glycine exists primarily in its fully protonated form, H₃N⁺-CH₂-COOH. As the pH increases and surpasses pKa₁, the carboxyl group loses a proton, resulting in the zwitterionic form, H₃N⁺-CH₂-COO⁻, which predominates around the isoelectric point (pI) of approximately 6.0. Upon further increase in pH and exceeding pKa₂, the amino group deprotonates, leading to the fully deprotonated form, H₂N-CH₂-COO⁻.
At pH 10.0, which is above pKa₂, the equilibrium favors the deprotonated amino group. Therefore, glycine molecules predominantly exist as H₂N-CH₂-COO⁻, carrying a net negative charge. This understanding is crucial in biochemical applications, such as protein purification and enzyme activity studies, where the charge state of amino acids influences molecular interactions and function.
For a visual representation of glycine’s titration curve and its species distribution across different pH levels, you can refer to the following resource: