Organic Chemistry: Concepts and Applications - Headley Allan D. 2020

Synthetic Polymers and Biopolymers
20.9 Acid—Base Properties of Amino Acids

As is obvious from the structure of amino acids, they are both acidic and basic molecules — they contain a carboxylic acid functionality and an amine functionality. In solutions of different pH, amino acids exist in different forms, as shown in Reaction (20-15).

(20-15)Image

Problem 20.4

1. Give the structure of (R)-alanine at a pH of 10 (basic solution).

2. Give the structure of valine at a pH of 1 (acidic solution).

Note that in solutions of low or high pH, the actual amino acid carries a net charge, i.e. at low pH, the charge is positive, and at high pH, the net charge is negative. For each amino acid, there is a pH where the net charge will be zero. This pH is close to neutral, but not necessarily pH 7. This pH is also referred to as the pI, the isoelectric pH. As we know from physics, like charges will repel each other and unlike charges will be attracted to each other. If amino acids are placed in a solution of specified pH and electrodes are immersed in the solution, migration of the charged amino acids will occur. At low pH, the protonated amino acid (positively charged amino acid) will migrate toward the negative electrode, and at high pH, the anionic amino acid will migrate to the anode. This process is known as electrophoresis.

Problem 20.5

To what electrode (anode or cathode) will leucine go in a solution of pH 11.0? Explain your answer.