Question

Amino acids are the building blocks for all proteins in our bodies. A structure for the amino acid alanine is

All amino acids have at least two functional groups with acidic or basic properties. In alanine, the carboxylic acid group has $K_a=4.5\times {10}^{-3}$and the amino group has. Because of the two groups with acidic or basic properties, three different charged ions of alanine are possible when alanine is dissolved in water. Which of these ions would predominate in a solution with$\left[H^{+}\right]=1.0M?$In a solution with$\left[O{H}^{-}\right]=1.0M?$

Short answer

The ions that would predominate in a solution

With $\left[H^{+}\right]=1.0M$

With $\left[O{H}^{-}\right]=1.0M$

Step-by-step solution

Definition of amino acids

Molecules that combine to form proteins are defined as amino acids.

The ions that would predominate in a solution with [H+]=1.0 M.

The predominant species of an amino acid depends on the pH of the solution.

$\begin{array}{l}pH=-log(1.0M)\\ pH=0\\ p{K}_{a}ofCOOH=-log(4.5\times {10}^{-3})\\ p{K}_{a}ofCOOH=2.35\end{array}$

$\begin{array}{l}{K}_{a}ofN{H}_3^{+}group=1.35\times {10}^{-10}\\ p{K}_{a}ofN{H}_3^{+}group=-log(1.35\times {10}^{-10})\\ p{K}_{a}ofN{H}_3^{+}group=9.87\end{array}$

The pH of the solution is much lesser than the of the carboxylic acid group and the amino group. Hence, both the carboxylic groups and the amino group of alanine are protonated. The alanine molecule has a net positive charge

The ions that would predominate in a solution with .

The pH is,

$\begin{array}{l}\left[O{H}^{-}\right]=1.0M\\ pOH=-log(1.0M)\\ pOH=0\\ pH=14\end{array}$

The pH of the solution is much higher than the of the carboxylic acid group and the amino group. Hence, both groups are deprotonated at this pH. The alanine bears a net negative charge.