L-Threonine is one of the essential amino acids. It is found in animal protein (for example, eggs and milk) but is missing in some grains, such as rice. It is now common practice to fortify grains with the essential amino acids that they lack. The structure of L-threonine is shown below.

Question

[latex]\begin{matrix} \underset{|}{OH} \quad\quad\quad\quad \ \ \underset{||}{O} \ CH_3-CH-CH-C-OH \ \ \ \overset{|}{NH_2} \ \end{matrix} [/latex]

[latex]pK_{a_1} = 2.15 \quad pK_{a_2} = 9.12[/latex]

Use the above data to determine the principal form of L-threonine and the direction that it will migrate under the influence of an electric field in a gel that is 0.25 M [latex]NaH_2PO_4 [/latex] and 0.50 M [latex]Na_2HPO_4[/latex].

Analyze
The key to this problem is to recognize that the [latex]NaH_2PO_4/Na_2HPO_4[/latex] mixture is a buffer that controls the pH of the gel. Once the pH of the gel is established we can identify the ionization state for the bulk of the L-threonine and hence the direction it will migrate.

Accepted Answer

Use [latex]pK_{a_2}[/latex] from Table 16.4 and the Henderson-Hasselbalch equation to calculate the pH of the gel.

[latex]pH = p K_{ a _2}+\log \left(\left[ HPO _4{ }^{2-} ight] /\left[ H _2 PO _4{ }^{-} ight] ight)[/latex]

and

pH = 7.20 + log(0.50/0.25) = 7.50

The structures of the three ionic forms in which L-threonine can appear are

[latex]\begin{matrix} Acidic solution: & & Isoelectric point: \ CH_3CH(OH)CHCOOH & & CH_3CH(OH)CHCOO^- \ \quad\quad \overset{|}{NH_3{ }^+} & \quad\quad\quad & \quad\quad \ \overset{|}{NH_3{ }^+} \end{matrix} [/latex]

[latex]\begin{matrix} Basic solution: \ CH_3CH(OH)CHCOO^- \ \quad\quad \ \overset{|}{NH_2} \end{matrix} [/latex]

The pH at the first equivalence point in the titration of a weak diprotic acid is given by equation (17.5). Applied to L-threonine, the result is

[latex] ext { for } H _2 PO _4{ }^{-}: \quad pH =\frac{1}{2}\left( p K_{ a _1}+ p K_{ a _2} ight)=\frac{1}{2}(2.15+7.20)=4.68[/latex] (17.5)

[latex]pH =\frac{ p K_{ a _1} + p K_{ a _2}}{2}=\frac{2.15 + 9.12}{2} = 5.64[/latex]

The predominant species at this first equivalence point of pH = 5.64 is the dipolar zwitterion, which would not migrate at all in an electric field. However, in the gel medium of pH = 7.50 the hydroxide ion concentration is 100 times as great. The predominant species is the anion with charge -1. This form of the L-threonine migrates toward the positive electrode, the anode.

Assess
This problem suggests two ways to establish the isoelectric point of an amino acid. The first, used here, is to calculate pI by combining [latex]pK_a[/latex] values of a polyprotic amino acid. Another way is to find the pH of a gel medium in which the amino acid does not migrate in the presence of an applied electric field, hence pH = pI. Additionally, migration through an electric field in a gel (gel electrophoresis) can be used to separate amino acid mixtures.

Question 28.Int.1: L-Threonine is one of the essential amino acids. It is found...

Related Answered Questions

Naming a Polypeptide What is the name of the polypeptide whose structure is shown below? ...

Verified Answer:

Verified Answer: