Question

Draw a structural formula for the form of each amino most prevalent at pH \(10.0\). (a) Leucine (b) Valine (c) Proline (d) Aspartic acid

Short answer

Question: Draw the structural formulas of Leucine, Valine, Proline, and Aspartic acid at pH 10.0.

Step-by-step solution

a) Leucine

First, let's examine Leucine's structure. Leucine has the following chemical formula: H\(_2\)N-CH- (CH\(_2\)CH\(_2\)CH)\(_2\)(CH\(_3\))_2\(. As the given pH is 10.0, the amino group (NH\)_2\() will accept a proton and become charged as NH\)_3^+\(, and the carboxyl group (COOH) will lose a proton and become charged as COO\)^-\(. Thus, the structural formula for Leucine at pH 10.0 is: H\)_3\(N\)^+\(-CH- (CH\)_2\(CH\)_2\(CH)\)_2\((CH\)_3\()_2\) -COO^-$.

b) Valine

Valine's chemical formula is H\(_2\)N-CH-CH\(_3\)-(CH\(_3\))\(_2\)(CH\(_3\)). Similar to Leucine, the amino group will accept a proton due to the given pH of 10.0, resulting in the charged form NH\(_3^+\). The carboxyl group will lose a proton and become charged as COO\(^-\). The corresponding structural formula for Valine is: H\(_3\)N\(^+\)-CH-CH\(_3\)-(CH\(_3\))\(_2\)(CH\(_3\)) -COO^-$.

c) Proline

For Proline, the chemical formula is H\(_2\)N-CH-(CH\(_2\))\(_3\)-NH. Due to the pH of 10.0, the amino group will accept a proton, and the carboxyl group will lose a proton. Thus, Proline's structural formula becomes H\(_3\)N\(^+\)-CH-(CH\(_2\))\(_3\)-NH -COO^-$.

d) Aspartic acid

Aspartic acid's chemical formula is H\(_2\)N-CH-CH\(_2\)-COOH. In this case, the amino group will accept a proton, and the carboxyl group will lose a proton, similar to the other amino acids. However, the side chain also contains a carboxyl group (COOH). Since the given pH is 10.0, this side chain carboxyl group will also lose a proton. The resulting structural formula for Aspartic acid is: H\(_3\)N\(^+\)-CH-CH\(_2\)-COO\(^-\) -COO^-$. You now have the structural formulas for Leucine, Valine, Proline, and Aspartic acid at pH 10.0!

Key concepts

Leucine structure

Leucine is an essential amino acid, meaning our body cannot produce it, and we must obtain it through our diet. It plays a crucial role in protein synthesis and muscle repair. The structural formula of Leucine is:

• General formula: C\(_6\)H\(_{13}\)NO\(_2\)
• Amino group: H\(_2\)N
• Alpha carbon bonded to: two CH\(_3\) groups, one H, and one side chain
• Side chain: Isobutyl group (CH\(_2\)CH(CH\(_3\))\(_2\))

At a pH of 10.0, which is alkaline, the amino group in Leucine accepts a proton and becomes positively charged as \( \text{NH}_3^+ \). Additionally, the carboxyl group (COOH) loses a proton and becomes negatively charged as \( \text{COO}^- \). These changes result in a zwitterionic form of Leucine, which is typical under these conditions. This property of amino acids enables them to act as buffers, resisting changes in pH.

Valine structure

Valine is another essential amino acid, and it is important for muscle growth, tissue repair, and energy production. Valine's structure is similar to that of Leucine but with a slightly different side chain. Here’s what it looks like:

• General formula: C\(_5\)H\(_{11}\)NO\(_2\)
• Amino group: H\(_2\)N
• Alpha carbon bonded to: one CH\(_3\) group, one H, and one side chain
• Side chain: Isopropyl group (\(\text{-CH(CH}_3\text{)}_2\))

Under the pH condition of 10.0, the structure of Valine changes like Leucine; the \( \text{NH}_2 \) group becomes \( \text{NH}_3^+ \) and the COOH group turns into \( \text{COO}^- \). This zwitterion state aids its function in cellular communication and metabolic processes.

Proline structure

Proline is unique among the amino acids due to its cyclic structure, which makes it a secondary amine. This distinct structure plays a role in protein folding because it introduces kinks in protein chains. Here’s an overview of Proline's structure:

• General formula: C\(_5\)H\(_9\)NO\(_2\)
• Secondary amine group: NH
• Side chain: A pyrrolidine ring (or a \(\text{-} (CH_2)_3\text{-} \) segment that links back to the nitrogen)

When exposed to a pH of 10.0, Proline's amine group takes on an additional proton, becoming \( \text{NH}_3^+ \), and its carboxyl group donates a proton to become \( \text{COO}^- \). This forms the stabilizing zwitterion that is less flexible but contributes significantly to structural features in protein.

Aspartic acid structure

Aspartic acid is an amino acid that plays pivotal roles in the citric acid cycle and in the synthesis of other amino acids. Its defining feature is its side chain, which contains an extra carboxyl group. Here’s a closer look:

• General formula: C\(_4\)H\(_7\)NO\(_4\)
• Amino group: H\(_2\)N
• Side chain: CH\(_2\)COOH

In a basic solution with a pH of 10.0, Aspartic acid undergoes deprotonation of both carboxyl groups. The amino group becomes \( \text{NH}_3^+ \), and both carboxyl groups convert to \( \text{COO}^- \). The dual presence of carboxyl groups allows Aspartic acid to balance its charge efficiently, which is crucial for its role in transmitting nerve signals and maintaining metabolic health.