Question

Amino acids are building blocks of proteins. These compounds contain at least one amino group ( \(-\mathrm{NH}_{2}\) ) and one carboxyl group (-COOH). Consider glycine (NH \(\left._{2} \mathrm{CH}_{2} \mathrm{COOH}\right)\). Depending on the \(\mathrm{pH}\) of the solution, glycine can exist in one of three possible forms: Fully protonated: \({ }^{+} \mathrm{NH}_{3}-\mathrm{CH}_{2}-\mathrm{COOH}\) Dipolar ion: \({ }^{+} \mathrm{NH}_{3}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\) Fully ionized: \(\mathrm{NH}_{2}-\mathrm{CH}_{2}-\mathrm{COO}^{-}\) Predict the predominant form of glycine at \(\mathrm{pH} 1.0,7.0\), and \(12.0 .\) The \(\mathrm{p} K_{\mathrm{a}}\) of the carboxyl group is 2.3 and that of the ammonium group \(\left(\mathrm{NH}_{3}^{+}\right)\) is 9.6 .

Short answer

At pH 1.0: fully protonated form; pH 7.0: dipolar ion; pH 12.0: fully ionized form.

Step-by-step solution

Understand the Formulas and Terms

Glycine can exist in three forms depending on the pH: fully protonated, dipolar, and fully ionized. The key indicators of which form predominates at a given pH are the pKa values of the carboxyl group (2.3) and the ammonium group (9.6).

Determine Predominant Form at pH 1.0

When the pH is below the pKa of a functional group, that group is predominantly in its protonated form. At pH 1.0, both the carboxyl group (pKa = 2.3) and the ammonium group (pKa = 9.6) are protonated. Therefore, at this pH, glycine is in the fully protonated form: \[ { }^{+} ext{NH}_3- ext{CH}_2- ext{COOH} \]

Determine Predominant Form at pH 7.0

At physiological pH (around 7.0), the carboxyl group (pKa = 2.3) is deprotonated, while the ammonium group (pKa = 9.6) remains protonated due to the pH being less than its pKa. Glycine will exist mainly as a dipolar ion or zwitterion form: \[ { }^{+} ext{NH}_3- ext{CH}_2- ext{COO}^{-} \]

Determine Predominant Form at pH 12.0

At high pH (above the pKa of the ammonium group, 9.6), the ammonium group is deprotonated along with the carboxyl group, leading to the fully ionized form of glycine: \[ ext{NH}_2- ext{CH}_2- ext{COO}^{-} \]

Key concepts

Glycine

Glycine is the simplest amino acid and an essential building block of proteins. It has the chemical formula \( \text{NH}_2\text{CH}_2\text{COOH} \) and features:

• an amino group \( (-\text{NH}_2) \)
• a carboxyl group \( (-\text{COOH}) \)

These functional groups allow glycine to participate in various chemical reactions.
Unlike other amino acids that have an asymmetric carbon, glycine's \( \text{-CH}_2 \) backbone makes it non-chiral, which means it does not exist in "mirror image" forms.
This property can affect how proteins fold and function since glycine can fit into tight spaces within protein structures.

Zwitterion

A zwitterion is a molecule that carries both a positive and a negative charge, but the overall molecule remains neutral. For glycine, this state is predominant at a physiological pH of around 7.0.
In this form, the carboxyl group \( (-\text{COOH}) \) loses a proton, resulting in a carboxylate ion \( (-\text{COO}^-) \). Simultaneously, the amino group \( (-\text{NH}_2) \) gains a proton to become \( (+\text{NH}_3) \).
Thus, the structure of glycine as a zwitterion is \( +\text{NH}_3-\text{CH}_2-\text{COO}^- \).

• This state of balancing charges allows glycine to be versatile in accepting or donating protons depending on the environment.
• This capability of existing as zwitterions makes amino acids unique and central to biochemical reactions.

Protonation

Protonation refers to the addition of a proton (\( \text{H}^+ \)) to a neutral molecule, a key concept in understanding amino acid behavior. Depending on the pH level of the environment, glycine can gain or lose protons, which affects its net charge.

• At lower pH levels, glycine is more likely to be fully protonated because the environment has a high concentration of \( \text{H}^+ \) ions.
• At higher pH levels, deprotonation occurs as \( \text{H}^+ \) ions are less available, meaning glycine loses protons from one or both of its groups.

This characteristic of gaining or losing protons in response to pH changes is essential for the biological functions of amino acids, as it influences protein structure and enzyme activity.

pKa Values

The \( \text{pKa} \) value is crucial in predicting how an amino acid like glycine will behave in different pH environments. It is a measure of the strength of an acid, representing the pH at which a particular group is 50% ionized. In glycine:

• The carboxyl group has a \( \text{pKa} \) of 2.3, indicating it readily loses a proton to form the \( \text{COO}^- \) group even at low pH.
• The ammonium group's \( \text{pKa} \) is 9.6, showing it holds onto its proton until a higher pH level is reached.

Understanding these \( \text{pKa} \) values allows us to predict the ionization state of glycine at different pH values.
This helps in determining the predominant form of glycine, which influences how it interacts with other molecules in biological systems.