Problem 237 - Introductory Chemistry Atoms First

Question

Actually, once in water, the amino acid glycine (see problem above) exists as a zwitterion, as shown below. A zwitterion is an overall electrically neutral species that nevertheless has \(\mathrm{a}+\) and \(-\) charge in it. Zwitterionic form of glycine Now, the \(\mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+}\) end is weakly acidic and the \(\mathrm{COO}^{-}\) end is weakly basic. Does this mean that the zwitterionic form cannot supply buffering action against strong acid or strong base in water? Explain.

Step-by-Step Solution

Answer

In summary, although the zwitterionic form of glycine possesses both weakly acidic and weakly basic properties, its limited ability to donate or accept protons (H+) makes it ineffective as a buffer against strong acids or bases in water.

Step 1: Understanding Zwitterions

A zwitterion is a molecule that has both positive and negative charges within it, while maintaining an overall neutral charge. In this exercise, we deal with glycine, which has a zwitterionic form. In this form, the glycine molecule has an \(\mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+}\) group representing a positive charge and a \(\mathrm{COO}^{-}\) group representing a negative charge. Let's analyze the properties of these groups.

Step 2: Examining Acidic and Basic Properties

The \(\mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+}\) end of the glycine molecule is weakly acidic, meaning it has the ability to donate a proton (H+) to the solution in a reaction like this: \[\mathrm{NH}_{3}^{+} \rightleftharpoons \mathrm{NH}_{2} + H^{+}\] On the other hand, the \(\mathrm{COO}^{-}\) end of the glycine molecule is weakly basic, meaning it has the ability to accept a proton (H+) from the solution in a reaction like this: \[\mathrm{COO}^{-} + H^{+} \rightleftharpoons \mathrm{COOH}\]

Step 3: Understanding Buffer Action

Buffer solutions have the ability to resist changes in pH when small amounts of acids or bases are added to them. A good buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid). When a strong acid is added to a buffer solution, the weak base present in the solution will neutralize the added acid by accepting the protons (H+) from the strong acid. When a strong base is added, the weak acid present in the solution will neutralize the added base by donating protons (H+).

Step 4: Analyzing Glycine's Buffering Ability

Now that we understand the basics of buffer action, let's consider the case of zwitterionic glycine. When a strong acid is added to a solution containing zwitterionic glycine, the \(\mathrm{COO}^{-}\) end (weakly basic) will try to accept the protons (H+) from the strong acid. However, it's important to note that the equilibrium lies quite far to the left, and the ability of \(\mathrm{COO}^{-}\) to accept H+ ions is quite limited. Similarly, when a strong base is added to the solution, the \(\mathrm{NH}_{3}{\underline{\phantom{xx}}}^{+}\) end (weakly acidic) will try to donate protons (H+) to the strong base, but again, this reaction also has a limited ability to donate H+ ions.

Step 5: Conclusion

Even though the zwitterionic form of glycine displays both weakly acidic and weakly basic properties which make it capable of reacting with strong acids and bases, the extent of the reaction is quite limited. Consequently, zwitterionic glycine would fail to provide a significant buffering action against strong acids or bases in a water solution.

Key Concepts

Buffer Solution

A buffer solution is a special type of solution that can resist changes in pH when small amounts of acid or base are added. This is crucial in biological and chemical processes where maintaining a stable pH is necessary. Buffer solutions usually contain:

• A weak acid and its conjugate base, or
• A weak base and its conjugate acid

When an acid is added to the buffer, the weak base in the solution will react with the hydrogen ions ( H^+ ) from the acid to form the weak acid, thus minimizing the pH change. Similarly, when a base is added, the weak acid will donate H^+ ions to neutralize the base, again resisting significant pH changes. This property is essential in many natural and industrial processes.

Acid-Base Reaction

Acid-base reactions involve the transfer of H^+ ions between different species. These reactions can occur in various forms, such as the reaction between a strong acid and a base, or between weak acids and bases.

• Strong acids/bases dissociate completely in solution, releasing or accepting H^+ ions freely.
• Weak acids/bases do not completely dissociate. They have a limited ability to donate or accept H^+ ions, which makes them ideal for forming buffer solutions.

In the context of glycine, the zwitterion form of the amino acid contains both acidic ( ext{NH}_3^+ ) and basic ( ext{COO}^- ) groups. The acidic group can donate H^+ ions, while the basic group can accept them, allowing it to participate in acid-base reactions, albeit weakly. This delicate balance is pivotal in various biological processes where glycine acts as a buffering agent.

Glycine

Glycine is the simplest amino acid found in proteins, and it uniquely can exist in a zwitterionic form. This means that it has both a positive and a negative charge within the same molecule, balancing each other to create a neutral molecule overall. Here is how it works in particular:

• The ext{NH}_3^+ end of glycine provides a weakly acidic center, capable of donating a hydrogen ion (proton).
• The ext{COO}^- end supplies a weakly basic site, allowing it to accept hydrogen ions.

In aqueous environments, such as within the human body, glycine can function as a minor buffer. Its zwitterionic nature is important for maintaining stability in biological systems, even if its buffering power is limited compared to dedicated buffer systems.

pH Resistance

The ability of a substance to resist changes in pH is highly significant for both chemical reactions and biological systems. The zwitterionic form of glycine contributes to pH resistance, though its ability to do so is limited. Its ext{NH}_3^+ group can donate H^+ ions when a base is present, and its ext{COO}^- group can accept them when an acid is present. Despite these attributes, glycine's capacity to act as a buffer against strong acids or bases is not very strong due to the equilibrium of reactions which predominately favors the non-reactive side.

This means that while glycine does provide some degree of pH resistance, it is not enough to stabilize significant pH changes in the environment of strong acids or bases. Therefore, stronger and more balanced buffer systems are often required for robust pH maintenance, particularly in biological contexts like blood or cellular environments.