Question

In a neutral solution, most amino acids exist as: (A) positively charged compounds. (B) zwitterions. (C) negatively charged compounds. (D) hydrophobic molecules.

Short answer

The correct answer is (B) zwitterions.

Step-by-step solution

- Understanding Amino Acids

Amino acids are the building blocks of proteins. Each amino acid contains both a basic amine group (-NH2) and an acidic carboxyl group (-COOH).

- What is a Zwitterion?

A zwitterion is a molecule with both positive and negative charges but is overall neutral. In amino acids, this occurs when the amine group is protonated (-NH3+) and the carboxyl group is deprotonated (-COO-).

- Amino Acids in a Neutral pH

At a neutral pH (around 7), the acidic carboxyl group tends to lose a proton, becoming negatively charged (-COO-), while the basic amine group tends to gain a proton, becoming positively charged (-NH3+).

- Conclusion

In a neutral solution, the amino acid exists in a state where it has both positive and negative charges but remains overall neutral — this state is known as a zwitterion.

- Choose the Correct Option

Based on the explanation, the amino acid in a neutral solution exists as a zwitterion. Hence, the correct answer is (B) zwitterions.

Key concepts

amino acids

Amino acids are essential molecules in biochemistry. They are the building blocks of proteins, which are vital for numerous biological functions. Each amino acid contains two key functional groups: a basic amine group (-NH2) and an acidic carboxyl group (-COOH). These groups allow amino acids to participate in various chemical reactions crucial for life.
A typical structure of an amino acid includes a central carbon atom bonded to:

• An amino group (-NH2)
• A carboxyl group (-COOH)
• A hydrogen atom (H)
• A unique side chain (R group) that determines the amino acid's properties

Understanding amino acids is fundamental for grasping more complex biochemical concepts, such as the behavior of proteins and their interactions in biological systems.

zwitterions

Zwitterions are special molecules with both positive and negative charges but an overall neutral charge. In the context of amino acids, zwitterions form when specific conditions cause the amine and carboxyl groups to ionize.
For most amino acids, the zwitterion form occurs when:

• The amine group (-NH2) gains a proton, becoming positively charged as -NH3+
• The carboxyl group (-COOH) loses a proton, becoming negatively charged as -COO-

This dual charge state makes zwitterions unique because they balance a positive and negative charge within the same molecule. Despite having charges, zwitterions are electrically neutral overall.
This capability of amino acids to form zwitterions is essential for their biological functions and interactions, especially in neutral pH environments.

neutral pH

The term 'neutral pH' refers to a pH level of approximately 7, which is considered neither acidic nor basic. Most biological processes occur optimally at or near this pH.
At a neutral pH, the behavior of amino acids changes because of the pH's influence on their ionizable groups. Specifically:

• The acidic carboxyl group (-COOH) tends to lose a proton, becoming -COO-
• The basic amine group (-NH2) tends to gain a proton, becoming -NH3+

These changes result in the amino acid adopting a zwitterion form, as described above. This is essential for understanding how amino acids behave in biological contexts since many bodily fluids and cellular environments maintain a roughly neutral pH to support various life-sustaining reactions.

protonation

Protonation is the process of gaining a proton (H+). In amino acids, this typically occurs with the amine group (-NH2), which has a lone pair of electrons that can attract and bind an extra proton.
When an amino acid's amine group protonates, it converts from -NH2 to -NH3+. This additional proton gives the amine group a positive charge, contributing to the zwitterion form in neutral pH environments.
Understanding protonation is crucial for predicting and explaining the behavior of amino acids and other molecules in different pH conditions. Protonation reactions play a significant role in biochemical pathways, enzyme function, and cellular signaling processes.

deprotonation

Deprotonation is the loss of a proton (H+), which typically occurs with the carboxyl group (-COOH) in amino acids. When the carboxyl group deprotonates, it loses a proton and forms -COO-, acquiring a negative charge.
This deprotonation contributes to the formation of the zwitterion form of amino acids at neutral pH. Simultaneously, the carboxyl group's negative charge offsets the positive charge from the protonated amine group (-NH3+).
Deprotonation is essential to grasp because it helps explain how molecules like amino acids can have properties that change with pH levels. Many biochemical reactions depend on deprotonation and protonation processes to proceed correctly and efficiently.