Zwitterion: Videos & Practice Problems

Understanding the structure of amino acids is fundamental in biochemistry, particularly the concept of zwitterions. At physiological pH, which is approximately 7, all 20 amino acids predominantly exist as zwitterions. A zwitterion is defined as a dipolar molecule that contains two groups with opposite charges, resulting from acid-base reactions or proton transfers.

In the common structure of amino acids, the backbone consists of an alpha carbon, a central hydrogen atom, a carboxylate group, an amino group, and a variable R group. At physiological pH, the amino group carries a positive charge, while the carboxylate group has a negative charge due to the loss of a proton. This dual charge character is what classifies the amino acid as a zwitterion.

The significance of zwitterions extends beyond mere classification; they play a crucial role in the behavior of amino acids in biological systems, influencing solubility, reactivity, and interactions with other biomolecules. Recognizing that the zwitterionic form is the predominant structure at physiological pH is essential for understanding protein structure and function, as well as the overall chemistry of amino acids.

As you continue your studies, keep in mind that this foundational knowledge about zwitterions will be referenced frequently, enhancing your comprehension of more complex biochemical concepts.

The predominant structure of amino acids varies with pH, particularly at physiological pH (around 7), where they exist primarily as zwitterions. A zwitterion has both a positively charged amino group and a negatively charged carboxylate group, resulting in a net charge of zero. However, understanding how amino acids behave at different pH levels is crucial for grasping their chemistry.

To determine the predominant structure of an amino acid at any given pH, one must compare the pH of the solution to the pKa values of the amino and carboxyl groups. The pKa is a quantitative measure of the strength of an acidic hydrogen; a higher pKa indicates a weaker acid. For amino acids, the pKa of the alpha amino group typically ranges from 9 to 10.5, while the pKa of the alpha carboxyl group is around 2.

When the pH is lower than the pKa, the corresponding group is predominantly in its protonated form. For example, at a pH of 1, both the amino group (pKa 9) and the carboxyl group (pKa 2) are protonated. The amino group exists as NH₃⁺, contributing a positive charge, while the carboxyl group is neutral (–COOH). Thus, the net charge of the amino acid backbone at pH 1 is +1.

Conversely, at a higher pH, such as 12, both groups are deprotonated. The amino group transitions to NH₂, which is neutral, and the carboxyl group becomes a carboxylate anion (–COO^–), contributing a negative charge. Therefore, at pH 12, the net charge of the amino acid backbone is -1.

In summary, the predominant structure of amino acids shifts from zwitterionic at physiological pH to positively charged at low pH and negatively charged at high pH. This understanding is essential for predicting the behavior of amino acids in various biochemical contexts and will be beneficial for solving related practice problems.

To determine the predominant structure of valine at pH 2, we need to analyze the pKa values provided: pKa1 (9.62) for the amino group and pKa2 (2.32) for the carboxyl group. Valine, a non-polar amino acid characterized by its V-shaped R group, requires us to assess the protonation states of both the amino and carboxyl groups at the given pH.

First, we consider the amino group. The pKa of the amino group (9.62) is significantly higher than the pH of 2. According to acid-base principles, when the pH is lower than the pKa, the protonated form predominates. Therefore, at pH 2, the amino group exists predominantly as NH_3^+, which is the conjugate acid form. This allows us to eliminate any answer options that do not feature NH_3^+.

Next, we evaluate the carboxyl group. The pKa of the carboxyl group (2.32) is also higher than the pH of 2. Again, since the pH is lower than the pKa, the protonated form of the carboxyl group predominates, meaning it will be in the form of COOH (the conjugate acid). This allows us to eliminate options where the carboxyl group is not protonated.

After analyzing the options, we find that only option D contains both the protonated amino group (NH_3^+) and the protonated carboxyl group (COOH). Thus, option D is the correct answer, representing the predominant structure of valine at pH 2.