Question

Draw condensed structural formulas for the following amino acids buffered at \(\mathrm{pH}\) 6.0: (a) aspartic acid; (b) lysine; and (c) alanine.

Short answer

The condensed structures of the three amino acids at pH 6.0 will be: aspartic acid in its deprotonated form (-COO-), lysine in its protonated form (-NH3+), and alanine in its neutral form.

Step-by-step solution

Identify the Isoelectric Point

First, determine the isoelectric points of each of the given amino acids. Aspartic acid has a pI of about 2.8, lysine has a pI of about 9.7, and alanine has a pI of about 6.0.

Compare the pH to the pI

Compare the given pH to the pI of each amino acid. At pH 6.0, the net charge on aspartic acid (pI 2.8) will be negative as pH > pI. For lysine (pI 9.7), it will be positive as pH < pI. For alanine (pI 6), the net charge will be zero as pH = pI. Determine the correct ionization state based on this.

Draw the Condensed Structural Formulas

Draw the condensed structural formulas for the amino acids in the identified ionization states. Since aspartic acid has a negative net charge, its carboxyl group (\(-COOH\)) is deprotonated to (\(-COO^-\)). For lysine, with a positive net charge, its amino group (\(-NH2\)) is protonated to (\(-NH3^+\)). Alanine will have a zero net charge, so it will not be ionized.

Key concepts

Isoelectric Point

The isoelectric point, often abbreviated as pI, is a key concept for understanding amino acids. The isoelectric point refers to the specific pH at which an amino acid carries no net electrical charge. This is important because amino acids can behave very differently depending on the pH of their environment. For example, at its pI, an amino acid is in its "zwitterion" form, where it carries both a positive and a negative charge, but these charges balance out to zero. Knowing the pI of an amino acid helps in predicting how it will react under different pH conditions.
Add to this, each amino acid has a unique pI value. For instance, aspartic acid has a pI of approximately 2.8, lysine has a pI around 9.7, and alanine's pI is roughly 6.0. These differences occur because of the varying side chains (R groups) of the amino acids, which influence their overall charge. Understanding the pI helps us assess the ionization state of amino acids at different pH levels.

Ionization States

To fully grasp how amino acids behave in different environments, it's essential to understand ionization states. Amino acids can exist in different forms depending on the pH of their surroundings, affecting their net charge. The ionization state changes as the pH changes because acids can donate protons and bases can accept protons.
Consider aspartic acid, which at a pH greater than its pI (2.8), will be deprotonated, giving it a negative net charge. Lysine, on the other hand, will have a positive net charge at a pH that is less than its pI (9.7) as its amino group accepts a proton. Alanine, with a pI of 6.0, is unique in that at pH 6.0, it will neither accept nor donate additional protons, resulting in a charge-neutral zwitterionic form. Recognizing these ionization states is crucial for drawing the appropriate structures of amino acids at various pH levels.

Condensed Structural Formulas

When depicting amino acids, especially in different ionization states, condensed structural formulas provide a simplified way to convey their structure. These formulas allow you to see the core structure of the amino acid and any significant modifications due to changes in pH or the state of ionization.
For example, aspartic acid, when buffered at a pH of 6.0, will show a deprotonated carboxyl group as \(-COO^-\), marking its negative charge. Lysine, at the same pH, will often show a protonated amino group depicted as \(-NH_3^+\), indicating its positive charge. Lastly, alanine, being neutral at pH 6.0, retains its basic \(-COOH\) and \(-NH_2\) without any additional charges. Understanding these formulas helps in comprehending how amino acids will look and behave in physiological and laboratory conditions.

pH Comparison

Comparing pH and pI is pivotal in predicting the behavior of amino acids. By understanding this relationship, you can determine the ionization state and subsequently the net charge of an amino acid.
When the pH is above the pI, amino acids tend to lose protons, and are likely to carry a negative charge. Conversely, when the pH is below the pI, amino acids tend to gain protons, making them carry a positive charge. However, when the pH equals the pI, they reach a state of equilibrium where they are uncharged, or in the zwitterion form.
This is why, at a pH of 6.0, aspartic acid will carry a net negative charge (pH > pI), lysine will be positively charged (pH < pI), and alanine will be neutral as the pH equals its pI at 6.0. Recognizing how amino acids interact with their environment helps in anticipating their behavior in biological systems and experiments.