Question

Electrophoresis can separate amino acids by subjecting them to an electric field. The elcctric field applies a force whose strength and direction is dependent upon the net charge of the amino acid. If a solution of amino acids at a \(\mathrm{pH}\) of 8 underwent electrophoresis, which of the following would most likely move the furthest toward the anode? A. lysine B. arginine C. glutamate D. histidine

Short answer

C. glutamate

Step-by-step solution

- Understand the amino acid properties at pH 8

Identify the net charge of each amino acid at pH 8. Look up the pKa values of the ionizable groups for lysine, arginine, glutamate, and histidine. Determine the ionization state of the amino acids at this pH.

- Determine charges of amino acids

At pH 8, lysine and arginine (basic amino acids) have a net positive charge because their pKa values for the side chain groups are above 8. Glutamate (an acidic amino acid) has a net negative charge because its pKa value is below 8. Histidine can be neutral or positively charged depending on its exact pKa value (close to 6).

- Predict the movement towards anode

Since anodes attract negatively charged particles, identify which amino acid will have the highest negative charge to predict which one will migrate the furthest towards the anode.

- Apply the knowledge

Given that glutamate has a net negative charge at pH 8 and other amino acids have either positive or less negative charges, glutamate will move furthest towards the anode.

Key concepts

amino acid properties

Amino acids are the building blocks of proteins. Each amino acid has a unique side chain, which gives it specific chemical properties. There are 20 standard amino acids commonly found in proteins.
Some of the important properties of amino acids include:

• Polarity: Amino acids can be polar or non-polar. Polar amino acids have side chains that interact well with water, whereas non-polar amino acids have side chains that are hydrophobic.
• Charge: The side chains of amino acids can carry positive, negative, or neutral charges depending on the pH of the surrounding environment.
• pKa values: The pKa is the pH at which half of the molecule's ionizable groups are protonated. Each amino acid has unique pKa values for its amino, carboxyl, and side chain groups.

At a given pH, an amino acid's charge depends on the protonation states of its ionizable groups. This is crucial in electrophoresis, where the movement of amino acids in an electric field is determined by their net charge.

isoelectric point

The isoelectric point (pI) of an amino acid is the pH at which it carries no net electrical charge. At this point, the positive and negative charges balance each other out. The pI values help in understanding the behavior of amino acids in different pH environments.
To determine the pI of an amino acid, one must average the pKa values of the ionizable groups that surround its zwitterionic form. For example:

• For an amino acid with no ionizable side chain, the pI is the average of the pKa values of the carboxyl group and the amino group.
• For amino acids with ionizable side chains (like lysine, arginine, or glutamate), the pI is derived from the pKa values of the most similar groups surrounding the isoelectric point.

Understanding the isoelectric point is key to predicting the movement of amino acids in an electrophoresis experiment. Amino acids will migrate towards the electrode opposite to their charge when the pH is different from their pI.

pH and charge

The pH of the solution in which amino acids are dissolved greatly influences their net charge. At acidic pH values, amino acids tend to carry positive charges because their amino groups are protonated, and the carboxyl groups remain protonated as well. Conversely, at basic pH values, amino acids usually carry negative charges because their carboxyl groups are deprotonated, and amino groups are deprotonated.
In electrophoresis, when subjected to an electric field:

• Amino acids with a net positive charge (at low pH) will migrate towards the cathode (negatively charged electrode).
• Amino acids with a net negative charge (at high pH) will migrate towards the anode (positively charged electrode).

For example, at pH 8, basic amino acids like lysine and arginine have net positive charges. In contrast, acidic amino acids like glutamate have net negative charges. The differential movement of these amino acids in an electric field helps to separate them based on their properties.