Explain the order of elution (with a buffer of pH 4) of the following pairs of amino acids through a column packed with Dowex 50 (Figure 21.3): a. aspartate before serine

Verified step by step guidance

1. Identify the functional groups of aspartate and serine. Aspartate has a carboxyl group in its side chain, making it a negatively charged amino acid at pH 4. Serine has a hydroxyl group in its side chain, which is neutral at pH 4.
2. Understand the properties of the Dowex 50 resin. Dowex 50 is a cation-exchange resin, which means it has negatively charged sites that attract and bind positively charged species.
3. Analyze the charge of the amino acids at pH 4. Aspartate, with an extra carboxyl group, will have an overall negative charge, while serine, with only the amino group being protonated, will have a neutral or slightly positive charge.
4. Predict the interaction between the amino acids and the resin. Aspartate, being negatively charged, will not bind strongly to the negatively charged cation-exchange resin and will elute first. Serine, with its neutral or slightly positive charge, may interact more with the resin and elute later.
5. Conclude that in a buffer of pH 4, aspartate will elute before serine when passed through a column packed with Dowex 50 due to the differences in their charges and interactions with the resin.

Key Concepts

Ion Exchange Chromatography

Ion exchange chromatography is a technique used to separate charged molecules, such as amino acids, based on their net charge. In this method, a column is packed with charged resin, allowing for the binding of oppositely charged species. The elution order is influenced by the pH of the buffer, which affects the ionization state of the amino acids.

Amino Acid Properties

Amino acids have distinct properties based on their side chains, which can be polar, nonpolar, acidic, or basic. At a specific pH, the ionization state of the amino acids changes, affecting their overall charge. For example, aspartate, being acidic, carries a negative charge at pH 4, while serine, being neutral, has no charge, influencing their elution order in ion exchange chromatography.

pH and Charge Relationship

The pH of a solution plays a critical role in determining the charge of amino acids. At pH 4, aspartate is negatively charged due to its carboxyl group being deprotonated, while serine remains neutral. This difference in charge affects their interaction with the positively charged resin in the Dowex 50 column, leading to aspartate eluting before serine.