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): b. serine before alanine

Verified step by step guidance

1. Understand the properties of Dowex 50: Dowex 50 is a cation-exchange resin, which means it has negatively charged sites that can attract and bind positively charged species (cations).
2. Consider the pH of the buffer: At pH 4, both serine and alanine are in their zwitterionic forms, but the overall charge of each amino acid can vary slightly depending on their side chain groups.
3. Analyze the side chain groups: Serine has a hydroxymethyl group attached to its side chain, which can engage in additional hydrogen bonding compared to the methyl group of alanine. This can affect their interaction with the resin.
4. Predict the interaction with the resin: Since serine can form more hydrogen bonds due to its side chain, it may be less retained on the cation-exchange resin compared to alanine, which has a less polar side chain and fewer opportunities for hydrogen bonding.
5. Conclude the order of elution: Based on the interactions and the structure of the amino acids, serine, with its ability to form more hydrogen bonds and thus potentially weaker interaction with the negatively charged resin, will elute before alanine.

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 influence their charge at a given pH. At pH 4, amino acids can exist in different ionization states; for instance, serine has a hydroxyl group that can participate in hydrogen bonding, while alanine has a non-polar side chain. Understanding these properties is crucial for predicting their behavior in ion exchange chromatography.

pH and Charge Relationship

The pH of the buffer affects the ionization of amino acids, determining their overall charge. At pH 4, amino acids with acidic side chains may be negatively charged, while those with basic side chains may be positively charged. This relationship is essential for predicting the order of elution, as amino acids will bind to the resin based on their charge at that specific pH.