Which of the following amino acids has the highest isoelectric point (pI)?

• A. Arginine
• B. Glutamine
• C. Aspartic acid
• D. Tyrosine

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the concept of isoelectric point (pI). The isoelectric point is the pH at which an amino acid has no net electrical charge, meaning the positive and negative charges balance each other. This depends on the ionizable groups in the amino acid's structure.
2. Step 2: Analyze the amino acids provided in the question. Each amino acid has different ionizable groups that contribute to its pI. For example, Arginine has a positively charged guanidinium group, Aspartic acid has a negatively charged carboxyl group, Tyrosine has a phenolic group, and Glutamine has an amide group.
3. Step 3: Recall the typical pI values for amino acids. Basic amino acids like Arginine generally have higher pI values because their side chains are positively charged at physiological pH. Acidic amino acids like Aspartic acid have lower pI values due to their negatively charged side chains. Neutral amino acids like Glutamine and Tyrosine have intermediate pI values.
4. Step 4: Compare the structures and ionizable groups of the amino acids. Arginine has a basic side chain, which contributes to a higher pI. Aspartic acid has an acidic side chain, leading to a lower pI. Tyrosine has a phenolic group, which is less acidic than Aspartic acid but still contributes to a moderate pI. Glutamine has an amide group, which does not ionize significantly, resulting in a neutral pI.
5. Step 5: Conclude that Arginine has the highest isoelectric point among the given amino acids due to its basic guanidinium group, which raises its pI value significantly compared to the others.