Which of the following structures represents a zwitterion?

• A. H\(_3\)N\(^+\)–CH\(_2\)–COO\(^-\)
• B. H\(_2\)N–CH\(_2\)–COOH
• C. H\(_2\)N–CH\(_2\)–COO\(^-\)
• D. H\(_3\)N\(^+\)–CH\(_2\)–COOH

Answer: (A)

Verified step by step guidance

1. Step 1: Understand the concept of a zwitterion. A zwitterion is a molecule that contains both a positive and a negative charge, but is overall electrically neutral. This typically occurs in amino acids at a specific pH (isoelectric point).
2. Step 2: Analyze the given structures. Look for the presence of both a positively charged group (e.g., H\(_3\)N\(^+\)) and a negatively charged group (e.g., COO\(^-\)) within the same molecule.
3. Step 3: Evaluate the first structure, H\(_3\)N\(^+\)–CH\(_2\)–COO\(^-\). This structure contains a positively charged amino group (H\(_3\)N\(^+\)) and a negatively charged carboxylate group (COO\(^-\)), making it a zwitterion.
4. Step 4: Evaluate the second structure, H\(_2\)N–CH\(_2\)–COOH. This structure has a neutral amino group (H\(_2\)N) and a neutral carboxylic acid group (COOH), so it is not a zwitterion.
5. Step 5: Evaluate the remaining structures. H\(_2\)N–CH\(_2\)–COO\(^-\) has a neutral amino group and a negatively charged carboxylate group, while H\(_3\)N\(^+\)–CH\(_2\)–COOH has a positively charged amino group and a neutral carboxylic acid group. Neither of these are zwitterions because they lack both charges simultaneously.