Question

Which of the following is a neutral amino acid?

• A. Aspartate
• B. Arginine
• C. Glycine
• D. Histidine

Hint:

Look for the amino acid whose side chain has no ionizable charged group.

Answer 1

The Correct Option is C

Step 1: Sort the four options by side-chain chemistry. The question asks for the amino acid whose R group is electrically neutral around pH 7.

Step 2: Glycine's side chain is simply a single $H$ atom, the smallest possible, with nothing to ionize. So it sits squarely in the neutral, nonpolar group and is the answer.

Step 3: Cross off the charged ones. Aspartate bears an extra $-COO^-$, making it acidic and negatively charged. Arginine carries a guanidinium group and histidine an imidazole ring, both of which can hold a positive charge, so both are basic.

\[\boxed{\text{Glycine}}\]