Which amino acid is basic?

• A. Valine
• B. Arginine
• C. Cysteine
• D. Proline

Answer: (B)

Basics in amino acids are defined by their side chains being positively charged at physiological pH. Arginine has a guanidinium group in its side chain that stays protonated over the body's pH range, giving it a definite positive charge and classifying it as basic. Valine and proline are nonpolar, hydrophobic residues with no positive charge at physiological pH. Cysteine has a polar thiol side chain that is typically neutral at around pH 7 and only becomes negatively charged under very high pH, so it’s not considered basic. Thus, arginine is the basic amino acid.