8. The most basic naturally occurring amino acid is:
a. lysine
b. arginine
c. serine
d. tryptophan
Most Basic Naturally Occurring Amino Acid: Arginine Explained
Arginine stands as the most basic naturally occurring amino acid among the options lysine, arginine, serine, and tryptophan. Basicity in amino acids depends on the side chain’s ability to accept protons, measured by higher pKa values of their conjugate acids.
Option Analysis
Lysine (a): Features a primary ε-amino group in its side chain with pKa ≈10.5-10.8, making it basic but less so than guanidinium groups due to single nitrogen stabilization.
Arginine (b): Contains a guanidinium group with pKa ≈12.5, the highest among standard amino acids, due to resonance delocalization across three nitrogens for exceptional proton retention.
Serine (c): Has a neutral hydroxyl (-OH) side chain (pKa ≈13 for deprotonation), classifying it as polar uncharged with no basic character.
Tryptophan (d): Possess an indole ring side chain, non-basic and non-ionizable at physiological pH, rendering it neutral.
pKa Comparison Table
| Amino Acid | Side Chain Group | Side Chain pKa | Basicity Rank |
|---|---|---|---|
| Lysine | ε-Amino (-NH3+) | ~10.5-10.8 | 2nd |
| Arginine | Guanidinium | ~12.5 | 1st (Most Basic) |
| Serine | Hydroxyl (-OH) | None (neutral) | Not Basic |
| Tryptophan | Indole | None (neutral) | Not Basic |
Answer: b. arginine.
