Most Basic amino acid is
Wait, the question is asking which one is the most basic. I remember that the basicity is determined by the pKa of the side chain. Lysine has a pKa around 10.5, arginine is around 12.5, and histidine's side chain pKa is around 6.5. So arginine has the highest pKa, meaning its side chain is more likely to be protonated and thus more basic compared to lysine and histidine.
But wait, histidine's pKa is lower than physiological pH (which is around 7.4), so at physiological pH, histidine's side chain is about 50% protonated. Lysine's pKa is higher than 7.4, so it's mostly protonated. Arginine's pKa is even higher, so it's almost fully protonated. Therefore, arginine's side chain has a stronger positive charge than lysine's. So arginine is the most basic among them.
Now, the options were A, B, C, D but the user didn't fill them in. Assuming that one of the options is arginine, then that's the correct answer. The wrong options would be lysine, histidine, and maybe another amino acid like aspartic acid, which is acidic. So for the incorrect options: lysine is a basic amino acid but less basic than arginine. Histidine is basic but only partially protonated at physiological pH. Aspartic acid is acidic, not basic. So the correct answer is arginine.
**Core Concept**
Basic amino acids have side chains that can accept protons (positive charge at physiological pH). The most basic amino acid is determined by the pKa of its side chain, with higher pKa values indicating stronger basicity due to greater protonation at physiological pH (~7.4).
**Why the Correct Answer is Right**
Arginine is the most basic amino acid because its guanidino group has a pKa of ~12.5, far exceeding physiological pH. This allows it to remain fully protonated and positively charged, creating a stronger positive charge than lysine (pKa ~10.5) or histidine (pKa ~6.5). The guanidino group's resonance stabilization enhances its ability to stabilize the protonated form.
**Why Each Wrong Option is Incorrect**
**Option A:** Lysine is a basic amino acid but less basic than arginine due to its lower pKa (~10.5).
**Option B:** Histidine has a pKa (~6.5) close to physiological pH, so its side chain is only partially protonated.
**Option C:** Aspartic acid is an acidic amino acid with a negatively charged side chain at physiological pH.
**Clinical Pearl / High-Yield Fact**
Arginine’s strong basicity is critical in enzyme active sites and protein-ligand interactions. Remember: **"Arg is the king of basic amino acids"** due to its highest pKa among basic residues.
**Correct Answer: D. Arginine**