Arg vs Lys

[Wolfpack93]

There's a question on a NS exam asking why arginine is a stronger base than lysine. The answer is because the electron donating group (which i'm assuming is the double bond NH group) stabilizes the conjugate acid. While I see why this would stabilize the conjugate acid, wouldn't that mean it would be a weaker base? A more stable CA would mean you wouldn't want to protonate the NH2 group because this would lead to loss of hybridization right?

---

Members don't see this ad.

 

[theonlytycrane]

The pKa of Lys ~ 10.5 and the pKa of Arg ~ 12.5. Lys is more acidic, so it's a weaker conjugate base.

Looking at the protonated forms of both residues, the protonated form of Arg is resonance stabilized whereas Lys isn't. This follows with above where Lys will more readily give up the proton because it's less stable in that form.

 

[Wolfpack93]

The pKa of Lys ~ 10.5 and the pKa of Arg ~ 12.5. Lys is more acidic, so it's a weaker conjugate base.

Looking at the protonated forms of both residues, the protonated form of Arg is resonance stabilized whereas Lys isn't. This follows with above where Lys will more readily give up the proton because it's less stable in that form.

Ahh I was thinking it was the NH2 group that was protonated on R not the =NH group. That makes more sense, so since that protonated form is resonance stabilized its less likely to donate this proton, which is what makes it a stronger base?

 

[theonlytycrane]

it's more stable in acid form, so it's a stronger base (in un-protonated form). Lys is less stable in acid form (stronger acid), so in un-protonated form it's a weaker conjugate base.

Just for reference: