Basicity and acidity question

[mariposas905]

It says on one of the AAMC flashcards that arginine is more basic than lysine, but how do we know this? how can you tell which amino acids are more basic or acidic just by looking at their structure?

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[FriedLemon]

It says on one of the AAMC flashcards that arginine is more basic than lysine, but how do we know this? how can you tell which amino acids are more basic or acidic just by looking at their structure?

You can definitely tell by going back to some of your orgo thinking; when they are both deprotonated, which one is more stable? Think resonance and inductive effects, for example.

 

[Osminog]

It says on one of the AAMC flashcards that arginine is more basic than lysine, but how do we know this? how can you tell which amino acids are more basic or acidic just by looking at their structure?

Arginine has several nitrogen groups on the same side chain, and these nitrogen groups have lone pairs of electrons. The lone pairs of electrons can stabilize a positive charge when a proton is bound to the proton-accepting nitrogen group.

Lysine doesn't have these extra lone pairs on its side chain, so a positive charge on the proton-accepting nitrogen group in Lysine is less stabilized.

Arginine is better able to accommodate a proton due to the stabilization effects described above, and therefore it is the stronger base. Does that make sense?

 

[mariposas905]

You can definitely tell by going back to some of your orgo thinking; when they are both deprotonated, which one is more stable? Think resonance and inductive effects, for example.

Arginine has several nitrogen groups on the same side chain, and these nitrogen groups have lone pairs of electrons. The lone pairs of electrons can stabilize a positive charge when a proton is bound to the proton-accepting nitrogen group.

Lysine doesn't have these extra lone pairs on its side chain, so a positive charge on the proton-accepting nitrogen group in Lysine is less stabilized.

Arginine is better able to accommodate a proton due to the stabilization effects described above, and therefore it is the stronger base. Does that make sense?

Thank you, that makes sense. I forgot about the fact that arginine has a double bond and lone pairs in its side chain, which can lead to resonance and hence more stability.

 

[BerkReviewTeach]

The quickest and easiest way to determine that arginine is a stronger base than lysine is to consider their pKbs. The side chain of arginine has a pKa of 12 when protonated, so its pKb is 2 when deprotonated. The side chain of lysine has a pKa of 10.8, so its pKb is 3.2 when deprotonated. A pKb of 2 is more basic than a pKb of 3.2, confirming that arginine is more basic than lysine.

 

[FriedLemon]

The quickest and easiest way to determine that arginine is a stronger base than lysine is to consider their pKbs. The side chain of arginine has a pKa of 12 when protonated, so its pKb is 2 when deprotonated. The side chain of lysine has a pKa of 10.8, so its pKb is 3.2 when deprotonated. A pKb of 2 is more basic than a pKb of 3.2, confirming that arginine is more basic than lysine.

True, but is remembering the pKbs and pKas really recommended for the MCAT? (Besides the case of histidine.)

 

[mariposas905]

The quickest and easiest way to determine that arginine is a stronger base than lysine is to consider their pKbs. The side chain of arginine has a pKa of 12 when protonated, so its pKb is 2 when deprotonated. The side chain of lysine has a pKa of 10.8, so its pKb is 3.2 when deprotonated. A pKb of 2 is more basic than a pKb of 3.2, confirming that arginine is more basic than lysine.

True, but is remembering the pKbs and pKas really recommended for the MCAT? (Besides the case of histidine.)

Yeah...I thought that was too much memorization which ones do we have to remember? Could you provide a list of the ones we need to know?

 

[Lawpy]

True, but is remembering the pKbs and pKas really recommended for the MCAT? (Besides the case of histidine.)

Yeah...I thought that was too much memorization which ones do we have to remember? Could you provide a list of the ones we need to know?

You don't need to memorize pKas or pKbs. You just need to be familiar with amino acid structure and one-letter/three-letter abbreviations.