Question of the Day

[MissionStanford]

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All amino acids have at least two pKa values. Which of the following functional groups would provide an amino acid a third pKa value (between 0 and 14)?

A) -CH2CH2CONH2
B) -C(OH)CH3
C) -CH2CH2SCH3
D) -CH2CH2CH2CH2NH2

The answer is choice D, but I don't get why.

 

[Gauss44]

All amino acids have at least two pKa values. Which of the following functional groups would provide an amino acid a third pKa value (between 0 and 14)?

A) -CH2CH2CONH2
B) -C(OH)CH3
C) -CH2CH2SCH3
D) -CH2CH2CH2CH2NH2

The answer is choice D, but I don't get why.

Is it because nitrogen can either gain or lose a hydrogen depending on the ph of the solution?

 

[LoLCareerGoals]

Hmm. I am assuming this means adding a third "branch" somewhere between the amino and carboxylic ends. I am guessing these 2 ends provide the 1st and 2nd pKa. So any answer that adds another carboxylic or amino end would sound good to me.
A) Amide. Meh
B) Huh? Is that an enol? Sounds unstable to me.
C) I have no idea wtf this is, but S has no Hs to lose and neither do alkyls surrounding it.
D) Amino end!!! yey!

I doubt I could answer without seeing your answer first though and not even sure if my posthoc logic is right.

 

[MissionStanford]

Is it because nitrogen can either gain or lose a hydrogen depending on the ph of the solution?

I think that's the reason why an amine group can provide a pKa value, but I was under the impression that we are supposed to see which functional group could provide a different pKa value from the ones the amino acid already has, and all amino acids already have an amine group.

 

[Temperature101]

I answered that question using POE.

 

[BerkReviewTeach]

Hmm. I am assuming this means adding a third "branch" somewhere between the amino and carboxylic ends. I am guessing these 2 ends provide the 1st and 2nd pKa. So any answer that adds another carboxylic or amino end would sound good to me.
A) Amide. Meh
B) Huh? Is that an enol? Sounds unstable to me.
C) I have no idea wtf this is, but S has no Hs to lose and neither do alkyls surrounding it.
D) Amino end!!! yey!

I doubt I could answer without seeing your answer first though and not even sure if my posthoc logic is right.

Good idea to change the answer choices into something easier to read. The second choice is a secondary alcohol. The OP forgot an H on that carbon.

Another way to translate the answer choices would be:

A) Glutamine
B) Threonine
C) Methionine
D) Lysine

 

[gettheleadout]

If you're familiar with the concept of the isoelectric point for amino acids then upon reading this question you should know you're looking for an R group here with either a carboxylic acid or amine functional group. Note that A is an amide, and D is the only possible answer.

Edit: Turns out C is called a thioether. I would've guessed that but wasn't sure.