Amino Acid question

[oneoneoneone]

This is from insight 2011 so if you're planning on doing it, don't read this.






Which of the following amino acids will have the highest pH when dissolved in solution?

B. Histidine
C. Lysine






The answers say C, but I thought it should be B, since there are two amino groups and they both have a lone pair, allowing them to accept the H+, making it di-basic?

[AllAboutTheLGs]

C has 2 amino groups, B has N-H but does not have a lone pair (its in a ring) and is therefore not an amine, look at the structure properly

[oneoneoneone]

How does it being in a ring mean it doesn't have a lone pair?
The histidine ring has a secondary and tertiary amino group, so I would have thought that it could accept 2 H+'s. :\

[AllAboutTheLGs]

Ooops my mistake. I read the structure wrongly and thought the N was double bonded to a carbon. My bad

[AllAboutTheLGs]

Well now that I think about it, the only logical reason would be different acidity constants which would cause a different pH ? I'm not too sure about this and definitely don't think its within the scope of the unit 3 course

edit did some research:
"The pK of the acid is 6.0 so that at neutral pH, histidine is about 90% in the basic form with about 10% still in the acid form" - I guess that could be a reason?

[oneoneoneone]

Yeah, looking at pKa's gives C, but they're not provided on the data sheet so I'm not sure how you would be able to figure that out in an exam.

[Mao]

The standard interpretation goes:

1. the amine group is basic because nitrogen has a lone pair of electrons, and this lone pair attracts H+.

2. The ring in histidine (this ring is called the imidazole ring) is quite similar to the benzene ring, in that the conjugated double bonds and lone pairs from the nitrogen also forms a delocalised ring of electrons.

3. Nitrogen in the imidazole ring 'donates' some of its electrons to the ring. This means that its lone pair is no longer just on the nitrogen, but rather spread all over the ring. This means the negative charge on the nitrogen is smaller, and thus it's less likely for the to attract/accept a H+, therefore less basic.

[thushan]

Surely they're not expected to know about imidazole rings are they?

[AllAboutTheLGs]

Yeah I've never seen anything like this lol. I really don't think its in the study design and it would be rat if VCAA asked something like this.

Thanks Mao for the explanation

[oneoneoneone]

Thanks Mao. I'd respect you but i dont know how.

[Mao]

Glad to help

Surely they're not expected to know about imidazole rings are they?

Of course not. Ah well

Though on a side note, why is this stickied? Unstuck.

[pi]

Haha, apparently you stickied it, was wondering the same thing