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Histidine - Hanabira.Kage - 2011-02-25
Well, this is kinda out of my syllabus, but I just wanted to know...
Histidine is an alpha-amino acid with an R group that resembles cyclopentane, except that two of atoms in the ring are Nitrogen atoms (at the 2 and 4 positions) instead of Carbon atoms. One of these two Nitrogen atoms (this one's at the 4 position) forms a double bond with an adjacent Carbon atom, and a single bond with another adjacent Carbon atom. The other Nitrogen atom (this one's at the 2 position) forms only single bonds with its two adjacent Carbon atoms, and has a Hydrogen atom attached to it.
My question is, when placed in a highly acidic solution, which one of these two Nitrogen atoms will accept a Hydrogen ion (my Chemistry teacher confirmed that only one of them will accept an ion, but refused to divulge any more information)?
My guess is that the Nitrogen atom with the double bond will accept the Hydrogen ion, because the high electron density within the double bond will attract the positively-charged Hydrogen ion...is this correct?
Histidine is an alpha-amino acid with an R group that resembles cyclopentane, except that two of atoms in the ring are Nitrogen atoms (at the 2 and 4 positions) instead of Carbon atoms. One of these two Nitrogen atoms (this one's at the 4 position) forms a double bond with an adjacent Carbon atom, and a single bond with another adjacent Carbon atom. The other Nitrogen atom (this one's at the 2 position) forms only single bonds with its two adjacent Carbon atoms, and has a Hydrogen atom attached to it.
My question is, when placed in a highly acidic solution, which one of these two Nitrogen atoms will accept a Hydrogen ion (my Chemistry teacher confirmed that only one of them will accept an ion, but refused to divulge any more information)?
My guess is that the Nitrogen atom with the double bond will accept the Hydrogen ion, because the high electron density within the double bond will attract the positively-charged Hydrogen ion...is this correct?
Histidine - Shidoshi - 2011-02-25
Hanabira.Kage Wrote:Well, this is kinda out of my syllabus, but I just wanted to know...
Histidine is an alpha-amino acid with an R group that resembles cyclopentane, except that two of atoms in the ring are Nitrogen atoms (at the 2 and 4 positions) instead of Carbon atoms. One of these two Nitrogen atoms (this one's at the 4 position) forms a double bond with an adjacent Carbon atom, and a single bond with another adjacent Carbon atom. The other Nitrogen atom (this one's at the 2 position) forms only single bonds with its two adjacent Carbon atoms, and has a Hydrogen atom attached to it.
My question is, when placed in a highly acidic solution, which one of these two Nitrogen atoms will accept a Hydrogen ion (my Chemistry teacher confirmed that only one of them will accept an ion, but refused to divulge any more information)?
My guess is that the Nitrogen atom with the double bond will accept the Hydrogen ion, because the high electron density within the double bond will attract the positively-charged Hydrogen ion...is this correct?
You just have to think which of the nitrogens is more basic. That will be the one to extract a hydrogen, the one linked to the carbons should be more basic (if I remember my chemistry right). For confirmation you can look pKa tables of different chemical groups.
http://evans.harvard.edu/pdf/evans_pKa_table.pdf
Edit: guess I was wrong with my first guess, not that strong with organic chemistry... anyway, checking pKa tables is a good way to know this.
Histidine - Hanabira.Kage - 2011-02-26
Uhh...I found the relative acidity of the ENTIRE -R group structure, but I can't find the relative basicities of the individual Nitrogens...
Histidine - Shidoshi - 2011-02-26
Search for structures that have a nitrogen like the one in your R group, that should give an idea of the basicity of that part of the R group.
Basicity = 1/acidity meaning the higher the pKa, the more basic the group is.
Histidine - Swerve - 2011-02-26
Hanabira.Kage Wrote:My guess is that the Nitrogen atom with the double bond will accept the Hydrogen ion, because the high electron density within the double bond will attract the positively-charged Hydrogen ion...is this correct?
http://wps.prenhall.com/wps/media/objects/726/744059/0110f.html
Answer (from source above): I believe delocalization confers a formal positive charge on the sigma bonded nitrogen and results in the other nitrogen to have a more basic lone pair.
![[Image: serprot02.jpg]](http://www.mikeblaber.org/oldwine/BCH4053/Lecture29/serprot02.jpg)
Pandering:I would think that delocalization (if caused by conjugation) would have delocalized the sigma-pi bonded nitrogen rather than the sigma nitrogen as the sigma nitrogen is not in conjugation with the pi-system. Hmm. This amino group does undergo a lot of rearrangement. Perhaps that could be a factor in this electropositive / electronegative designation. Anyway, was fun. Please post an explanation when you get one. Would be much appreciated.
Histidine - hadriel - 2011-02-26
The first to be protonated in a solution (i.e. the more basic nitrogen) is the nitrogen in the imidazole group (the 5 membered ring with a 1,3-nitrogen) that is double-bonded to carbon, although in reality the two nitrogens in the ring are really equivalent.
Simply put, the protonated nitrogen on the ring can be stabilised by resonance, whereas a protonated amino group at the alpha-carbon down there (not the R-group) does not possess any stabilisation mechanisms. Equilibrium shifts to the right (getting protonated i.e. more basic) the better your conjugate acid (the protonated stuff) can be stabilised. Resonance is a very good way of stabilising an ion because the positive charge is no longer localised over 1 atom, but in this case 3 atoms (N -- C --N).
The two nitrogens on the imidazole ring are (almost) equally basic because both takes part in the resonance structures of each others' protonation stabilisation, and from there you can draw a side step to show that the double bond is equally likely to land on either nitrogens, so either nitrogens are equally basic.
BUT why don't we use that nitrogen that's single-bonded only? Because at that instance, the lone pairs on that nitrogen are oriented perpendicular to the plane in a p-orbital (not sp3), and this allows the lone pairs to interact with the two double bonds to form an aromatic pi-system (4n + 2 = 6, n = 1). Conversely, the lone pair becomes unavailable for protonation, because that would mean breaking aromaticity (and it's associated resonance stabilisation energy like for benzene).
By the same arguments, the double-bonded nitrogen's lone pair is pointing out of the ring i.e. it's in the sp2 orbital. This is very clearly not going to have an effect on aromaticity because protonation of this nitrogen doesn't disturb the p-orbital.
Hanabira - DRAW THE RESONANCE STRUCTURES AND THE ARROW PUSHING OR YOU DON'T GET ALL YOUR MARKS. It should look like this one:
![[Image: hisH-resonance.gif]](http://guweb2.gonzaga.edu/faculty/cronk/CHEM440/images/hisH-resonance.gif)
Of course remember to draw the arrow pushing for the two structures on the right.
Curious why you didn't ask this in the Backyard. No funky heteroaromatic chemistry here. Just simple resonance stabilisation.
Hadriel
Histidine - Hanabira.Kage - 2011-02-27
hadriel Wrote:Hanabira - DRAW THE RESONANCE STRUCTURES AND THE ARROW PUSHING OR YOU DON'T GET ALL YOUR MARKS. It should look like this one:
I already said that it isn't in my H2 Chemistry syllabus. o.o
It just happened to be in my Organic Nitrogen Compounds tutorial under electrophoresis, and my teacher said to replace Histidine with Lysine because Histidine happens to be fairly complex. So I was just wondering how complex it was. >.>
Anyways, thanks!
It just happened to be in my Organic Nitrogen Compounds tutorial under electrophoresis, and my teacher said to replace Histidine with Lysine because Histidine happens to be fairly complex. So I was just wondering how complex it was. >.>
Anyways, thanks!
Histidine - hadriel - 2011-02-27
Strictly speaking, it is in your syllabus. It is an extension of aromatic compounds - why some groups are activating and why some aren't. The same idea applies here.
Bah you're not taking H3. No fun.
Hadriel