Zwitterions

[Martoman]

When zwitter, it will also was be a overall neutral charge right?


TSFX seem to think that glutamic acid will be overall negative.

[Studyinghard]

I think this is because they make the other COOH also into a negative therefore overall charge being negative.

[Martoman]

Yeah but that will only happen in highly basic conditions. It says its at pH 6 and mentions that it is in a zwitterion form. This means overall net charge has to be 0. Not negative.

[stonecold]

So if we get asked to draw a zwitterion which has an amine group or carboxyl group on the Z chain, we don't protonate/deprotonate that?

http://chemistry.about.com/library/weekly/aa080801a.htm
According to this, everything will protonate...

[chansthename]

I think this is because they make the other COOH also into a negative therefore overall charge being negative.

that is it

in an basic solution it would dontae both H+, a zwitterion is defined as: A dipolar ion formed when the proton on the
carboxy group of an amino acid molecule is transferred to
the amino functional group, as in +H3NCH2COO–. (from the textbook)

so I am not sure if a zwitterion can be anything other than neutral (since it is di-polar)

[Martoman]

So if we get asked to draw a zwitterion which has an amine group or carboxyl group on the Z chain, we don't protonate/deprotonate that?

http://chemistry.about.com/library/weekly/aa080801a.htm
According to this, everything will protonate...

Godamn this, now i'm even more confused.

I have been taught that Zwitter is neutral. Now you're saying its not>

[stonecold]

I just thought at isoelectric point, everything either protonates/deprotonates.  I could be wrong.  We need Mao to answer this one...

That article says at pH 7.4, the side chains will lose or gain H+.

The ambiguity is giving me the shits...

[m@tty]

Isoelectric point is where the zwitterion exists. The zwitterion has an overall charge of zero. I think that only the amine and carboxyl groups on the second carbon are involved(ie. the ones in every amino acid). Where I get confused is whether side group will be affected if a net charge of zero can still exist, ie. there is both and amine group and carboxyl on the side chain...

But a zwitterion has a net charge of zero.

[stonecold]

^I think you guys are right.  Charge has to equal 0.

What about if you put glutamic acid in an alkaline solution.  -1 charge or -2 charge?

[chansthename]

^^ yeah, what I am thinking is that if the question says something like pH 10 or 12 then everything is donated but if it says zwitterion only draw both without any side groups involved.

On a related question...

if you are asked to draw an amino acid at a neutral pH then do you draw a zwitterion or a basic amino acid without any charges anywhere.

edit: @stonecold I'm thinking -2

[fady_22]

^I think you guys are right.  Charge has to equal 0.

What about if you put glutamic acid in an alkaline solution.  -1 charge or -2 charge?

According to the uni text book that I have, as a zwitterion, the side chains do not donate or accept protons.

However, in basic conditions, the charge would be -2, as all protons that can be donated are donated.

[m@tty]

If it asks at a neutral pH then draw the zwitterion. They don't expect us to know the isolelectric points of different amino acids, that was just NEAP being tricky. I don't know if they'll ask that way though, as it is a bit ambiguous.

[stonecold]

^I think you guys are right.  Charge has to equal 0.

What about if you put glutamic acid in an alkaline solution.  -1 charge or -2 charge?

According to the uni text book that I have, as a zwitterion, the side chains do not donate or accept protons.

However, in basic conditions, the charge would be -2, as all protons that can be donated are donated.

Sweet, this is what I am leaning towards then, unless the blessed Mao comes and overules haha....

[Martoman]

haha.

My understanding of it is basically:

Always use the zwitter as the main building block when considering what happens as pH changes.

So in glutamic at zwitter the Z group isn't touched.

Now in basic it will donate, so one of the the 3 H's on the N will go (the carboxyl is already given it away so don't touch this) and the Z group carboxyl will act as an acid.

Yes, -2 charge.

Doing it this way makes sense to me. I don't know how you guys do it.

[stonecold]

I just think if you put an acid in an acid, nothing happens to the acid, and if you put a base in a base, nothing happens to the base.