Zwitterions

[Toothpaste]

Amino acids with non-ionisable side chains are considered to exist in a zwitterion form at 'neutral pH'. Zwitterion = dipolar ion = COO-/NH3+

TSFX is wrong in that sense by saying it's a zwitterion at pH 6. The overall charge at pH 6 for Glu is actually -1 where the side chain is ionised to COO- as you already know (NH3+/COO-/COO-, not a zwitterion).

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.

Does TSFX realise that it's actually -ve? If not, they seem to be generalising it down to "all amino acids have the zwitterion form around that pH".

Apparently VCE doesn't require you to know the following but it might clear up a few issues.

Side note. The amino acids with ionisable R groups are glutamic acid, aspartic acid, arginine and lysine. First two having the acidic R group and last two having basic (and ionised at ~pH 11 if you must know). Histidine is another one (side chain ionised ~pH 6, which makes it 0 charge). Histidine is just special because at a lower pH it can have a +2 form. Refuse.. to use.. "Z".

With glutamic acid, three parts of it can be ionised depending on the pH of the environment it's in. So it can have:

• NH3+ (+1)
• NH3+/COO- (0, zwitterion)
• NH3+/COO-/COO- (-1, side chain now ionised)
• NH2/COO-/COO- (-2)

(in order of increasing pH, acidic to basic environment)

Updated Glu.

[Greggler]

I asked my teacher about this and she was adamant that you will not be asked to draw a zwitterion of an amino acid which contains either a NH2 or COOH in its side chain.

[Mao]

Here's how I think about it, may not be completely rigorously correct (us physical chemists haven't done anywhere near as much on biochemistry as pharmaceutical scientists such as Toothpaste).


To make sense of Toothpaste's data:

• at , NH3+ (+1) and NH3+/COO- (0, zwitterion) are in equal ratio.
• at , NH3+/COO- (0, zwitterion) and NH3+/COO-/COO- (-1, side chain now ionised) are in equal ratio
• at , NH3+/COO-/COO- (-1, side chain now ionised) and NH2/COO-/COO- (-2) are in equal ratio

Thus, acid form (add H+ to everything) exists at low pH (pH~1). The zwitterion (R group unreacted) exists at pH~3. At higher pH (between 5 and , the Z group has lost the H+. At higher pH (greater than 10), the basic form predominates (no H+ anywhere).

To generalize:


The alpha-carboxyl group is stronger than normal carboxyl groups, and loses its H+ pretty much all the time (except for when the pH is ridiculously low, like pH = 1).
The beta-amine group is -NH3+ for pH less than 10~11.

The Z group can contain an acid group (-COOH), but this carboxyl group is about the average strength, which is very weak. At low pH (less than 3), it is COOH. At high pH (greater than 3), it is COO-. (Think of it like, it can only lower the pH to 3 at the lowest, thus any lower and it loses the competition with other acids, thus not able to donate H+, martoman you can come up with an adequate analogy).
The zwitterion of these amino acids exist at pH~5.

The Z group can also contain an amine group (-NH2). There are only three of these: arginine, lycine and histidine. For the purpose of VCE, arginine won't be tested. At low pH (less than 12), it is NH3+.
The zwitterion of these amino acids are a lot more complicated to deal with. I'm not actually sure if zwitterions of these actually exist. I'll hand it over to toothpaste from here.

Hope that helped.

[mankay]

If we were asked to draw a dipeptide at neutral pH, should we draw the net -1 charge (i.e. NH3+, COO-)?

[chansthename]

If we were asked to draw a dipeptide at neutral pH, should we draw the net -1 charge (i.e. NH3+, COO-)?

do you mean net charge 0 (zwitterion)?

[mankay]

If we were asked to draw a dipeptide at neutral pH, should we draw the net -1 charge (i.e. NH3+, COO-)?

do you mean net charge 0 (zwitterion)?

Well i meant if the question specifically said "neutral pH", so i guess it could imply zwitterion (at VCE level)

[Toothpaste]

Well i meant if the question specifically said "neutral pH", so i guess it could imply zwitterion (at VCE level)

At 'neutral pH', yes you'd draw the zwitterion for the dipeptide.
Crap text diagram but: (NH3+) Amino Acid <peptide link> Amino Acid (with COO-)
Use your imagination to pretend it's drawn out.

If we were asked to draw a dipeptide at neutral pH, should we draw the net -1 charge (i.e. NH3+, COO-)?

chansthename was pointing out the "should we draw the net -1 charge (i.e. NH3+, COO-)?" since if you're drawing a zwitterion, the net charge would be 0 and not -1.

[mankay]

My bad

Cheers again!

[Studyinghard]

Can someone just summarise everything in this thread in one sentence. Too cbf to read everything

[stonecold]

That would be awesome.  If we got say glutamic acid, what do we draw at:

-pH less than 7
-pH 7
-pH greater than 7

?

[Toothpaste]

Can someone just summarise everything in this thread in one sentence. Too cbf to read everything

Basically, in VCE, ignore real stuff like how Glutamic Acid has a side chain that can ionise into COO-. The information provided above is if you really want to extend your knowledge about what happens to Glutamic Acid. So yes, you pretend it's just like any other amino acid.

Shorter: In VCE, ignore Glutamic Acid side chain.

I asked my teacher about this and she was adamant that you will not be asked to draw a zwitterion of an amino acid which contains either a NH2 or COOH in its side chain.

That would be awesome.  If we got say glutamic acid, what do we draw at:

-pH less than 7
-pH 7
-pH greater than 7

?

VCE version (oh how it pains to do this to glutamic acid)
pH less than 7, only NH3+
pH 7, both NH3+ and COO- (zwitterion)
pH greater than 7, only COO-

Ignore side chain, pretend it's neutral lalala. Use:

I think of it like this.

~ We know that bases accept protons. So in a basic solution, the amino acid has its protons "sucked in" by the surrounding solution. Hence it is in the form NH2 - CHZ - COO-

~ We know that acids donate protons. So in an acidic solution, the amino acid is "fattened" with protons by the surrounding solution. Hence it is in the form NH3+ - CHZ - COOH

[stonecold]

Thanks for clarifying

[Martoman]

The Z group can contain an acid group (-COOH), but this carboxyl group is about the average strength, which is very weak. At low pH (less than 3), it is COOH. At high pH (greater than 3), it is COO-. (Think of it like, it can only lower the pH to 3 at the lowest, thus any lower and it loses the competition with other acids, thus not able to donate H+, martoman you can come up with an adequate analogy).
Hope that helped.

Sooooo the alpha carboxyl is like a slut, always putting out and the other is a more conservative catholic, cautiously waiting until the time is right.


mmmm.

[Toothpaste]

The Z group can contain an acid group (-COOH), but this carboxyl group is about the average strength, which is very weak. At low pH (less than 3), it is COOH. At high pH (greater than 3), it is COO-. (Think of it like, it can only lower the pH to 3 at the lowest, thus any lower and it loses the competition with other acids, thus not able to donate H+, martoman you can come up with an adequate analogy).
Hope that helped.

Sooooo the alpha carboxyl is like a slut, always putting out and the other is a more conservative catholic, cautiously waiting until the time is right.


mmmm.

LOLOL.

[chansthename]

Sooooo the alpha carboxyl is like a slut, always putting out and the other is a more conservative catholic, cautiously waiting until the time is right.


mmmm.

Wow you think up the weirdest ideas, I love them