Post by: Martoman on June 06, 2010, 01:37:29 pm
TSFX seem to think that glutamic acid will be overall negative.
Post by: Studyinghard on June 06, 2010, 01:38:53 pm
Post by: Martoman on June 06, 2010, 01:44:12 pm
Post by: stonecold on June 06, 2010, 01:48:41 pm
http://chemistry.about.com/library/weekly/aa080801a.htm
According to this, everything will protonate...
Post by: chansthename on June 06, 2010, 01:49:51 pm
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)
Post by: Martoman on June 06, 2010, 01:52:47 pm
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>???
Post by: stonecold on June 06, 2010, 01:56:04 pm
That article says at pH 7.4, the side chains will lose or gain H+.
The ambiguity is giving me the shits...
Post by: m@tty on June 06, 2010, 02:01:44 pm
But a zwitterion has a net charge of zero.
Post by: stonecold on June 06, 2010, 02:06:34 pm
What about if you put glutamic acid in an alkaline solution. -1 charge or -2 charge?
Post by: chansthename on June 06, 2010, 02:08:39 pm
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
Post by: fady_22 on June 06, 2010, 02:10:22 pm
^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.
Post by: m@tty on June 06, 2010, 02:12:25 pm
Post by: stonecold on June 06, 2010, 02:13:31 pm
^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....
Post by: Martoman on June 06, 2010, 02:17:15 pm
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.
Post by: stonecold on June 06, 2010, 02:19:05 pm
Post by: Toothpaste on June 06, 2010, 02:39:14 pm
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:Updated Glu.(in order of increasing pH, acidic to basic environment)
- NH3+ (+1)
- NH3+/COO- (0, zwitterion)
- NH3+/COO-/COO- (-1, side chain now ionised)
- NH2/COO-/COO- (-2)
Post by: Greggler on June 06, 2010, 07:13:17 pm
Post by: Mao on June 06, 2010, 08:01:49 pm
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 8), 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.
Post by: mankay on June 06, 2010, 08:13:05 pm
Post by: chansthename on June 06, 2010, 08:33:08 pm
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)?
Post by: mankay on June 06, 2010, 09:58:13 pm
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)
Post by: Toothpaste on June 06, 2010, 10:10:00 pm
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.
Post by: mankay on June 06, 2010, 10:11:05 pm
Cheers again!
Post by: Studyinghard on June 06, 2010, 10:25:17 pm
Post by: stonecold on June 06, 2010, 10:30:30 pm
-pH less than 7
-pH 7
-pH greater than 7
?
Post by: Toothpaste on June 06, 2010, 11:23:15 pm
Can someone just summarise everything in this thread in one sentence. Too cbf to read everything :PBasically, 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:VCE version (oh how it pains to do this to glutamic acid)
-pH less than 7
-pH 7
-pH greater than 7
?
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
Post by: stonecold on June 06, 2010, 11:25:10 pm
Post by: Martoman on June 06, 2010, 11:50:43 pm
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.
Post by: Toothpaste on June 06, 2010, 11:53:03 pm
LOLOL.
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.
Post by: chansthename on June 07, 2010, 04:36:44 pm
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