Buffering action of amino acids

[Cuntryboner]

I was doing a problem in TSFX book 4 and I am stumped with trying to undrstand the  buffering states of amino acids in certain pH's. For example if I had glutamic acid solution made up to a pH of 9, I thought the net charge on the amino acid would be -1, as the amine group donates in the basic solution. However the net charge is +2, this is very confusing to me ATM :uglystupid2:, any clarification would be greatly appreciated. :laugh:

[Martoman]

I had a similar problem. I'm still not clear on this aspect but I think it donates only one of its H from the COOH which is NOT in the z group.

[vexx]

I was doing a problem in TSFX book 4 and I am stumped with trying to undrstand the  buffering states of amino acids in certain pH's. For example if I had glutamic acid solution made up to a pH of 9, I thought the net charge on the amino acid would be -1, as the amine group donates in the basic solution. However the net charge is +2, this is very confusing to me ATM :uglystupid2:, any clarification would be greatly appreciated. :laugh:

pretty much all we need to know about amino acids and pH, is that:
- if it's placed in a basic solution, the amino acid becomes an acid and donates a proton to become (COO-) on the carboxylic acid side and the amine stays the same (-NH2)
- if it's placed in an acidic solution, the amino acid becomes a base and accepts a proton to become (-NH3+) on the amine group and the carboxyl stays the same (COOH)
- if it's placed in a neutral solution of 7, it will donate and accept at the same time (zwitterion) which my teacher (not sure if she was serious) said it accepts what it donates, so the COOH gives a proton to the NH2 which then becomes COO- and NH3+ respectively.

Hope this helped:P

[stonecold]

Yeah, that's what I thought vexx, but apparently they can make it more complicated if they want to be assholes.

They were talking about this in my class but I wasn't listening :p

[m@tty]

I had a similar problem. I'm still not clear on this aspect but I think it donates only one of its H from the COOH which is NOT in the z group.

Why wouldn't any Z group carboxyl group donate?

[kyzoo]

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

[Mao]

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

^^THIS.

As well as what vexx said.

[Toothpaste]

In regards to the question, a +2 net charge at pH 9 sounds wrong for glutamic acid though. There's a possibility for ionising groups to NH3+, COO- and the side chain COO- only. That is, the only net charge combos are: +1, 0, -1 & -2. A -ve charge would be expected as mentioned?

[mankay]

On this, in pH < 7 (i.e. acidic solution), does the Z group's carboxy function group of glutamic acid donate a proton and change to COO- ?

And for the amine group (on the Z group) in asparagine?

[andy456]

On this, in pH < 7 (i.e. acidic solution), does the Z group's carboxy function group of glutamic acid donate a proton and change to COO- ?

And for the amine group (on the Z group) in asparagine?

If the amino acid is in acidic solution then it will accept protons; that is act as a base.

[mankay]

If the amino acid is in acidic solution then it will accept protons; that is act as a base.

I know that, but does that include the side chain (z group) as well? (any carboxy and amine groups on that)

[andy456]

Yes it should do....
but I'm not 100% certain.

[m@tty]

Don't see why not.

One thing to note, though, is that Z group does not change in a zwitterion; there must be a net charge of zero.

[Toothpaste]

If the amino acid is in acidic solution then it will accept protons; that is act as a base.

I know that, but does that include the side chain (z group) as well? (any carboxy and amine groups on that)

The side chain of glutamic acid is actually ionised (along with the other groups) around pH 4.5 and the zwitterion exists around the pH 3.22 mark (that's the NH3+ and the COO-, side chain COOH).

In reality, yes it should ... but in the VCE course, no idea what you actually have to know (no memorising of amino acid pKa).

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".

So you're probably thinking, who cares bro what about our exam?! In a sense this is a bit over the top for VCE because all I remember learning was the 'general' ionisation of the amino acid parts without all these R group exceptions ^see kyzoo's post. I think it's something for someone to confirm with their chemistry teacher in case VCAA decides to dick you all with glutamate.

And for the amine group (on the Z group) in asparagine?

It's an amide. That side chain is actually considered neutral.

[mankay]

Cheers.

So you're probably thinking, who cares bro what about our exam?! In a sense this is a bit over the top for VCE because all I remember learning was the 'general' ionisation of the amino acid parts without all these R group exceptions ^see kyzoo's post

Yeah i'd came to the same method that kyzoo was using myself, but i wasn't sure whether or not it applied to the R group as well.

I think it's something for someone to confirm with their chemistry teacher in case VCAA decides to dick you all with glutamate.

I might just do that

It's an amide. That side chain is actually considered neutral.

Amide? Don't they have a C=O? So it stays as NH2?