Buffering action of amino acids

[Martoman]

I always like to think of this stuff in terms of the zwitterion.

In neutral you will have: .

In basic this will donate its H from N.

In acidic it will gain a H to its COOH.

I like this slightly better as its more *realistic* as you will hardly ever find amino acids in the way they show it in the data book.

[m@tty]

What about the side chain?

[Martoman]

In zwwwwwitter ions (german accent) say in glutamic acid the side chain can't have ionised as we need a net charge of 0. So it stays with its COOH. In acidic it will do nothing. In basic, it will donate its proton as well as the ammonia will change to an amine group.

[Toothpaste]

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

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

Yep, that chemical group there is called an amide and is different to amines. They're weaker as a base than amines (the C=O takes the lone pair of electrons from N and that stabilises it via resonance) so the R group on asparagine won't act like lysine or arginine. So yeah it stays as NH2.

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

Report back if you can, might make life easier for everyone


EDIT: see http://vcenotes.com/forum/index.php/topic,26424.msg267995.html#msg267995

[mankay]

Report back if you can, might make life easier for everyone Tongue

Well I asked my teacher and although she's not an organic chemist, she said for the exam, that if the amino acid is in acidic solution, any amino groups on the R group should be ionised, and if it's in a alkaline solution, the carboxy groups should be drawn ionised.

Does that sound okay?