Functional groups on AAs at different ph levels

[jadams]

Hi all,

I hate to be one to ask questions 12 hours before the exam, just one little thing I'd love to have clarified if anyone can do so.

At say a neutral ph, amino acids with non-polar side chains can just be drawn as either the zwitterion or as written in the data book. At basic ph levels, the coo- group will be deprotonated, and the alpha amino group will remain unchanged. At acidic ph levels, the alpha amino group will become protonated, and the acidic group will remain the same.

IF i were drawing aspartic acid/ another amino acid with either a NH2 or COOH functional group in its Z side chain, how would I do so in basic, neutral, and acidic environments?

According to this: http://chemistry2.csudh.edu/rpendarvis/aminoacids.html         Aspartic Acid would be drawn with all functional groups on the molecule acting as either acids or bases compared to their standard form.

Appreciate anyone's clarification.

Cheers

[Mao]

For AA with ionisable side chains (-COOH or -NH2, NOT amides -CONH2), the easiest way is imagine if you were doing a titration. Let's for now completely ignore the amino acid part, and only look at the Z group.

e.g. Aspartic acid.
We start at pH=1, highly acidic environment, a lot of H+ in solution, so we cannot keep the anion form. Just like the acidic form of the backbone, we have -COOH.
We now increase the pH by adding base. This means -COOH reacts with the base as pH increases. At ~neutral pH, we will have -COO(-)
We add more base, and pH keep increasing, but we don't have any acidic groups to react with anymore, so we remain at -COO(-)

:. at highly acidic environments, -COOH
:. at neutral environments, -COO(-)
:. at basic environments, -COO(-)

Similar argument for basic Z-group.

at highly basic environments, a lot of proton acceptors, so we cannot be a cation -NH3+ as something else will take the proton away
:. at basic environments, -NH2
:. at neutral environments, -NH3+
:. at acidic environments, -NH3+

The behaviour of the Z-group doesn't affect the behaviour of the backbone amino-acid much, so use the same method as always to determine whether you have acid/zwitterion/basic form for the amino-acid group.

[jadams]

Mao, you're a legend mate.

That's a much easier way to look at things. Time to sleep.

Thank you very much.

[bentennason]

So COOH and NH2 in the ionisable side chains also behave the same way as the amino and carboxyl group in the amino acid backbone when exposed to acidic/basic conditions?

[nina_rox]

When in a pH of 7 don't all the COOH and NH2 become ions? So the overall charge say for aspartic acid is -1 (-1 from the two COOH groups and +1 from the NH2 group) is this similar to zwittor ion?

[Mao]

@bentennason, not quite the same way, but for the purposes of VCE they're roughly the same.

@nina_rox, that's right.

[nina_rox]

Okay so for zwittor ions do only the backbone COOH and NH2 change? So zwittor ions are always neutral?