Ammo acids

[sam0001]

How do you draw glutamic acid at a neutral pH?

[charmanderp]

I could just tell you but I feel like a bit of understanding would be prudent...

At neutral pH amino acids become what is known as a 'zwitterion'. That is, they are ionised yet maintain a neutral charge (an overall charge of 0). We know that amino acids have both acidic and basic properties, due to the presence of a -COOH and -H2N group respectively, so they have the ability to effectively ionise themselves.

What happens is that the acidic -COOH group donates a proton to the basic -H2N group to become -COO(-) and -H3N(+) respectively. Glutamic acid, from memory, has two -COOH groups (as much is suggested by the name) but only that on the alpha carbon (the one which has both an -H2N group and a -COOH group attached to it) actually loses a proton in neutral ph. Remember, we have to maintain neutral charge, so if both were to lose a H+ the overall charge would be -1!



That is how you draw any zwitterion. The R group represents the side chain which makes each amino acid unique - in the case of glutamic acid that is the -CH2CH2COOH chain which is attached the alpha carbon.

Now, what do we know about pHs below 7? They mean acidic conditions! Hence the negatively charged atoms on the zwitterion will receive electrons from the solution. We have a negative -COO(-) group which will now become -COOH, getting back the H+ it previously lost. However, the -H3N+ maintains its positive charge. So we have H3N+--C(R)H--COOH, with the R representing the side chain -CH2CH2COOH.

In basic solution (pH above 7), the -OH(-) molecules which are representative of a base would rip off all protons from the zwitterion. Thus it would become H2N--C(R)H--COO(-), only this time the R group is -CH2CH2COO(-) as it has lost its H+ ion.

Sorry for the crappy explanation, this would be easier in person. If you have any further inquiries, give me a buzz

[thushan]

I could just tell you but I feel like a bit of understanding would be prudent...

At neutral pH amino acids become what is known as a 'zwitterion'. That is, they are ionised yet maintain a neutral charge (an overall charge of 0). We know that amino acids have both acidic and basic properties, due to the presence of a -COOH and -H2N group respectively, so they have the ability to effectively ionise themselves.

What happens is that the acidic -COOH group donates a proton to the basic -H2N group to become -COO(-) and -H3N(+) respectively. Glutamic acid, from memory, has two -COOH groups (as much is suggested by the name) but only that on the alpha carbon (the one which has both an -H2N group and a -COOH group attached to it) actually loses a proton in neutral ph. Remember, we have to maintain neutral charge, so if both were to lose a H+ the overall charge would be -1!

(Image removed from quote.)

That is how you draw any zwitterion. The R group represents the side chain which makes each amino acid unique - in the case of glutamic acid that is the -CH2CH2COOH chain which is attached the alpha carbon.

Now, what do we know about pHs below 7? They mean acidic conditions! Hence the negatively charged atoms on the zwitterion will receive electrons from the solution. We have a negative -COO(-) group which will now become -COOH, getting back the H+ it previously lost. However, the -H3N+ maintains its positive charge. So we have H3N+--C(R)H--COOH, with the R representing the side chain -CH2CH2COOH.

In basic solution (pH above 7), the -OH(-) molecules which are representative of a base would rip off all protons from the zwitterion. Thus it would become H2N--C(R)H--COO(-), only this time the R group is -CH2CH2COO(-) as it has lost its H+ ion.

Sorry for the crappy explanation, this would be easier in person. If you have any further inquiries, give me a buzz

50 in chem, right here

[pi]

Sorry for the crappy explanation

Which explanation does this refer too?

[sam0001]

Thanks! It wasn't crappy.

What about at a ph of 4 though? It's above the isoelectric point?

[jasoN-]

At pH 7 the side group will be deprotonated as well, so you will have an overall charge of -1.
At a pH of 4, the alpha carboxylic acid group would be deprotonated, however the side chain group will be protonated, i.e. it has an overall charge of 0
I'm pretty sure this is right.

Edit: fixed

[charmanderp]

50 in chem, right here

A pH of 4 is that pH below 7, where the solution the amino acid is in is acidic. Hence all basic/negatively charged parts of the amino acid will be augmented by a H+ ion and become either -H3N+ or -COOH.

[sam0001]

So, at a ph of 4, over all charge +1 ?

[jasoN-]

A pH of 4 is that pH below 7, where the solution the amino acid is in is acidic. Hence all basic/negatively charged parts of the amino acid will be augmented by a H+ ion and become either -H3N+ or -COOH.

You're thinking is somewhat correct. A pH of under 7 is considered acidic, however you have to take into account the acidity/basicity of the functional groups.
So the carboxylic acid group on the alpha carbon is already moderately acidic (with a pKa ~2). At a pH of 4, even though the solution is acidic, the carboxylic acid group will exist primarily in it's ionised (deprotonated form).
I don't think you guys have learnt about pKa's, you just need to know that at a pH = pKa, the group will exist half in its ionised form, and half in its unionised form.
So to take the above example, at a pH ~ 2, the alpha carboxylic group will exist 50% unionised (COOH) and 50% ionised (COO-)
A pH < 7 does not mean EVERY acidic group will remain in its protonated form.

Glutamic acid's functional groups exist in its ionised form at a pH of 7.
At a pH of 4 (acidic), the alpha COOH group exists in the DEPROTONATED (COO-), however the side chain carboxylic group exists primarily in its PROTONATED (COOH) form.

I'm not entirely sure if you learn this in VCE chemistry, someone shed some light because the idea that in an acidic environment, all acidic groups are protonated is incorrect.

edit: grammar fix

[AllAboutTheLGs]

Types up 10/10 essay to explain zwitterions, then proceeds to refer it as crappy. TYPICAL CHEMDERP.

[thushan]

Types up 10/10 essay to explain zwitterions, then proceeds to refer it as crappy. TYPICAL CHEMDERP.

Choong, think you're just a tad harsh on this guy's essay. I'd give it a 12/10...

[pi]

I'm concerned about the spelling of "amino" in the title of this thread actually.

[sam0001]

Lol, sorry! Typed it on the iPad too quickly.

[Destiny]

Ammo....

AMMO ACIDS LET'S KILL VCAA~

[ggxoxo]

Would COOH and amino groups in teh R groups be affected by changes in pH as well?