[ducthinh]

    In some simple aminoacids such as Gly , Ala ... , they are only contain one cacboxyl group and one amine group so their isoelectric point can easily calculated by pHi = 1/2*(pKa1 + pKa2) . But in some more complicated aminoacids such as diaminocacboxylic or aminodicacboxylic ... they have more than one amino group or more than one cacboxyl group so they have three pKa's and I dont know how to choose which pKa to calculate the pHi of that aminoacid... thanks for any explanation...

[Albert]

You always have to consider what the zwitterion is: it's a condition where an amino acid shows no net charge. You can think of it as a state in between a mono-cationic and a mono-anionic form.

Whenever you have more than two pKa-values, you have to work out what condition they represent. For example, aspartic acid has 2.1, 3.9 and 9.8 as pKa values.
The first one 'represent' the cation: 1 net positive charge.
The second one is owed to the presence of 1 net negative charge.
The third is because of a possible second negative charge.

Now, going back to what I said in the beginning, you have to find where your zwitterion is. The answer is, of course, between pKa1 and pKa2:

pI = (pK1 + pKa2) / 2

in this example.

[ducthinh]

 ah`...do you mean that aspartic acid may have three pHi values ??

[Albert]

Not even once in a blue moon!

Every amino acid and protein has only ONE isoelectric point.
I was trying to link pKa-values to net charges.

[Albert]

...in a nutshell, the isoelectric point is the mean of the two closest pKa-values.

In my first post, I tried to give you an explaination. Maybe, with the help of a picture, it'll be more simple.

[Borek]

What about isoelectric points in case of R(NH₂)₂(COOH)₂?

RNH₂NH₃(COOH)(COO) - effective charge 0

R(NH₃)₂(COO)₂ - effective charge 0

No idea if such aminoacid exists, but it doesn't look completely impossible.

So is there always only one isoeletric point, or is it possible there is more?

[sdekivit]

What about isoelectric points in case of R(NH₂)₂(COOH)₂?

RNH₂NH₃(COOH)(COO) - effective charge 0

R(NH₃)₂(COO)₂ - effective charge 0

No idea if such aminoacid exists, but it doesn't look completely impossible.

So is there always only one isoeletric point, or is it possible there is more?

this aminoacid doesn't excist

--> there are only 20 different amino acids with all the same structure:

H2N - CHR - COOH

in this amnioacid, there must be 1 NH2 and 1 COO-group in the restgroup and that's impossible to be an aminoacid.

[Albert]

there are only 20 different amino acids

Yes, indeed. And before anyone points it out, also the other less common or not present in proteins amino acids have the same structure sdekivit drew...and one isoelectric point.

[Yggdrasil]

What about isoelectric points in case of R(NH₂)₂(COOH)₂?

RNH₂NH₃(COOH)(COO) - effective charge 0

R(NH₃)₂(COO)₂ - effective charge 0

No idea if such aminoacid exists, but it doesn't look completely impossible.

So is there always only one isoeletric point, or is it possible there is more?

RNH₂NH₃(COOH)(COO)

The carboxyl group of an amino acid has a pKa of about 2 while the amino group of a carboxyl group has a pKa of about 9.5.  So, the compound would quickly react in an intramolecular acid/base reaction to form the first compound you listed.  Therefore, I don't think that the compound above would be stable at any pH.  So even the amino acid you listed would have one isoelectric point.

[Borek]

RNH₂NH₃(COOH)(COO)

The carboxyl group of an amino acid has a pKa of about 2 while the amino group of a carboxyl group has a pKa of about 9.5.  So, the compound would quickly react in an intramolecular acid/base reaction to form the first compound you listed.  Therefore, I don't think that the compound above would be stable at any pH.  So even the amino acid you listed would have one isoelectric point.

I am not asuming alpha-aminoacid (neither did OP, at least he never stated it)! R can stand for anything - both amino and carboxylic groups can be separated and their properties will depend on their closest neighbours. Thus you can't say 2 or 9.5 - and it  seems very likely that more complicated aminoacid will have such pKa values that they will allow two isoelectric points.

[ducthinh]

Cystein residues can form dissulfide bridge , this disulfide-linked dimer of cystein is cystine.It has two cacboxyl groups and two amine groups , but in my reference book , I saw that it has only one isoelectric point .

[Albert]

First of all, I'd like to know if ducthinh understood how to calculate isoelectric points, because this is what this topic is about.

Then, Borek's structure reminded me of ampholytes (synthetic polyamino-polycarboxylic acids): in a mixuture of them, each different acid has its own single isoelectric point and this is a basic principle of IEF.

However, if I were an admin, I'd move all posts dealing with this theme in a different and new topic.

[ducthinh]

Oops... I am studying aminoacids and actually dont know much about it ... thanks for your help , I have understood more .
Maybe my chem book wrong since it listed Cystine in the table of common aminoacids ...huh?

[Albert]

It's wrong. YOU are right.

Maybe it's a misprint: they probably confused cystine with cysteine...as I often do