isoelectric point of aspartic acid

[steezmonster]

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For an amino acid with an acidic R group how do we get to an isoelectric point?

For example, aspartic acid has a CH2COOH group so when titrating, it would lose both COOH protons first giving it a 2- charge then the amino group would become protonated giving it a -1 overall charge... how can this ever average out to a neutral charge regardless of concentrations?

Thank you for any help in advance.

 

[FeinMS]

Side chain acidic proton and alpha-carboxyl acidic proton have different pKa. They are not ionized at the exact same pH. I can't recall which one has lower pKa and thus gets deprotonated first though. But once you find out 3 pKa value, you should be able to find isoelectric point just like any other amino acids.

 

[Vurday]

Let's first take a look at the pKa of the protons on Aspartic Acid:

pKa1: Carboxyl end: 2.10
pKa2: R group: 3.86
pKa3: Amino end: 9.82

Assume that pH is 0 or 1. At this point, pH < pKa1,2,3
So, all three positions would be protonated at this pH.
So at a very low pH, the charge on Aspartic acid is +1 (due to the protonated amino group)
To be able to find the pI, you will need to deprotonate just one acid. This occurs at the first equivalence point. To find the pH where this occurs, you can take:

pI = (pKa1 + pKa2)/2
= 2.10 + 3.86 = 5.96/2 = 2.98

This general formula works for ALL amino acids that don't have basic side chains.

For Histidine, Arginine, and Lysine (Basic AAs), you will need to deprotonate one more time, and so neutral pH will be at the second equivalence point, so:
pI = (pKa2 + pKa3)/2

 

[steezmonster]

Thanks man! Somehow I got confused with the starting conditions and let myself think that the amino group wouldn't be protonated at the beginning... I start thinking some ridiculous stuff after studying all day lol.