Calculating the isoelectric point of amino acids

[StarryNights]

Given that aspartic acid has the following pkas, how would you calculate the ip? For the COOH group pka is 2.1, for the NH3 group pka is 9.8, and the R group pka is 3.9

I know to average the pkas of carboxyl and amino groups for simple a.acids, but what happens when the R group contributes as well? Kaplan has a 3 paragraph explanation for this, and it might as well be a verbal passage cause I have no clue what they're trying to say. If anyone can help, I'd really appreciate it. Thanks!

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[loveoforganic]

Average the pKa of the functional group that's protonated right after the neutral state and the pKa of the functional group that's protonated right before the neutral state.

 

[Eye Cue]

For neutral amino acids: pKa(N) & pKa(C)
For acidic amino acids: use the two more acidic pKa's
For basic amino acids: use the two more basic pKa's

 

[Eye Cue]

Woops, sorry loveoforganic, didn't see that you beat me to the post!