[fighter127]

In my o chem book it describes calculating the pI as taking the pka of similar ionizing groups on an amino acid and averaging the 2. However it mentions that there is an exception to this method for the amino acids tyrosine and cysteine. Can someone explain this in more detail to me? Does it have something to do with the pka of the side chains?

[Babcock_Hall]

Do you know how to calculate the pI for glutamic acid, for example?  Can you explain the logic behind it?