[help784]

Hi!

aspartic acid is a weak acid that supposedly completely dissociates its side chain at a pH of 5.5.  This is according to my teacher.  I don't understand why.  When I asked the teacher, they said God made it that way.  The question is bothering me since I will probably see this on a test or a quiz at some point.

I do understand that aspartate is a charged amino acid, very hydrophilic and the pKa value is for the side chain.  But why does it dissociate completely?  Are there calculations using the Henderson-Hasselbach equation or other equations to find the net charge. Please help, this lack of explaination is bothering me and I don't get what it going on.

[Wreath]

Try to look at this: http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/Amino_Acids/Fig5_10GlyTitr.GIF
Focus at problems what does pK mean, what pI stands for. What and how much is dissociated/deprotonated around the pKs, how much is protonated/deprotonated at pI. Although this is not picture for aspartate, it might lead you to the right idea. Or, try to find titration curve for some triprotic acid.