Ionisation points of amino acids, pKa, pH?

Question

I have the experimental data (pH, when HCl and NaOH added) of various amino acids and need to plot the data on graphs, draw the aa's at their ionisation points and calculate pKa values. Do the structures of the aa's actually change at their ionisation points?

Answer 1

I guess that you know the simplified structure of amino acids: they contain a carboxyl group, an amino group and a R group.

Proton of the carboxyl group tend to be transferred into amino group which leads into the formation of a "salt" and over 90% of the amino acids are in the form of this peculiar ion, having both positive (NH3+) and negative (COO-) charge. This results an ion which is not electrically charged towards its environment.

Real ionisation of aa happens when OH- or H+ acts with it, resulting R-CHNH2COO- or R-CHNH3COOH+, respectively. aa's are thus amphoteric and react with hydroxyls or protons only in the -CHNH2COOH group. This causes no modifications of the R- group.

If the subject of your study are proteins, other things can happen. But observing only the chemical reactions of amino acids, no modification of R- group happens, I guess.

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Answer 2

I assume that you're going to calculate pKa from the point of half-neutralisation each time.
At the "ionisation point", proton transfer from the carboxylic acid group to the amine group is complete. Look up "zwitterionic form" of amino acids.