How to read an amino acid titration curve

As you add base, you strip protons off the amino acid one group at a time. The flat, gently sloped stretches are buffering regions, each centered on a pKa — that is the half-equivalence point, where the group is half protonated and resists pH change best. The steep vertical jumps are equivalence points, where a proton has been fully removed. The isoelectric point (pI), where the molecule carries no net charge, sits at the midpoint of the two pKa values flanking the neutral zwitterion: for a simple amino acid like glycine that is just (pKa₁ + pKa₂)/2, and for one with an ionizable side chain you average the two pKas around the net-zero species.

pKa values are standard textbook values (Lehninger); sources vary by a few hundredths.