What is the titration curve of glycine?

The titration curve for glycine looks like the titration curve for a weak diprotic acid.
Below is a typical curve for the titration of glycine with NaOH.

(from elte.prompt.hu)
Although we often write glycine as NH₂COOH, it is really a zwitterion, ##stackrel(+)(“N”)”H”_3″CH”_2″COO”^⁻##.
The fully protonated form of glycine is ##stackrel(+)(“N”)”H”_3″CH”_2″COOH”##.
The protonated form of glycine ionizes in two steps:
Step 1 is the loss of ##”H”^+## from the carboxyl group.
##stackrel(+)(“N”)”H”_3″CH”_2″COOH” + “H”_2″O” ⇌ stackrel(+)(“N”)”H”_3″CH”_2″COO”^⁻ + “H”_3″O”^+##
Step 2 is the loss of ##”H”^+## from the less acidic ##”NH”_3^+## group.
##stackrel(+)(“N”)”H”_3″CH”_2″COO”^⁻+ “H”_2″O” ⇌ “NH”_2″CH”_2″COO”^⁻ + “H”₃”O”^+##
The first equivalence point, at 50 % titration, is at ##”pH” = 5.97##.
Halfway between 0 % and 50 % titration (i.e. at 25 %) ##”pH” = “p”K_”a1″##.
The second equivalence point, at 100 % titration, is at ##”pH” = 11.30##.
Halfway between 50 % and 100 % (i.e. at 75 %), ##”pH” = “p”K_(
“a2″##.
At 50 % titration, the glycine exists as a zwitterion.
This is the isoelectric point ##”pI”##.
At this point, ##”pH” =”pI”##.
##”pI” = ½(“p”K_”a1” + “p”K_”a2″)##
For glycine, ##”p”K_”a1″ = 2.34##, ##”p”K_”a2″ = 9.60##, and ##”pI” = 5.97##.
Each amino acid has a characteristic set of ##”p”K## and ##”pI”## values.
Thus, you can use a titration curve to identify an unknown amino acid.