= Glycine protonation
The three principal aqueous <glycine> forms differ by loss of one <proton> at each step:
$$
\begin{aligned}
\mathrm{H_3N^+{-}CH_2{-}COOH}+\mathrm H_2\mathrm O&\rightleftharpoons\mathrm{H_3N^+{-}CH_2{-}COO^-}+\mathrm H_3\mathrm O^+,\\
\mathrm{H_3N^+{-}CH_2{-}COO^-}+\mathrm H_2\mathrm O&\rightleftharpoons\mathrm{H_2N{-}CH_2{-}COO^-}+\mathrm H_3\mathrm O^+.
\end{aligned}
$$
Call these <glycine> forms $G^+$, $G^0$ and $G^-$, respectively: a <cation>, a <zwitterion> and an <anion>, with net <electric charges> $+e$, $0$ and $-e$, where $e$ is the <elementary charge>. $G^0$ specifically denotes the <zwitterion>, not an uncharged amino/carboxyl drawing. The first dissociation removes the carboxyl <proton>; the second removes a <proton> from the positively charged <nitrogen>. The <chemical equilibrium> changes are reversible, not three permanently separate kinds of <glycine>. <Organic Chemistry by John McMurry>[§26.1, aqueous zwitterion protonation/deprotonation; §26.2, two successive amino-acid dissociations]{ref}
For all calculations and both diagrams here, explicitly stipulate the <pKa> values $\mathrm pK_{a1}=2.35$ and $\mathrm pK_{a2}=9.78$. These are illustrative model inputs, not measurements from the <glycine titration experiment> or a claim of universal constants. Actual <acid dissociation constants> depend on <temperature> and on the convention used for <thermodynamic activity>; apparent <concentration>-based values also depend on <ionic strength>. For comparison, the inspected textbook tabulates $2.34$ and $9.60$ for <glycine>, which would give a different <isoelectric point>. <Organic Chemistry by John McMurry>[§26.1, Table 26.1, glycine row: pKa 2.34, 9.60 and pI 5.97]{ref}
\Image[/glycine-protonation-speciation-model.png]
{title=<Glycine protonation> and calculated <chemical species> fractions}
{description=Original three-state ideal <glycine speciation model>, with stipulated <pKa> values $2.35$ and $9.78$. The structures show net <electric charges> $+1$, $0$ and $-1$ in units of the <elementary charge>. Curves are calculated fractions, not measured populations. The vertical markers locate the two <pKa> values and the model <isoelectric point> $6.065$.}
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