Editing Amino acid (section)

===Isoelectric point===
[[File:Titration Curves of 20 Amino Acids Organized by Side Chain.png|class=skin-invert-image|thumb|right|upright=1.5|Composite of [[titration curve]]s of twenty proteinogenic amino acids grouped by side chain category]]

For amino acids with uncharged side-chains the zwitterion predominates at pH values between the two p''K''<sub>a</sub> values, but coexists in [[Chemical equilibrium|equilibrium]] with small amounts of net negative and net positive ions. At the midpoint between the two p''K''<sub>a</sub> values, the trace amount of net negative and trace of net positive ions balance, so that average net charge of all forms present is zero.<ref>{{cite book | vauthors = Fennema OR |title=Food Chemistry 3rd Ed |publisher=CRC Press |pages=327–328 |isbn=978-0-8247-9691-4 |date=1996-06-19 }}</ref> This pH is known as the [[isoelectric point]] p''I'', so p''I'' = {{sfrac|1|2}}(p''K''<sub>a1</sub> + p''K''<sub>a2</sub>).

For amino acids with charged side chains, the p''K''<sub>a</sub> of the side chain is involved. Thus for aspartate or glutamate with negative side chains, the terminal amino group is essentially entirely in the charged form {{chem2|\sNH3+}}, but this positive charge needs to be balanced by the state with just one C-terminal carboxylate group is negatively charged. This occurs halfway between the two carboxylate p''K''<sub>a</sub> values: p''I'' = {{sfrac|1|2}}(p''K''<sub>a1</sub> + p''K''<sub>a(R)</sub>), where p''K''<sub>a(R)</sub> is the side chain p''K''<sub>a</sub>.<ref name="Vollhardt-2007">{{Cite book | vauthors = Vollhardt KP |title=Organic chemistry : structure and function |date=2007 |publisher=W.H. Freeman |others=Neil Eric Schore |isbn=978-0-7167-9949-8 |edition=5th |location=New York |pages=58–66 |oclc=61448218}}</ref>

Similar considerations apply to other amino acids with ionizable side-chains, including not only glutamate (similar to aspartate), but also cysteine, histidine, lysine, tyrosine and arginine with positive side chains.

Amino acids have zero mobility in [[electrophoresis]] at their isoelectric point, although this behaviour is more usually exploited for peptides and proteins than single amino acids. Zwitterions have minimum solubility at their isoelectric point, and some amino acids (in particular, with nonpolar side chains) can be isolated by precipitation from water by adjusting the pH to the required isoelectric point.