Editing Isoelectric point (section)

== Calculating pI values ==
For an [[amino acid]] with only one [[amine]] and one [[carboxyl]] group, the pI can be calculated from the [[mean]] of the [[pKa]]s of this molecule.<ref>For derivation of this expression see [[acid dissociation constant#Isoelectric point|acid dissociation constant]]</ref>

: <math> \mathrm{pI} = \frac{\mathrm{p}K_\mathrm{a1} + \mathrm{p}K_\mathrm{a2}}{2} </math>

The [[pH]] of an electrophoretic gel is determined by the [[Buffer solution|buffer]] used for that gel.  If the [[pH]] of the buffer is above the pI of the protein being run, the [[protein]] will migrate to the positive pole (negative charge is attracted to a positive pole).  If the [[pH]] of the buffer is below the pI of the [[protein]] being run, the [[protein]] will migrate to the negative pole of the gel (positive charge is attracted to the negative pole).  If the [[protein]] is run with a buffer pH that is equal to the pI, it will not migrate at all. This is also true for individual amino acids.

=== Examples ===
{|style="float: right; border: 1px solid #BBB; margin: .46em 0 0 .2em;"
|-
|[[File:Glycine pI-en.svg|250px]]

|[[File:AMP pI-en.svg|250px]]
|-
|align=center|glycine pK = 2.72, 9.60
|align=center|adenosine monophosphate pK = 0.9, 3.8, 6.1
|}
In the two examples (on the right) the isoelectric point is shown by the green vertical line. In [[glycine]] the pK values are separated by nearly 7 units. Thus in the gas phase, the concentration of the neutral species, glycine (GlyH), is effectively 100% of the analytical glycine concentration.<ref>{{Cite journal|last1=Jensen|first1=Jan H.|last2=Gordon|first2=Mark S.|date=August 1995|title=On the Number of Water Molecules Necessary To Stabilize the Glycine Zwitterion|url=https://pubs.acs.org/doi/pdf/10.1021/ja00136a013|journal=Journal of the American Chemical Society|volume=117|issue=31|pages=8159–8170|doi=10.1021/ja00136a013|issn=0002-7863}}</ref> Glycine may exist as a [[zwitterion]] at the isoelectric point, but the equilibrium constant for the isomerization reaction in solution
:<chem>H2NCH2CO2H <=> H3N+CH2CO2-</chem>
is not known.

The other example, [[adenosine monophosphate]] is shown to illustrate the fact that a third species may, in principle, be involved. In fact the concentration of {{chem2|(AMP)H3(2+)}} is negligible at the isoelectric point in this case.
If the pI is greater than the pH, the molecule will have a positive charge.