= Isoelectric point
{title2=$\mathrm{pI}$}
{wiki}
The <isoelectric point> is the <pH> at which a specified <molecule>'s mean net <electric charge> is zero. Different <protonation> states can still carry nonzero <electric charges>; their contributions cancel in the average. For the two-step <glycine speciation model>, this gives $\mathrm{pI}=(\mathrm pK_{a1}+\mathrm pK_{a2})/2$. Additional ionizable <amino acid side chains> require their own <chemical equilibrium> calculations; averaging every <pKa> is not a general rule. <Organic Chemistry by John McMurry>[§26.2, discussion following Figure 26.2: isoelectric points and the dissociations surrounding the neutral form]{ref}
Below its <isoelectric point>, free <glycine> has positive mean <electric charge> and an <electric field> drives it toward the negative <electrode>; above its <isoelectric point>, the mean <electric charge> is negative and the direction reverses. At its <isoelectric point> the charge-driven contribution vanishes. <Diffusion> and bulk movement of the <chemical solution> can still move the sample. <Gel electrophoresis> applies this charge dependence to separation, although <protein> size, shape and interactions also affect migration. The <pKa> values of free <glycine> do not determine a <protein>'s <isoelectric point>: its free <N-terminus>, <C-terminus> and ionizable <amino acid side chains> supply a different inventory. <Organic Chemistry by John McMurry>[§26.2, Figure 26.3 and accompanying electrophoresis discussion]{ref}
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