Hey, so we're
going to be talking about the isoelectric point,
or pI as it's abbreviated. Now, the isoelectric point is
the point along the pH scale at which a molecule,
and in this case we're going to be talking
about an amino acid, exists in a neutral
form with zero charge. In other words, it is neither
positively or negatively charged overall. It is isoelectric,
and "iso" means equal. And it's nice to know
the isoelectric point for an amino acid, because then
we can predict whether or not it will be charged
at a certain pH. And who doesn't want
the power of prediction? So how do we figure out
the isoelectric point for an amino acid? Well, let's start with the
generic amino acid structure here. So now let's take a look at
the two functional groups on this amino acid. Ignoring the R group, or the
side chain, for the time being, we're going to be talking
about the amino group and the carboxylic acid group. So the amino group here,
it has this nitrogen, which is a very happy
proton acceptor. So we're going to
write that here. And because it's a
happy proton acceptor, it is considered to be basic. And we've drawn it out
in its protonated form here after it's accepted an
extra hydrogen, or proton. So now coming over to our
carboxylic acid group, this group is a very
willing proton donor. And because it is a proton
donor, we call this acidic. And so we've drawn it out
here after it's already donated its protons, so
it has a negative charge. And now looking at the overall
net charge of our amino acid, we can see that we have
a positive charge here and a negative charge here,
and so the overall charge is 0. And we have a
special name for when you have a molecule
that has both a positive and a
negative charge present. And that special word is
called a "zwitterion," which comes from the
German word for "hybrid." So now what would happen
if we take our amino acid and we put into a
solution that is a very low pH, say a pH of 1? In other words, an
acidic solution. Well, we can think
of acidic solutions as having a lot of
excess protons around. So anything that can be
protonated on our amino acid is going to be
protonated, and so it's going to look like this. And now if you
take a look at both of the groups on
our amino acid, you can see that our amino group
is still in its protonated form and carries a positive charge. But now our carboxylic acid
group has gained a proton and lost its negative charge. And now you can see that
the overall net charge on this molecule
is now positive 1. So now let's come over to the
other end of the spectrum. Let's put our amino acid in a
solution with a very high pH, say a pH of 12. And so this is going to
be really basic solution, and we can think of
really basic solutions as having a lot of excess
hydroxide anions around. And so now, everything
that can be deprotonated on our amino acid will be, so
it's going to look like this. And if we look at our overall
net charge of our amino acid now, our amino group
has been deprotonated so now it is neutral, and the
carboxylic acid group has been deprotonated and so it has
a negative charge again. And so it has an overall
net charge of negative 1. So now we know that we
have a range of forms that our amino acid can take. We have the positively
charged version at low pHs all the way up to
the negatively charged version at high pHs. Now back to our question
about the isoelectric point. So the isoelectric
point is the pH at which we go from the
positive to the negative form. In other words, it's where
we find the zwitterion. And to find out the
exact pH, we have to take the average of the pKa's
of our two functional groups. And recall that the pK
is just the negative log of the acid
dissociation constant. So on average, and
it varies between all the different amino acids, but
on average, the amino group has a pK of around 9. And then on average, the pK
for the carboxylic acid group is right around two. So now if we just give ourselves
a little bit more room here, we can calculate what the
pI, or isoelectric point, would be for our
generic amino acid. So taking the average
pK for the amino group and then the average pK for
the carboxylic acid group, then we divide by 2,
then you get 11 over 2. And we come to an
isoelectric point of 5.5. But say our amino
acid has a side chain or an R group that is
also a functional group? Then, we would also have to
take the pK for that group into account when we calculate
the isoelectric point. So what have we learned? Well, we've learned that
the isoelectric point is the pH at which a molecule's
found in neutral form, in this case, when an amino
acid is in its zwitterion form. And we also learned
how to calculate this isoelectric point
for an amino acid by taking the average of the
pKs of all the functional groups in that amino acid.