Isoelectric point and zwitterions

hey so we're going to be talking about the isoelectric point or PII as it's abbreviated now the isoelectric point is the point along the pH scale at which a molecule and in this case you're going to be talking about an amino acid exists in a neutral form with zero charge in other words it is neither positively nor negatively charged overall it is ISO electric 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 a 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 sidechain 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 proton 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 zero 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 as what I on which comes from the german word for hybrid so now what would happen if we take our amino acid and we put it 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 a really basic solution and we can think of really basic solutions as having a lot of excess hydroxide I and ions 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 pH is 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 is where we find these whitter ion 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 pKa is just the negative log of the acid dissociation and so on average and it varies between all the different amino acids but on average the amino group has a pKa of around nine and then on average the pKa 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 P I or isoelectric point would be for our generic amino acid so taking the average PKA for the amino group and then the average PK for the carboxylic acid group and we divide by two then we get 11 over two 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 pKa for that group into account when we calculate the isoelectric point so what have we learned well we learned that the isoelectric point is the pH at which a molecules found in neutral form in this case when an amino acid is in its whitter ion form and we also learned how to calculate this isoelectric point for an amino acid by taking the average of the pka's of all the functional groups in that amino acid