pH, pKa, pI and protein charge
The isoelectric point or pI of an amino acid is the pH at which an amino acid has a net charge of zero. (Learn more about amino acid side chains link) The pKa of the protonated methylamine conjugate acid is like this: at. The relationship between pH, pK and charge for individual amino acids can be In proteins the isoelectric point (pI) is defined as the pH at which a protein has. Thus, the isoelectric point is the value of pH at which of zero charge at the particle surface, but this difference is pK− and pK+ to define the two conditions in terms of the.
And who doesn't want the power of prediction? So how do we figure out the isoelectric point for an amino acid?
Amino Acid Charge in Zwitterions and Isoelectric Point MCAT Tutorial
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.
A Quick Guide to pH, pKa and pI
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. 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.
Ch Isoelectronic point
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 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.
As the pH continues to rise, more and more molecules will deprotonate till the neutral uncharged form dominates. Finding Charge on Amino Acids in Preparation for Isoelectric Point Calculations While we started by analyzing acetic acid and methylamine independently, the same concept applies when analyzing the amino and carboxyl groups on an amino acid.
The key to understanding isoelectric point is to understand how to find what the charge is at any pH, including when the net charge is zero. With just a hydrogen in place of its variable group, we only have the backbone to examine. As we analyze the structure of glycine at different pH levels, we see only two values, one each for carboxyl and amino groups, on the amino acid pKa table.
Since pKa relates to an equilibrium constant, you will always have one more structure than the number of pKa values; for example, if there were two pka values, we would expect three structures. When the pH is considerably lower than the pKa we expect both sides to be fully protonated.
Isoelectric point and zwitterions (video) | Khan Academy
When we raise the pH a few units above the first pKa, and still well below the second pKa value, the carboxyl group will lose its proton; however, the amino group is still protonated. This is the zwitterion form, with a positive and negative to cancel out.
When you raise the pH to well above the amino value, the nitrogen will lose its proton and thus its charge. We now have negative and zero for a net charge of The zwitterion form can exist anywhere between the the 2 pKa values.
So how does this relate to the isoelectric point? Do we randomly pick a value? As explained in the buffer video abovewhen the pH is exactly at the pKa value, we have an ideal buffer where the molecules exist in equilibrium. Now if we raise the pH to 9. The isoelectric point is the average of the 2 pKa values that have a neutral molecule as one of its equilibrium species.
In other words, find the pKa that takes the amino acid from neutral to -1 9. This sounds like a great deal of work for an amino acid with just 2 side chains. This is especially critical when dealing with acidic or basic amino acids that have a third pKa value for their side chain. Do we take the average of all three? If just two, which two? Find the pKa which represents the equilibrium between the positive and neutral form. Find the pKa which represents the equilibrium between the negative and neutral form.
And average those two. Since 1 is less than every given pKa, we have too many protons in solution and EVERY potential group will be protonated. This pKa should automatically pop out at you as the pKa between zero and positive 1.
Now raise the pH to 3. There will be a The most acidic carboxyl will be deprotonated and negatively charged, the less acidic carboxyl remains protonated and thus neutral.
The pH is still too acidic for the amino group, which remains protonated and positive. This gives us a net charge of 0 and our zwitterion form. This pKa represents the equilibrium between the side chain protonated neutral carboxyl and the deprotonated net negative form. Remember, 3 pKa values implies 4 structures. This results in a net charge of Now if we raise the pH well above the highest pKa value to a pH of 12, the solution will be too basic for any protons to remain.
With so much OH- in the solution, every possible acidic proton will be grabbed off the amino acid and will react with OH- to form water.