Isoelectric Point - Video Tutorials & Practice Problems
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concept
Isoelectric Point of Amino Acids
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Video transcript
In this video, we're gonna begin our discussion on the isoelectric point of amino acids. So the isoelectric point can actually be abbreviated with the letters p I, which is shown here in parentheses. And, really, the isoelectric point is really just a pH value. It's really just a specific or an exact pH at which a molecule is going to have no net charge. And if it has no net charge, what that means is that if there are any positive charges, that they're gonna be completely balanced by negative charges because the overall net charge of the protein is going or of the amino acid is going to be 0 when the pH is equal to the isoelectric point. And so, in our practice videos for this topic, we're going to be asked to calculate the isoelectric point of amino acids. And so it's really important to keep in mind that the isoelectric point is always just gonna be the midpoint or the average between just 2 p k a values. And so that's important to keep in mind because even if there are 3 pka values given to you, or 4, or 5, or however many they give it give you, the isoelectric point is always just gonna be an average of just 2 pKas. And so that's really, really important to keep in mind. And specifically, the way that you know which 2 pKas to average is gonna have to do with, the fact that you're gonna wanna average the 2 pKas pKas for the 2 ionizations that involve the neutral species. And so that might sound like a little bit of gibberish to you guys, but really, all I want you guys to know is that when you're calculating the p I in our practice videos later on, you really just wanna make sure that you're gonna only average 2 p k a's. And so that's really all I want you guys to know for now. Now, another important trend to keep in mind is that more acidic amino acids so the more acidic amino acids actually have more acidic p I values, or more acidic isoelectric points, and the same applies with basic. So the more basic amino acids have more basic p I values, so that's an important trend to keep in mind. And then, the last thing I want to leave you guys off with is that when the p h is equal to the p PI of a molecule, we already know that the molecule is going to have no net charge like we already stated earlier. But what's also important to know is that the molecule when it has no net charge, it's actually not going to migrate in an electric field. And that's really why the isoelectric point is such a valuable property because scientists can adjust the pH to try to get it to equal the exact p I of a molecule. And that means means that it's gonna have no net charge, and they can use the isoelectric point of a molecule in a a technique called electrophoresis. And electrophoresis basically allows scientists to separate proteins that have a net charge using an electric field. And we'll talk a lot more about electrophoresis in another topic later on in our course in a different video. But for now, all I want you to know is that the p I or the isoelectric point of a a protein or an amino acid is a useful property because it allows them to separate and isolate specific amino acids and proteins of interest using electrophoresis. And so, in our next video, we're gonna talk about how do you go about calculating the isoelectric point of an amino acid with a non ionizable r group. So, I'll see you guys in that video.
2
concept
Isoelectric Point of Amino Acids
Video duration:
2m
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Video transcript
So in our last video, we said that the isoelectric point or the p I is just a pH value. It's the exact pH pH where a molecule is gonna have an overall neutral charge. And so now that we know that, we can talk about how to calculate the isoelectric point of amino acids acids with non ionizable r groups. And so the isoelectric point can actually be pretty easily calculated just by using the p k a values of a molecule. And the amino acid isoelectric point is really just a simple average. It's really just a pretty simple average of the appropriate 2 p k a values. And that's what we said in our previous video too, that even if you're given multiple p k a values, that the p I is always just gonna be an average of 2 p k a's. And so the good thing is that with non ionizable R groups, these amino acids only have 2 pKas. So it makes it pretty easy to figure out which 2 pKas you need to average. And so down here, what we have is the formula for the isoelectric point. And what you can see is that the isoelectric point is really just the average of the correct, p K a values, the proper 2. So we have p K a 1 and p K a 2, and it's it's just gonna be an average of these 2. And so in our example, it asks us to calculate the isoelectric point of valine. And recall that valine is one of the 13 amino acids that does not have an ionizable R group. Valine has a non ionizable r group, which means it only has 2 p k a values, and the 2 p k a values are given to us. So all we need to do to calculate the isoelectric isoelectric point is to take the average of the 2. So the isoelectric point is just gonna be 9.62 plus 2.32 32 divided by 2. And so this comes out to 11.94 divided by 2. And if we type this into our handy dandy calculators, the answer comes out to be 5.97. And so what we can see is that the p I availing is gonna be 5.97, which matches up with option c, and all the other options are incorrect. So we can give c a check mark here to indicate that c is correct. And so we'll be able to get more practice calculating the isoelectric point of non ionizable r groups, of amino acids with non ionizable r groups in our next practice video. So I'll see you guys in those videos.
3
Problem
Problem
Draw the structure of Leu and determine its pI. pK a1 = 9.6. pKa2 = 2.4.
a. 7.59
b. 6.0
c. 3.91
d. 5.5
A
7.59
B
6.0
C
3.91
D
5.5
4
Problem
Problem
At any pH below the pI, the population of amino acids in solution with non-ionizable R-groups will have:
A
a net negative charge.
B
a net positive charge.
C
no charged groups.
D
no net charge.
E
positive and negative charges in equal concentrations.