In this video, we're going to talk about the titrations of amino acids with nonionizable R groups. So recall that way back in our previous lesson videos, we reviewed titrations from your old chemistry courses. And really, this lesson here on the titrations of amino acids with nonionizable R groups is just an extension of those older lesson videos. A lot of the information that we're going to cover in this video is just going to be review. A lot of the principles and concepts that we already learned about titrations are just going to be applied specifically to the titrations of amino acids with nonionizable R groups. But if you don't remember much about titrations at all, be sure to go back and rewatch those older videos before you continue here. Now, that being said, it's important to note that all amino acids are polyprotic acids. We know that polyprotic acids are acids that have multiple acidic hydrogens, and we know that each acidic hydrogen has its own pKa value, so all amino acids have multiple pKa values. At least 2 pKa values, one for the alpha amino group on the backbone, and one for the alpha carboxyl group on the backbone.

Also, recall from our previous lesson videos that titrations can reveal the pKa values of weak acids, and that point on the titration curve is referred to as the inflection point, which is also called the midpoint. Because this is exactly where half of the molar equivalents of titrant have been added to neutralize half of an acid. And so the inflection point, or the midpoint, corresponds to each pKa value. Also, recall from our previous lesson videos that the equivalence point is also known as the endpoint because that is exactly where 100% of an acid has been neutralized, and that is the end of an acid. It represents the point of neutralization of an acid.

Down below in our example, we're going to do some titration curve review. You can see that we have a titration for an amino acid with a nonionizable R group. The surrounding titration curve, we have all of these colored boxes that correspond to the colors of specific regions found within our titration curve. We're just going to go around and fill out these boxes as we review the titration curve. We'll actually start off with this black box here in the bottom right, which is asking what does the black curve represent? The black curve is just referring to this black curve that we see going up through our graph. We know that this is going to be alanine's titration curve. We know that it's alanine because it says so at the top left here—it tells us that it's alanine's titration curve.

Now we'll move on to this green box here in the bottom left. This is asking what the green dot and lines represent. Notice that they're showing up at exactly 0.5 molar equivalents of titrant being added. So that means this has got to be a midpoint or an inflection point. Before this point, alanine's titration curve is curving downwards. After the titration curve, notice that the curve changes to an upward curve, so we know that this is an inflection point. Each inflection point and midpoint corresponds with a pKa value, so we can see we have a pKa value of 2.4 here. Carboxyl groups on the backbones of amino acids have pKa values in the ballpark range of about 2. This is really close to 2 and right in that ballpark range.

So, this must be the pKa value for the carboxyl group. Alright, moving on, we'll skip this yellow box really quickly and jump up to this dark blue box at the top left. You'll notice this is another midpoint because it's asking us what the dark blue dot and lines represent corresponding to this dark blue dot here and dark blue lines coming up and down. It's another midpoint because, again, what we see is a curve. Notice that it's curving downwards like this. After the point, it curves and changes its curvature to going upwards. This lets us know that this is an inflection point or a midpoint. Each midpoint corresponds with a pKa value, and this is a pKa value of 9.7. Amino groups on the backbones of amino acids have pKa values in the ballpark range of about 9 to 10.5.

this falls right in that ballpark range, so this must be the pKa value of the amino group. We can jump down to the yellow box here, which is asking us what the yellow dot and line represent. Here we see the yellow dot in the center and the yellow line going across to the left. This is referring to the isoelectric point. We know that the isoelectric point, also known as just the pI, is an average of the 2 pKa values for an amino acid with a nonionizable R group. Each pKa value corresponds with the y-axis of pH shown right here at this point. For amino acids with nonionizable R groups, we can take the average of these 2 pKa values, and that'll give us the pI, or the isoelectric point, which is what's showing up here, right with this yellow line here. We know from our previous lesson videos that the isoelectric point is the exact point where alanine's net charge is 0; it's going to have a neutral net charge.

We're moving on to the pink box at the top right, which is asking what the pink line represents. It's this pink vertical line going down like this. It appears at exactly 1 molar equivalent of titrant being added, which means that we must be neutralizing 100% of an acid. That means this has got to be an equivalence point or an endpoint because we're neutralizing 100% of an acid. Since we have the carboxyl group acid on the left, down below this pink line, this must be the neutralization of the carboxyl group.

This is the equivalence point of the carboxyl group. We're neutralized at this point here, this pink vertical line; we are essentially neutralizing 100% of the carboxylic acid, conjugate acid. This means that there's no more conjugate acid form of the carboxyl group, so no more COOH at any point after this pink line over here, moving to the right.

Now we're moving on to the next box over here, corresponding to the light blue boxes that we see in the background. We have 2 light blue boxes referring to the effective buffering ranges. We know that effective buffering ranges are within plus or minus one of the pKa. You can see that this blue box is referring to pKa plus or minus 1. This point up here and this point up here make up the effective buffering range, essentially, this whole area here between these pHs. The same applies to this one down here. The effective buffering range would be within plus or minus one of the pKa, so between 1.4 and 3.4. The effective buffering range is plus or minus 1 of the pKa.

Now for this red box, which is asking what the red line represents. It might be a little tough to see, but it's this red line over here on the far right. Note that it's showing up when we add an entire second molar equivalent of titrant. This means that we must be neutralizing another acid. This is going to be another equivalence point or another endpoint. Since we already covered the equivalence point of the carboxyl group, this must be the equivalence point of the amino group. We know that the equivalence points represent the end of an acid, and so that means that there's going to be no more of this conjugate acid form of the amino group. It has been completely neutralized at this red point over here on the far right.

Now you can see that we've filled out all of the boxes, and we've broken down the titration curve for alanine. In our next lesson video, we're going to talk about how to draw the structures of the predominant structures of amino acids just by looking at the titration curves of amino acids with nonionizable R groups. So I'll see you guys in that next lesson video.