3. Amino Acids
Titrations of Amino Acids with Non-Ionizable R-Groups
1
concept
Titrations Of Amino Acids With Non-Ionizable R-Groups
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in this video, we're going to talk about the titrate shins of amino acids with non ionizing bill are groups, so recall that way Back in our previous lesson videos, we reviewed titrate Asians from your old chemistry courses. And really, this lesson here on the Thai traces of amino acids with non invisible are groups is just an extension of those older lesson videos. And a lot of the information that we're going to cover in this video is just going to be review. And a lot of the principles and concepts that we already learned about titrate shins are just going to be applied specifically to the Thai tray shins of amino acids with non ionized our groups. But if you don't remember much about Thai Trish ins at all, be sure to go back and re watch those older videos before you continue here. Now, that being said, it's important to note that all amino acids are Polly pro dick acids, and we know that Polly Protic acids are acids that have multiple acidic hydrogen, and we know that each ascetic hydrogen has its own PK, a value, and so all amino acids have multiple PK a values at least two PK A values one for the Alfa Amino group on the backbone and one for the Alfa Car box. Will group on the backbone now also recall from our previous lesson videos that titrate Asians can reveal the PK, a values of weak acids and that point on the Thai tray. Shin curve is referred to as the inflection point, which is also called the midpoint. Because this is exactly where half of the moller equivalence of Titra have been added to neutralize half of an acid. And so the inflection point or the midpoint corresponds to each PK a value now 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 asset. So it represents the point of neutralization, of an asset and so down below. In our example, we're gonna do some titrate shin curve review so you can see that we have a tie trey shin for a amino acid with a non ionizing, our group and surrounding our titrate in curve, we have all of these colored boxes that correspond to the colors of specific regions found within our tight tray shin curve. And so we're just gonna go around and fill out these boxes as we review the Thai tray shin curve. And so we're actually going to start off with this black box here in the bottom, right? Which is asking, What does the black curve represent? And the black curve is just referring to this black curve that we see going up through our graph. And of course, we know that this is going to be Alan means Thai tradition curve, and we know that it's challenging because notice at the top left here. It tells us that it's Alan Jones titrate in curve. Now we'll move on to this green box here on the bottom left, which is asking us, what do the green dots and lines represent? So it's asking us about this green dot and these green lines here and so notice that they're showing up at exactly 0.5 Moeller equivalents of Thai trahant being added. So that means that this has got to be a midpoint or an inflection point. And so we also know that this is an inflection point because noticed that before this curve before this point, the, uh al needs titrate in curve is curving downwards like this. And then after the titrate in curve noticed that the curve changes to an upwards curve, and so that lets us know that this is an inflection point. Now we know that each inflection point and midpoint corresponds with the PKK value so we can see that we have a peek a value of 2.4 here. And we know that Alfa Car box of groups or the car box of groups on the backbones of amino acids have PK a values in the ballpark range of about two. So this is really close to two and right in that ballpark range. So this must be the PKK value for the car boxful group. All right, so now what we'll do is we'll skip this yellow box really quick and jump up to this dark blue box at the top left, because what you'll notice is that this is another midpoint because it's asking us what do the blue dot The dark blue dotted lines represent corresponding toe this dark blue dot here in this dark blue lines coming up and down. So it's another midpoint because again, what we see is, uh, there's a curve, so notice that it's curving downwards like this. And then after the point, it curves and changes its curvature to going upwards. So that lets us know that this is going to be an inflection point or a midpoint. And again, each midpoint corresponds with the peak a value, and this is a PK value of 97 And we know that Alfa amino groups or amino groups on the backbones of amino acids have PK A values in the ballpark range of about 9 to 10.5. And so this falls right in that ballpark range. So this must be the P. K. A value of the amino group. All right, so now we can jump down to this yellow box here, which is asking us what do the yellow dotted line represent. So you can see we have this yellow dot here in the center and the yellow line going across the left, and so this is actually referring to the ISO electric point. And so the reason that we know that this is the ISO electric point because we know that the is electric point, which is also known as just the P. I is just going to be an average of the two p k A values for an amino acid with a non ionizing our group. And so notice that this PK value here is corresponding with our y axis a pH shown right here at this point. And this PK a value over here is corresponding with the Y axis right at this point here. And so for amino acids with non ionized bill are groups. We know that we can take the average of these two p k A values and that will give us the p i or the is electric point, which is what's showing up here, right with this yellow line here. And so again, this is the is electric point, and we know that the is electric point is the exact point where AL innings net charge is going to be zero. It's gonna have a neutral net charge. We know that again from our previous lesson videos. So now we're moving on to the next box at the top appear the pink box which is asking us, what does the pink line represent? And it's this pink vertical line that's going down like this. And so notice that it's showing up at exactly one Moeller equivalent of Thai trahant being added, which means that we must be neutralizing 100% oven acid. And so that means that this has got to be an equivalence point or an endpoint, because we're neutralizing 100% of the NASA. And so because we have the car boxful group acid on the left, uh, down below, here, on the left of this pink line, this must be the neutralization of the car boxful group. And so we can put here This is the equivalence point of the car boxing group and so were neutralized at this point here, this pink vertical line we are essentially neutralizing 100% off the car box Cilic acid conjugated acid. So that means that there's no mawr congregate acid form of the car box group. So no more C o h at any point after this pink blind over here, moving to the right. So now we're moving on to the next box over here, which is corresponding to the light blue boxes that we see in the background, so we have to light blue boxes. And so these two light blue blocks boxes are referring to the effective buffering ranges. And so we know that the effective buffering ranges are within plus or minus one of the PKK. And so you can see that this blue box here is just referring to plus or minus one of the PKK so you can see our PK showing up at this blue dot that we indicated. But the blue boxes are referring to plus or minus one. So this point up here and this point up here, and that's the effective buffering range. Essentially, this whole area here between these pH is and so that's what it's referring to. The effect of buffering range and the same applies for this one. Down here, the effect of buffering range would be within plus or minus one of the PKK, so between 3.4 and 1.4 would be the effect of buffering range and just add numbers to this one. Up here is well, it would be between 10.7 and 8.73 Effect of buffering range plus or minus one of the P K. So now we're moving on to the next box here, which is the red box, which is asking, What does the red Line represent? The red hot line might be a little bit tough to see, but it's this red line that's over here on the far right over here, and so notice that it's showing up when we add an entire second Moeller equivalent of Thai trahant. So that means that we must be neutralizing another, um, acid. And so this is going to be another equivalence point or another endpoint. And because we already covered the equivalence point of the car Boxer group, this must be the equivalence point of the amino group. And so we know that the equivalence points represent the end off oven acid. And so that means that there's going to be gnome or off this conjugate acid form of the amino group. And so it has been completely neutralized at this red point over here on the far right, And so now you can see that we filled out all of the boxes. We've broken down the titrate in curve for Allan E. And 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 titrate in curves off, not of amino acids with non ionized Volare group. So I'll see you guys in that next lesson video.
2
concept
Titrations Of Amino Acids With Non-Ionizable R-Groups
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So now that we've reviewed titrate in curves again, we can talk about how to draw amino acids with non ionized able are groups just from their titrate in curves. But before we get there, I first wanna highlight two very important pieces of information that applies for all amino acids with non ionized able are groups. And so the first piece of information is that the ISO electric point is always found at the car boxful group equivalence point and just like we said in our previous video. So the second important piece of information is that the Thai tray shin curves for amino acids with non ionized able are groups on Li have to inflection points and two equivalence points. And that's because amino acids with non ionizing Balart groups on Li have to ionized herbal groups Thea Amino Group and the Car Boxing Group. And so there's one set of inflection, point and equivalence point for each of the invisible groups. So if there were ah, third ionized able groups, such as with the amino acids with ionized able are groups and there are groups. What that would mean is that there would be a third inflection point and equivalence point, but we'll talk about the Thai tray. Shin curves of amino acids with ionized able are groups in another video a little later in our course. And so now let's get to our example what were and what we're gonna do is draw the predominant structure of Alan. I mean, just from Allen and titrate in curve and notice that Alan is tight. Trish in curves has these colored regions. So we have this pink colored region over here. We have three yellow colored region up here, and then we also have the green colored region at the top. And so we're going to draw the predominant structure of Al inning at each of these colored regions of the Thai trey shin curve. And in addition to that, what we'll do is calculate the ISO electric point of Alan in at the end. So first in this example, what we're gonna do is look at this first region, this pink region down here. And so the pink region corresponds with this pink box over here where we're going to draw the predominant structure of Albany. And so before we actually do that, we need to remember that this green dot right here represents the car boxful group P k A. And so, um, the PKK here is 2.4. And we know this because it tells us that the p. K s 2.4 and it has to be the car box a group because car boxes groups have p k is in the ballpark of to. And so this corresponds on the Y axis with 2.4. And so this pink region again, that's down here on the titrate curve all corresponds with pH. Is that air below 24 And so we know that when the pH is below the peak a that that means that the conjugate acid form is gonna predominate. And so the conjugate acid form is going to be protein ated. And so that a plot and this pink region down here, the pH is below the peak A of the car boxer group. And it's below the pH. The PK of the amino group, which is way up here at 9.7, which corresponds to this point way over here, a 9.7. And so again, this piece this pink range down here is below 2. point seven. And so the protein ated forum is going to exist at this pink region for both of these, um, groups. So for the amino group, the protein ated form or the conjugate acid form is gonna be the NH three plus and then for the car boxful group. So drawing the car boxful group on this end, we know that the pro donated form is actually just gonna be an O. H. And it's not gonna have a charge. And then, of course, we can draw in Allen in our group, which is pretty easy to remember because it's just a method group. So it's just a ch Let's do that in Green. It's just a CH three group. And so, in this pink region right here, the net charge of the predominant structure, which we drawn right here, is gonna have just a plus. One charge is gonna have a positive one charge. And so we can put that down here next to this pink region. It's gonna have a positive one charge. So now what we're gonna do before we actually get to the Yellow region is we're gonna talk about the predominant structure at this, uh, pink dot at this green dot right here in the middle and this green dot remember, corresponds to the p k A of the car boxing group. So this is the car boxes group PK. And so at the car box of Group K, remember that, uh, the car the car boxer group is going toe have an equal concentration of its conjugate acid and its conjugate base. But the amino group that's way up here, it's PK is so far away from the PK of the car box, a group that when the P H is equal to 2.4, all of the amino group is gonna be protein ated and its conjugate form. So what we can say is that all amino group is charged. And so when all amino groups are charged, they're gonna be in their NH three plus form. And so that means that, uh, the amino group is going to contribute a plus one charge at the peak a pH. So when the P H is equal to 2.4, but at when the P H is equal to 2.4, the car boxful group On the other hand, we know that half of it is going to be in the kind of get acid form, but the other half is going to be in the conjugate base form. So Onley half of car boxful groups are gonna be pro donated Onley half of car boxer groups are charged. And so what that means is that there's only gonna be half a charge that's gonna be contributed. So negative 0.5 charge is gonna be contributed to the overall charge. And so we're looking for the total charge at the peak A of the car Boxer group. Then it's just gonna be the some of the charges on these two groups. So positive one minus 10.5 comes out to just positive 0.5 positive 0.5. And that's what we can put in over here. So, um, when the P H is equal to the PK of the car boxer group, the net charge is gonna be positive. Five. So we can put that as like, a little arrow here and say that the pH is going to be equal to 0.5, but positive 0.5. So now we can check the yellow region, which is shown right here between these two p. K. S and notice that Theis Electric Point is found in this yellow region. And that's right where the again the car boxful group equivalence point is gonna be. And so, in this yellow region here notice that we can pick any pH that's in this yellow region. So let's try maybe a ph of seven. And so at a ph of seven, Uh, basically, what's gonna happen is the amino group is still gonna be pro donated because the ph of seven is still less than the PK of the amino groups. So the amino groups still gonna be protein ated, so it's gonna be NH three plus. And then the car boxing group, on the other hand, is going to be deep rotated because the ph of seven is greater than the peak A of the car boxful group. And when the pH is greater than the peak A. That means that the conjugate base predominate and the conjugate base of the car boxer group is the car Boxley and I am, which has a negative charge and so we can draw in the our group for Alan in which again is just gonna be a methyl group ch three. And so what we're saying here is that the net charge of the structure of Allen is predominant structure is gonna have a neutral net charge or a net charge of zero um, in this yellow region. And that makes sense because that's where we find our ISO electric point, which is right here. So in this yellow region, that's where the electric point is. And that's where the predominant structure is gonna have a net charge of zero or neutral net charge. So now what we're gonna do before we get to the Green Region is do the same. And we're gonna talk about what is the charge? The total charge at the peak A of the amino group. So we know that, uh, the car boxing group is gonna be completely ionized at the PK of the Amino group because we've already passed the car boxing group equivalence point. So all of the protein ated amino group is gone, and the only thing that exists at this blue point in terms of the car Boxer group is just that charged car Boxley and I am so at this PK down here we could say all car boxful group So all car boxful group are charged. And so what that means is that they're going to contribute a total of a negative one charge a complete negative one charge. And for the amino group, though at the peak A of the amino group, we know that the amino group is only gonna be half charge. So only half of amino groups are going to be charged. So only half of amino groups are are charged. And so what that means is that they can Onley contribute a positive 0.5 charge when the P H is equal to the amino group PK. And so if we're looking for the total charge, if we're looking for the total charge, then it's basically gonna be a some of these two charges. So negative one plus 0.5. It's just gonna be negative 0.5. So that's the charge, um, the net charge of the predominant structure at the p k A. When the pH equals the peak A of the amino group. And so now that we've got that part or moving on to our last region here, which is the Green region way up here. And so in this green region noticed that that corresponds with pH values that are above both the P. K of the Amino Group and the PK of the car Boxer group. And so what that means is that they're both going to be deep throat nated because when the pH is greater than the PKK, the conjugate base predominates. And so the conjugate base form of the amino group is just in NH two, which is neutral. The conjugate base form of the car boxful group is just going to be a negatively charged car Boxley, Anna and then the. Our group, of course, is going to be non ionized able, it's just gonna be a ch three. So in the green region up here, the net charge of the predominant structure is just gonna have a negative one charge is gonna have a negative one charge. And so what we can do is say in this yellow region here. So, in the yellow region, let's mark, uh, the charges on here. So in this yellow region, the charges just zero and the dark blue region here and the peak a region. We know that the charges just negative 0. and then in the green region up here. We know that the charges just negative one. And so what you can see is, uh the charge goes from positive one toe positive 10.5 to 0 to negative 00.5 to negative one. So as the pH increases, notice how the charge of the, uh, predominant structure gets more and more negative. And so that's the relationship that I want you guys to be able to see here. And so the very last thing that we're going to do is to calculate the ISO electric point. And remember that Theis Electric Point is just the midpoint or the average between the two p. K. S for amino acid with a non invisible argument. So we've got P k A. One here, which is to four and P k two, which is 97 And so to get the p I Theis Electric point, all we need to do is take the average, so we'll take 9.7 plus 2.4, divided by two. And this comes out to 12.1 divided by two, which is equal to 6.5 And so for our is electric point here, what we can say is that it is 6.5 and that's our ISO electric point. R. P I is 6.5 and so this concludes this example, and we'll be able to get a lot of practice drawing the structures of amino acids just from the Thai tray shin curves in our practice video, so I'll see you guys there.
3
Problem
ProblemDraw the predominate structures of Leu at each of the indicated sections ( 1, 2, 3) on its titration curve.
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4
Problem
ProblemCalculate the pI of Ile using its titration curve. Mark the approximate position of the pI on the titration curve.
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5
Problem
ProblemWhich of the following compounds would make for the best buffer at pH 8?
A
Acetic acid, pKa = 4.8
B
Tricine, pKa = 8.15
C
Glycine, pKa = 9.9
D
Tris, pKa = 8.3
6
Problem
ProblemIdentify the region(s) on glycine's titration (I, II, III, IV, or V) that corresponds with each statement below:
a. Region where Gly predominant species has net charge of +1. ________
b. Region where the average net charge of Gly is +½. ________
c. Region where ½ of Gly's amino groups are ionized. ________
d. Region where the pH = pKa of carboxyl group. ________
e. Region where the pH = pKa of amino group. ________
f. Regions where Gly has its maximum buffering capacity. _____________
g. Region where the average net charge of Gly is 0. ________
h. Region where Gly's carboxyl group has been completely titrated. ________
i. Region where Gly has been completely titrated. ________
j. Region where Gly's predominant species is a zwitterion. ________
k. Region where the average net charge of Gly is -1. ________
l. Region where Gly is a 50:50 mixture of protonated & deprotonated carboxyl group. ________
m. Region indicating Gly's isoelectric point (pI). ________
n. Region indicating the end of Gly's titration. ________
o. Regions where Gly has poor buffering power. ____________________
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