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3. Amino Acids

1

Titrations Of Amino Acids With Ionizable R-Groups

9m

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in this video, we're going to talk about the titrate shins of amino acids with Ayan Izabal. Our groups so recall that amino acids with ionized herbal are groups actually have three ionized able groups. Their amino group, their car boxing group and there are group and also recall that there are seven amino acids with ionized well, our groups and are pneumonic to remember. Those seven amino acids is just yucky. Crazy dragons eat nights riding horses. And so if you can remember this pneumonic, you'll remember those seven amino acids with ionized our groups. And so because they have three ionized able groups, that also means that they're going toe, have three inflection points and equivalence points on their titrate in curves. And really, that's the biggest difference between the Thai tray. Shins of amino acids with ionized herbal are groups and titrate shins of amino acids with non ionizing bill are groups because remember amino acids with non ionizing Well, our groups on Li have to immunize able groups so they only have two inflection points and equivalence points in their tight tracing curves. But again, ionized able are groups are going toe have three, not two inflection points and equivalence points. And so, in our example, what we're gonna do is do some titrate in curve review, but specifically for an amino acid with an eye on Izabal, our group. And so what you can see here is that we've got the tie tray shin curve of histamine, which is indeed one of our seven amino acids with an eye on Izabal, our group, because histamines one letter code is an H. And so if we take a look at our example down here, what you'll see is that we've got these boxes all around the titrate in curve again that are asking us questions about the tight rations. And they're color coordinated to, uh, specific regions on the Thai tray shin curve. And so we're going to do is start with this black box in the bottom right again. And it's asking us, what does the black curve represent? And so the black curve is referring to this black curve that goes through the entire graph. And of course, we know that this black curve is referring to histamines, Titra, Asian curve. And so we know its history and again because it says history ing in the top left. And so the next box that we're gonna go to is this green box in the bottom left and it's asking us, what did the green dotted lines represent? And so this is referring to this green line this green dot here and this green line here. And so what you'll notice is that the curvature of history is tight. Trey Shin Curve is changing at this dot So first the curve ist like this. But then after the dot, the curve changes. And so when the curve changes in that fashion, we know that it's going to be a midpoint, and we know that mid points are associated with PK A values. And so here. What you see is that PK a value of about 1.82 And so we know this has gotta be the car boxing group PKK. Because car boxer groups have P K is in the ballpark range of about two, and so 1.82 is pretty close to two. So this must be the midpoint and peek a of the car Boxful Corp Now the next box that we have is this blue box, and it's asking us, what do the blue dotted lines represent? So it's referring to this blue line here, this blue dot here and this blue line here and so again. What we're seeing is that there's a change in the curvature before the dot so that here there's, uh it's curving in this fashion. And then after the dot it curves in that fashion. So this means that this is going to be another midpoint and again, mid points are associated with PK A values, and we see a PK a value here, specifically a pkr. So this is the peak a of the our group. So the next box that we have is this orange box, and so it's asking us, what do the orange dotted lines represent? So it's referring to this orange line going across here, this orange dot here and this orange line here and so again, what we're seeing is that the curvature is changing again for the tie Trey shin curve. So it's like this before, and then afterwards it's going up, and so again, this is going to be another midpoint. So mid points are associated with P. K s and we can see that there's a PK a here right at 9. So we know this has got to be the PK of the Amino group, and that's because amino groups have. PK is right in the ballpark range of about 9 to 10.5, so this falls right into that range. And so this is referring to the midpoint and Peca of the Minnow Group. And so, really, this is the biggest difference between the Thai trey shin. Curves of amino acids with ionized able are groups because they actually have three mid points, whereas with our previous curves with the amino acids with non invisible are groups, there were only two midpoint, so keep that in mind here. Now the next boxes asking us about this pink line. So it says, What does the pink line represent? And so this pink line here is going vertical up and down right at this position, and notice that it's showing up at one Moeller equivalent of the hydroxyl group titrate as being added. And so what that means is that when you add exactly one Mueller equivalent, that means that there's going to be a neutralization of an acid And when there's a neutralization, we know that there's going to be an equivalence point. That's what that's referring to. But the question is, which acid is being neutralized? Is that the car box? A group, the our group or is it the Amino Group? And so all we need to do to figure that out is look at the positioning of this equivalence point. So notice Here's the equivalence point and notice that it's showing up before the P. K s of the our group and the Amino Group, but it's showing up after the peak A of the car boxer group. And so, because that is true, this has to be the equivalence point of the car boxful group. And that's what this is. And so at thes equivalents point here of the car boxer group. But this is saying, is that at any point after, uh, this equivalence point is reached, the Thai trey shin curve is just gonna Onley have, uh, Deep throat nated car Boxer group. And so that means there's gonna be gnome or C 00 h. There's gonna be no more protein ated for him of the car Boxer group after this equivalence point is reached. So this next yellow boxes asking us what does the yellow line represent? And the yellow line is going vertical right here at this position. And so notice that the yellow line corresponds with exactly two molar equivalents of tightrope being added, which means that we must be neutralizing another acid. And so this has got to be another equivalents point. This is another equivalence point. And the question is, is this the equivalence point of the our group or the amino group? So we already got the equivalence point of the car Boxer group. So all we need to do to answer that question is look at the positioning of the equivalence point. So, uh, notice that it's coming before the PKK of the amino group, But after the PK of the our group. So this has to be the equivalence point for the our group, and indeed it is. So this is the equivalence point of the our group. So what that means is, once this line has reached, once this equivalence point is reached, the rest of the curve is on. Lee going toe have, uh, the deep protein ated form of the our group and we know that the deep protein ated form of the our group is going to be neutral. It's gonna be uncharged. And so what that means is that once this equivalence point is reached, there's gonna be gnome or charged our group. So the next boxes asking us about the light blue line and the light blue line is over here on the far far right of our graph. And so notice that its corresponding with three exactly three molar equivalents of titrate being added, which means that we are neutralizing another acid and so that this has gotta be another equivalence point. So this is our third equivalence point. And so this equivalence point must be for the neutralization of our last assets. So we've already neutralized our car box, a group and our our group. So this must be for the neutralization of the amino group. And so what that means is, once this, uh, equivalence point has been reached, there's going to be no more protein ated form of the amino group. And so it will all be the deep protein ated form. And so we've answered all the questions here and in our next video. What we're going to do is talk about how to draw amino acids with ionized able are groups just by looking at their titrate in curves. And so again, the biggest difference between this titrate curve and our previous ones is that there are three mid points and three equivalents points, whereas previously we only had two mid points and two equivalence points. So that concludes this lesson. I'll see you guys in our next video.

2

Titrations Of Amino Acids With Ionizable R-Groups

7m

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So in our last lesson video, we reviewed the Thai trey shins of amino acids with Ayan Izabal are groups, and in this video we're going to talk about how to draw the predominant structures of amino acids with ionized able are groups straight from their titrate in curves, and we'll also review how to calculate. There is electric points and so the P I or the is electric point is always going to be equal to an equivalence point. But it will not always be equal to the car boxful group Equivalence point like it was with amino acids with non invisible, are groups, but nevertheless it's still equal to an equivalence point, and it will always be equal to the equivalence point that allows for the neutralization of the net charge, meaning that there's going to be a net charge of zero at the ISO Electric Point or the P I now, in our example below, were first going to draw the predominant structures of history ing at each of its colored regions on its titrate in curve. And then we'll go back and calculate its P I and so notice down below. In the center, we have the tie tracing curve for his sitting and it's colored into these different regions. And so we're gonna start off here with the red region and draw the predominant structure of history ing in this red region in this red box over here on the left. And so notice that this red region here corresponds with pH values that are less than about two and these pH values that it corresponds with our less than the P K s for its three ionized groups. And that's exactly what's being indicated over here on the left that the pH is less than the value of all of the other P. K s and P K. R s. And so what that means is when the pH is less than the P. K s. That means that the conjugate acid zehr gonna predominate in the conjugate acids are all protein ated. So let's go ahead and draw. Historians are, um, structure with protein ated groups, and so the amino group is gonna be protein ated, so it will be NH three plus the car box. A group is also going to be protein ated, so it'll be o h neutral and then it's our group Recall is literally just a winning So ch two with ah five member grin branching off. So it will be, uh, a five member ID ring like that. And then remember that history in our group is, uh, comparable to a horse. So it's got these back legs, thes front legs and then to nitrogen is one of them's at the neck, and the other one is not being stepped on by the horse and recall that it's the nitrogen that's at the neck that's going to be the one that's ionized able. So the one that's going toe have the positive charge on it. And so notice that all three of these groups are protein ated. When the pH is less than the peak A and the total net charge here there's two positive charges, so the total net charge is gonna be plus two in this red region now moving on to the next region. What we have is the yellow region, which corresponds with pH values between six and two, and these pH values that correspond with the Yellow region are still lower than the PKR and the PKK of the amino group. But Now it's greater than the PK of the car boxing group. So on Lee, the Car Boxer group is going to change in this next yellow region down below. And that's exactly what's being indicated here that the pH is now greater than the PK of the car Boxer group. And so, essentially, what we could do is copy the same exact structure that we had drawn earlier and bring it down. Uh, here and essentially the only thing that's gonna change that the car Boxer group is now going to be deep protein ated and in the conjugate base form, since the pH is greater than it's PK. And so now notice that the total net charge here we have two positives and one negative. That means that the total net charge is gonna be plus one. So now moving on to the next region, which is the light blue region, corresponds with pH values that are between nine and about six. And these pH is are now greater than both the PKR and the P K of the car Boxer group, but still smaller than the PK of the Amino group. And so that's exactly what's being indicated. here. And so all we need to do is draw the same exact structure that we had over here on the left and just change the PKR. Since that's the only one that's being, uh, change from this one that's on the left to this one over here, that's on the right. And so when we change this, what you'll see is that we have two charges and they cancel each other out this positive in this negative charge. And so that means that we have a neutral net charge of zero on this blue structure. And then, of course, in the pink structure corresponds with pH values that are above nine here, and these pH values are greater than all of the other P. K s. And that's exactly what's being indicated here. And so when the pH is greater than the PK, the conjugate base is predominate. And so it's gonna be exactly the same structure that we drew down below. Except the amino group is now finally going to change. So now we're going to have a deep protein ated conjugate base form of the amino group, and now the total net charge is just going to be negative one. And so essentially, what you'll see here is that this red region here has a net charge of plus two indicated by this over here, the yellow region and here has a net charge of plus one indicated down below here. So we can color code this with yellow. And then in the blue region. Of course, we said that there was a net charge of zero so we can put in zero here and make this blue. And then in the pink region, of course, we said that there was a net charge of minus one. And so one thing that's important to note here is that the net charges of the predominant structures decrease Notice that they're going down. Uh, they're going from plus two down to negative one as the pH is increasing and also noticed that the net charges go down by one as we cross a PK. So it goes from plus two in the red region. And once we cross this green PK into the yellow region, it goes down one charge and the same goes for each of these other P. K s. When you cross, the PKK goes down one charge as you go from left to right. And so now we're going to calculate the is electric point, which we know is going to again be at the equivalence point that allows for neutralization of the net charge. And so again, the region that has a neutral net charges the light blue region. And so we're going to essentially take the average of the two PK sandwiching the region that has a neutral net charge on the predominant structure. So it's this Pekka and this PK here that we want to average so we can put the is electric point calculation as being the average of these two PK so six plus nine point one seven divided by two and this comes out to about seven point five nine. And so this is going to be the is electric point of histamine. And so up above here we can indicate that seven point five nine as Theis electric point, and you can see that it's showing up right at the equivalence point here of the, uh, our group shown over here. And so essentially, this concludes our lesson on how to draw the amino acids of ionized well our groups from their titrate in curves, and we'll be able to get some practice utilizing these techniques here and our next practice video, so I'll see you guys there.

3

Problem

Determine the pI of Glu & mark it on the provided titration curve.

a. 6.96

b. 5.93

c. 7.48

d. 3.22

Use the titration curve to draw the predominate structures of Glu at pH = 1.0, pH = 3.0, pH = 7.0, & pH = 11.0.

__Glu at pH = 1.0__ __Glu at pH = 3.0__ __Glu at pH = 7.0__ __Glu at pH = 11.0__

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4

Problem

Draw the predominate structures of K at the indicated sections of its titration curve. Mark the pI on the curve.

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5

Problem

Label each arrow on Asp’s titration curve & draw its predominate structure at each colored region.

A) Amino group pK_{a}.

B) Carboxyl group pK_{a}.

C) R-group pK_{a} (pK_{a} = 3.9).

D) Amino group equivalence point.

E) Carboxyl group equivalence point.

F) R-group equivalence point.

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