Label each arrow on Asp's titration curve & draw its predo...

Question

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

Titration curve of Asp showing pH vs equivalents of OH with labeled regions for net charge.

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.

Accepted Answer

Alright. So this practice problem wants us to label each arrow on aspartic acid's titration curve and then to draw aspartic acid's predominant structure at each of the colored regions on the titration curve. And so we'll start off first by labeling each arrow, and the first arrow here is pointing right at this position, which corresponds with half of the molar equivalents of titrate being added, which means that this is some kind of midpoint or inflection point. And every midpoint or inflection point corresponds with a p k a value. And so we know that this is gonna be some kind of p k a value and looking at what pH it corresponds with, it corresponds with a pH of about 2. And we know that alpha carboxyl groups have p k a's in the ballpark range of about 2. So this must be the pKa of a carboxyl group which matches up with answer option b. So we can go ahead and say b here in this first arrow and cross off b from our list. Now this next arrow here is pointing at this position which corresponds with exactly 1 molar equivalent of titrant being added. And so that means that this is some kind of equivalence point, since this is, the end of an acid being completely neutralized. And so because this is an an equivalence point, and the only pKa that shows up before it is the pKa of the carboxyl group, this must be the equivalence point of the carboxyl group. And so the equivalence point of the carboxyl group is answer option E, so we can go ahead and put e here and cross this off from our list. Now, this next position here shows up exactly at 1 and a half molar equivalents, which means that we're at another midpoint since half of another acid is being neutralized. And so essentially what you'll see is that, this point here is corresponding with a pH right just under 4, or right about 4. And so, notice option C here says that the R group has a pKa just below 4, right at 3.9, and so that this must be the midpoint in the pKa for the R group. So, we can go ahead and put C here and cross it off. Now the next point here is showing up right at 2 molar equivalents of titrant being added. So that means that we've neutralized another acid. And so, at this position here, what you can see is that because this is an equivalence point and the only pKa that comes before it that has not already been neutralized is going to be the R group. So this must be the equivalence point for the R group. So we can put in f at this position, since it corresponds with the R group equivalence point. And then, of course, what we've got next is another midpoint, because it's showing up at 2 and a half molar equivalent. So that's showing up that's showing that another, acid is being half neutralized. And so, again, midpoints correspond with pKas, and, this pKa is showing up right around 9 ish. And so we know that alpha carboxyl groups have pKas in the ballpark range of about 9 to 10 point 5, so this must be the pKa of an amino group. So we can put a here and cross it off. And then, of course, the last option here is gonna be the amino group equivalence point, option d, which is showing up exactly at 3 molar equivalents of titrant being added. Alright. So now that we've labeled each arrow in aspartic acid's titration curve, we're gonna draw aspartic acid's predominant structure at each of these colored regions that we see. We'll start off with this green region here. And so in this green region, notice that it corresponds with pH values that are really, really low and it's lower than all of these other pKas. And so when the pH is lower than all of the pKas, the conjugate acid forms are gonna predominate for all of the ionizable groups, and the conjugate acid forms are protonated forms with extra hydrogens. And so the amino group is gonna be protonated as an NH 3 plus, the carboxyl group is gonna be protonated as an Oh minus, and then the R group of aspartic acid is also going to be protonated as an Oh neutral. And so we only have one charge here, and this one charge is gonna give our, structure a net charge of plus 1 in the green region. And so now moving on to the next region which is this red region in here, we'll draw it in this red box. Notice that the only thing that's changing now is that this red region corresponds with pH values that are essentially higher than the p k a of the carboxyl group. So only the carboxyl group is gonna change from, this structure over here on the left to this structure over here on the right. And all of the other p k a's are greater than this pH region, so they're gonna stay exactly the same. So we'll change the carboxyl group to, a conjugate base form and then everything else is gonna remain exactly the same. And, essentially what you'll see here is that now we've got 2 charges, and these two charges here cancel each other out. So now we have a net charge of 0 in this red region. So now moving on to the next region, which is this blue region, so notice that this blue region here on our graph corresponds with pH values that are showing up right somewhere in between this region here. And so now this pH region is greater than both the pKa of the R group and the pKa of the carboxyl group. And so now, essentially the difference between this structure here and the next structure in the blue region is is gonna be the R group and everything else is gonna remain the same. And so we can go ahead and reflect that change. So now the R group is going to be deprotonated in the conjugate base form with a charge, and the amino group is still gonna be the same as it was, and, the carboxyl group is also still gonna be the same as it was. And so now what you'll see is that we've got 2 negative charges and one positive charge. So the net charge is negative one. Alright. So now we're moving on to our last and final region here, and in this region we've got this pink region which corresponds with really, really high pHs. And so, these pH's are all greater than all of the pKa's. When the pH is greater than the pKa's, the conjugate base forms predominate. So all of these structures, all of these ionizable groups are gonna be in the conjugate base form. The conjugate base of amino group is an NH 2 that is deprotonated and is neutral. The carboxyl group is gonna still be a carboxylate anion with a charge. And this carboxyl group in the R group is also gonna be charged with a negative charge. So now we have 2 negative charges, and that gives our structure a negative 2 charge. And so that concludes this practice problem, and I'll see you guys in our next video.

Struggling with Biochemistry?

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.

Watch next

Titrations of Amino Acids with Ionizable R-Groups practice set

Struggling with Biochemistry?

Label each arrow on Asp's titration curve & draw its predominate structure at each colored region.A) Amino group pKa.B) Carboxyl group pKa.C) R-group pKa (pKa = 3.9).D) Amino group equivalence point.E) Carboxyl group equivalence point.F) R-group equivalence point.

Watch next

Titrations of Amino Acids with Ionizable R-Groups practice set

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.