The titration curves for the amino acids glutamic acid and...

The titration curves for the amino acids glutamic acid and lysine are given below. (a) Using the structural formulas given in Table $12.8$, write the structural formulas for the forms of each amino acid corresponding to points on the titration curve when 0, 1.0, $2.0$, and $3.0$ equivalents of $\mathrm{OH}^{-}$ have been added. An equivalent in this case is the amount of base added that neutralizes an amount of $\mathrm{H}_{3} \mathrm{O}^{+}$ ions equivalent to the amount of amino acid present. (b) The isoelectric point is the $\mathrm{pH}$ at which an amino acid has no net charge. Write the structural formulas of glutamic acid and lysine at their isoelectric points. (c) What relationship exists between the conjugate acid and conjugate base forms of these amino acids at each of the $\mathrm{p} K$ points noted on the titration curve?

The titration curves for the amino acids glutamic acid and lysine are given below. (a) Using the structural formulas given in Table $12.8$, write the structural formulas for the forms of each amino acid corresponding to points on the titration curve when 0, 1.0, $2.0$, and $3.0$ equivalents of $\mathrm{OH}^{-}$ have been added. An equivalent in this case is the amount of base added that neutralizes an amount of $\mathrm{H}_{3} \mathrm{O}^{+}$ ions equivalent to the amount of amino acid present.
(b) The isoelectric point is the $\mathrm{pH}$ at which an amino acid has no net charge. Write the structural formulas of glutamic acid and lysine at their isoelectric points.
(c) What relationship exists between the conjugate acid and conjugate base forms of these amino acids at each of the $\mathrm{p} K$ points noted on the titration curve?

Key Concepts

Isoelectric Point

The isoelectric point (pI) is defined as the pH at which an amino acid has no net electrical charge, meaning that the positive and negative charges on the molecule balance each other. At this point, the amino acid exists predominantly as a zwitterion. Knowing the pI is crucial for understanding the behavior of amino acids in various biochemical and separation processes such as electrophoresis.

Conjugate Acid-Base Equilibria and pK Values

At each pK point on the titration curve, the amino acid exists in a dynamic equilibrium between a conjugate acid form and a conjugate base form. The pK value is the specific pH at which the concentrations of these two forms are equal. This relationship underpins the buffering capacity of the amino acid and is fundamental to understanding how protonation states shift with pH changes.

Amino Acid Titration Curve

An amino acid titration curve displays the sequential deprotonation (or protonation) of the ionizable groups as a titrant is added. This curve illustrates the change in the net charge of the amino acid, transitioning from fully protonated species (with a positive charge) to the zwitterion (net neutral) and then to the fully deprotonated form (with a negative charge), thereby reflecting the amino acid’s acid-base behavior in solution.

Equivalents in Titration

In the context of titrations, an equivalent refers to the amount of titrant needed to neutralize one mole of reactive hydrogen ions associated with the amino acid. As each ionizable group donates or accepts a proton, the titration progresses in steps corresponding to the addition of these equivalents. Understanding these steps is essential for correlating the changes observed in the titration curve with specific structural changes in the amino acid.

Transcript

00:01 For this question, we have two titration curves.

00:04 One for glutamic acid and the other one for lysine.

00:09 Each of them have three pca values.

00:13 So in theory, there would be three equivalence points.

00:16 However, you can't always see each equivalent point dependent upon the value of the ka.

00:23 If it's too close to a previous one, you can't see it.

00:27 If it's too small, you can't see it.

00:28 If it's too large, you can't distinguish it from a k.

00:31 Strong acid, and so forth.

00:33 So we don't see all three equivalence points, but they are identified at 0, 1, 2, and 3 equivalents, equivalents of sodium hydroxide that have been added.

00:47 First, it wants you to go back to table 12 .8, and using the structural formulas, draw the structural formulas for each of these, glutamic acid and lysine, at each of the equivalents, 0, 1, and 2.

01:06 So let's start with glutamic acid.

01:12 When glutamic acid is fully protonated at the lowest ph value, so at equivalent 0, sodium hydroxide, we are going to have the 2 carboxylic acids protonated, and we're even going to have the amine protonated.

01:27 When we protonate the amine, it takes on another hydrogen ion and becomes positively charged.

01:34 Then what we have to do is look up some information about glutamic acid in order to determine which hydrogen corresponds to which k -a.

01:46 Is this k -a -1, this k -a -2, this k -a -3, or is it some different order? it turns out that this carboxylic acid hydrogen corresponds to k -a -1, so that's the one that's going to be removed first as the ph increases.

02:04 So at the equivalence of hydrogen number one, that hydrogen will be, i'm sorry, equivalence of sodium hydroxide, one, this hydrogen will be removed.

02:21 Then at equivalence of sodium hydroxide two, we're going to have the hydrogen removed from glutamic acid that corresponds to its pka2, which is the one on the amine.

02:36 I'm sorry, it's the one on the other carboxylic acids, because carboxylic acids are typically stronger acids than protonated amines.

02:46 Then for the last one, at equivalence three, where the ph is high enough to completely deprotonate all of the hydrogens that are acidic from glutamic acid, we finally have that last hydrogen on the amine removed.

03:04 Now for lysine, it turns out that lysine has a substance.

03:09 Somewhat similar structure.

03:11 This portion is the same.

03:13 It's just this r group that's different.

03:16 Instead of another carboxylic acid, we have another amine.

03:21 K .a .1 corresponds to this hydrogen right here on lysine, the hydrogen corresponding to the carboxylic acid group.

03:32 So at the very low ph, where we are starting, the lysine molecule is fully protonated...

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