a. Why is the p Ka of the glutamic acid side chain greater than the p Ka of the aspartic acid si

A. Why is the p Ka of the glutamic acid side chain greater...

Key Concepts

Resonance Stabilization

Resonance stabilization refers to the delocalization of electrons over a conjugated system, which can stabilize a charged species. In amino acid side chains, if a protonated or deprotonated form benefits from resonance stabilization, it will typically exhibit a higher pKa because the charge is more delocalized and the deprotonated state is less favored.

Inductive Effects

Inductive effects involve the transmission of electron density through sigma bonds due to the electronegativity of atoms or groups attached to the reactive center. Variations in side chain composition can lead to differing inductive stabilization or destabilization of the charged form of the side chain, thereby shifting the pKa value.

Acid Dissociation Constant (pKa)

The pKa is a measure of the strength of an acid, indicating the pH at which half of the molecules are deprotonated. In the context of amino acid side chains, differences in pKa values reflect how the local electronic environment and molecular structure influence the stability of the charged versus uncharged forms, thereby affecting the amino acid's acid-base behavior.

Side Chain Structure and Chain Length Effects

The molecular structure of amino acid side chains, including the length and connectivity of atoms between the functional group and the backbone, significantly affects pKa values. For instance, an extra methylene group in one side chain versus another can alter the distribution of electronic charge along the chain, influencing how readily a proton is lost or retained.

Transcript

00:01 So in order to answer this question, it's good to know or touch based on the five characteristics that influence acid strength.

00:10 So the first characteristic is atom size.

00:18 And this is pertaining to the size of the atom that the acidic hydrogen is bonded to.

00:26 So with atom size, as that as size increases, you will see an increase in acid strength.

00:40 Okay.

00:40 Now the second factor, the ambivalves as acid strength is electronegativity.

00:52 Okay.

00:53 And that relationship is with an increase in electrobe -nege, you will see an increase in acid strength.

01:07 The third factor characteristic is hybridization.

01:16 Hybridization.

01:19 With this, you will see that you have an increase in strength, increased strength with sp, which is your triple bond, which is stronger than sp2, which is your double bond, which is stronger than sp3, which is stronger than sp2, which is your double bond, which is stronger than sp3.

01:44 Three, which is your single bond.

01:47 And this is rather than increased strength, this side is stronger.

02:05 The side, keeper.

02:09 Okay, so that's how hybridization influences mass as strength.

02:14 Then your fourth factor has to do with the inductive electron withdrawal.

02:29 And what this means is that if there is a presence, which i'll write plus for presence of electron with drawing group, you automatically increased acid strength.

03:22 Then on top of that, if your electron with drawing group has increased electrone negativity, you will see an increase in strength.

03:42 And if you see this electron withdrawing group close to your acidic proton, you get stronger acids.

03:59 Whereas if it's farther away, it becomes weaker.

04:03 So proximity of the electron withdrawing group also contribute to acid strength.

04:18 And again, let me scroll down just a little bit.

04:26 If it's close, increased strength, far, you get decreased strength.

04:38 And finally, we have electron delocalization, which is pretty much saying that the conjugate base with delocalized electrons is stronger than a conjugate base with localized electrons.

05:21 So i'll denote as e minus.

05:25 Okay, so those are the five factors that can influence acid strength.

05:32 And with that in mind, let's go ahead and tackle this question.

05:35 So i've highlighted in red the acidic proton or where that would be...

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