What is the predominant form of each of the following amino...

Key Concepts

Acid-Base Equilibria

Amino acids contain both acidic (carboxyl) and basic (amino) groups that can donate or accept protons depending on the pH of the environment. This concept explains how the ionization state of these groups changes as the pH moves above or below their respective pKa values, ultimately affecting the overall charge of the molecule.

pKa and pH Relationship

The pKa value of a functional group is the pH at which half of the species are deprotonated and half are protonated. Understanding the relationship between pH and pKa is crucial, as it allows one to predict whether a given amino acid group will be protonated or deprotonated under specific conditions, such as at pH 11.

Zwitterionic Form

Amino acids in solution can exist as zwitterions, meaning that they possess both a positive and a negative charge simultaneously. This form is typical near the isoelectric point where the overall charge is zero, but at extreme pH values, the balance of charges shifts as groups fully deprotonate or protonate.

Side Chain Ionization

Many amino acids have side chains that can be acidic or basic in addition to the main carboxyl and amino groups. The ionization state of these side chains, determined by their own pKa values, plays a key role in determining the overall charge of the amino acid at different pH levels.

Determination of Net Charge

To determine the overall charge on an amino acid at a specific pH, one must consider the ionization states of both the backbone groups and the side chain groups. This involves comparing the pH to each group's pKa to see if it will be in a protonated or deprotonated state, thereby establishing the cumulative charge of the molecule.

Transcript

00:01 Hello everyone.

00:01 Today we're doing chapter 29 problem 38 and this one asks us to determine the predominant form of each of the following amino acids at ph 11.

00:10 So and asks us also what's the overall charge on the amino acids.

00:14 So the first one is valiant.

00:15 So let's just draw the backbone of all these amino acids.

00:22 So at ph 11, what do we know? well, we know right off the bat that your carbonylaclylyacic group is going to be depotanated because that has a p .k of around two.

00:31 So obviously a ph 11.

00:32 What do we know? well, we know right off the bat that your carbonylacic acid group is going to be depototinated because that has a p.

00:32 So obviously a ph.

00:32 So we have a ph.

00:32 Of 11 is a lot more proton poor than when it is to be at ph 2.

00:38 So therefore, it's a proton deficient environment.

00:41 So dukaryngosilic acid is going to be in a deprotonated form.

00:44 But now what about the amin group? so amyons typically have a pk around 9 to 10.

00:49 So once again, this ph being ph 11, is going to be more proton poor than the pqa of this nitrogen backbone.

00:58 So this is also in existence, de -protonated form, having only two protons attached to it and being neutral.

01:06 So we can do that for all of the amino acids.

01:08 So this is valine, and then we have proline, and then we have glutamic acid, and lysine...

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