amino acid titration

[SaintJude]

Kaplan discrete:

View attachment Amino acid qbank .jpg

---

Members don't see this ad.

 

[chiddler]

Kaplan discrete:

View attachment 18918

that's the half titration point where pH = pKa

pKa + pKb = 14

pKa = 14 - pKb

pH = 14 - pKb

 

[MedPR]

that's the half titration point where pH = pKa

pKa + pKb = 14

pKa = 14 - pKb

pH = 14 - pKb

Yup. The buffering region is where you find the half-equivalence point. The definition of half-equivalence point is where the [HA]=[A-]. Plugging into HH equation:

pH=pKa + log[A-]/[HA]

pH=pKa + log1
pH=pKa + 0

then you can do what chiddler said.

 

[SaintJude]

Ok, I understand that. But what threw me off was the subscript no.2" Why is pKb2 and not just pKb?

 

[MedPR]

Ok, I understand that. But what threw me off was the subscript no.2" Why is pKb2 and not just pKb?

Amino acids are polyprotic acids. Since this one only has 2 humps, it is a diprotic amino acid, meaning its side chain is not acidic or basic. I had some trouble with this until a few days ago as well.

So for a polyprotic acid, H2A, we assume that only one proton dissociates at a time. So for H2A in water you get:

1. H2A + HOH ---> HA- + H3O+

2. HA- + HOH ---> A2- + H3O+

In the first reaction, the first proton is lost and in the second reaction the second proton is lost. Now, if you consider the regaining of protons (when the base reacts with water) you get:

3. A2- + HOH ---> HA- + OH-

4. HA- + HOH ---> H2A + OH-

Notice how step 4 is just the reverse of step 1? This means that the first proton lost (pKa1) corresponds to the second proton gained (pKb2). And the second proton lost (step 2, pKa2) corresponds to the first proton gained (step 3, pKb1).

 

[SaintJude]

Notice how step 4 is just the reverse of step 1? This means that the first proton lost (pKa1) corresponds to the second proton gained (pKb2). And the second proton lost (step 2, pKa2) corresponds to the first proton gained (step 3, pKb1).

Ok. Whoa. Let me think about that overnight and get back to you. MedPr, you've restored my faith in pre-meds...

 

[MedPR]

Ok. Whoa. Let me think about that overnight and get back to you. MedPr, you've restored my faith in pre-meds...

Yea it's a little complicated at first, but once you get it you'll want to smack yourself for missing it before.

 

[pfaction]

That's a bit confusing MedPR. I understand that point A is when the pKa of the acid = pH of the solution. That eliminates A, and D doesn't even make sense as it would be a negative #. I would have chosen B to be honest, since I don't know what this numbering system is. Looking at it though, I would have said that the pkb is a base therefore the acid is the "2nd base" here? No idea.

 

[MedPR]

That's a bit confusing MedPR. I understand that point A is when the pKa of the acid = pH of the solution. That eliminates A, and D doesn't even make sense as it would be a negative #. I would have chosen B to be honest, since I don't know what this numbering system is. Looking at it though, I would have said that the pkb is a base therefore the acid is the "2nd base" here? No idea.

My second post is only to explain why pKa1 + pKb2 = 14. It's almost irrelevant to why the answer is the answer.

 

[pfaction]

I'm gonna have to digest what you said because it's still confusing to me! That's what I meant to say by confusing, confusing--to me.

Boom, clicked, just got it.
1st proton: COOH
2nd proton: NH3
reverse it to get 1st proton GAINED, 2nd proton GAINED

 

[SaintJude]

😳 Umm, I just got to this.

So the general pKa + pKb =14 is only entirely true for monoprotic acids. For polyprotic acids there is no direct relationship between pKa1 and pKb1!

But b/c amino acids pass through at least two buffering stages, the real relatinship one has to use is

pKa1 + pKb2 = 14 and pKa2 + pKb1 = 14

Here is a question that I just came up with:

So say an MCAT question gave you the pI and the pKa1. The question is: Find pKb1.

To find the pI, you average the two pKa values on either side of the neutral form of the polypeptide
You would do pI = ( pKa1 + pKa2)/ 2 . That would get you pKa2.

Then you'd calculate 14 - pKa2 = pkb1.

Is that accurate?? Any other way to do it?

 

[MrNeuro]

😳 Umm, I just got to this.

So pKa + pKb =14 is only entirely true for monoprotic acids. For polyprotic acids there is no direct relationship between pKa1 and pKb1!

But b/c amino acids pass through at least two buffering stages, the real relatinship one has to use is

pKa1 + pKb2 = 14 and pKa2 + pKb1 = 14

Here is a question that I just came up with:

So say an MCAT question gave you the pI and the pKa1. The question is: Find pKb1.

To find the pI, you average the two pKa values on either side of the neutral form of the polypeptide
You would do pI = ( pKa1 + pKa2)/ 2 . That would get you pKa2.

Then you'd calculate 14 - pKa2 = pkb1.

Is that accurate?? Any other way to do it?

👍

 

[MedPR]

😳 Umm, I just got to this.

So the general pKa + pKb =14 is only entirely true for monoprotic acids. For polyprotic acids there is no direct relationship between pKa1 and pKb1!

But b/c amino acids pass through at least two buffering stages, the real relatinship one has to use is

pKa1 + pKb2 = 14 and pKa2 + pKb1 = 14

Here is a question that I just came up with:

So say an MCAT question gave you the pI and the pKa1. The question is: Find pKb1.

To find the pI, you average the two pKa values on either side of the neutral form of the polypeptide
You would do pI = ( pKa1 + pKa2)/ 2 . That would get you pKa2.

Then you'd calculate 14 - pKa2 = pkb1.

Is that accurate?? Any other way to do it?

This is only true if the amino acid is diprotic. If it is triprotic:

pKa1 + pKb3 = 14

pKa2 + pKb2 = 14

pKa3 + pKb1

Chiddler has a pretty good handle on this stuff, hopefully he'll come in here and make sure this post is correct!

 

[MrNeuro]

This is only true if the amino acid is diprotic. If it is triprotic:

pKa1 + pKb3 = 14
pKa2 + pKb2 = 14
pKa3 + pKb1 = 14

And assuming its not basic.

 

[MedPR]

And assuming its not basic.

It's different if the side chain is acidic or if it is basic? 😕

 

[MrNeuro]

It's different if the side chain is acidic or if it is basic? 😕

Yeah pI=pka2+pka3/2

Whereas everything excluding basic side chains is
pI=pka1+pka2/2

 

[SaintJude]

This is only true if the amino acid is diprotic. If it is triprotic:

pKa1 + pKb3 = 14

pKa2 + pKb2 = 14

pKa3 + pKb1

Chiddler has a pretty good handle on this stuff, hopefully he'll come in here and make sure this post is correct!

Yeah, that makes sense. The first proton lost will the be the last proton gained.

The last proton lost will be the first proton regained.

For a triprotic amino acid like histidine, will the pI then be closest to its pKA2 ?

Nevermind it'll be an average--didn't see Mr. Neuro's reply.

 

[MedPR]

Yeah pI=pka2+pka3/2

Oh in regard to pI. Yea I have absolutely 0 understanding of isoelectric point. Do you know of any online sources that have a decent explanation? TBR didn't talk much about it.

 

[MrNeuro]

I dont really have a strong grasp of it either....if I come across something I'll share

 

[jsp132]

I thought isolectric point was when the net charge on the amino acid was equal to 0

so just protonate and deprotonate the appropriate side chains according to pH with the according solvent until you can get a charge of 0

To calculate it I thought you just took the average of the pKa on the amino acid that was +1 or -1 on either side

like glycine

when you deprotonate the COOH above the pKa of 2.34 the charge is -1 to deprotonate the NH3 group you have to raise the pH above 9.60 then the amino acid charge will be +1

2.34 + 9.6 / 2 = pI = 5.97

 

[SaintJude]

I don't think we'll need to spend a lot of time on it. (Now watch there be a whole passage on it...lol) I don't think it would show up as a discrete. Kaplan & EK explained what we needed to know

1. Be able to estimate the relative pI values.
Given that the pI is the average of the two pKa values on either side of the neutral form of the polypeptide, it's quick to estimate. pI is proportional to pKa values. So amino acids with more acidic side chain groups will contribute a relatively low pKa. So the more acidic a side chain, the lower the pI. Conversely, an amino acid with more basic groups, will have higher pI.

2. And know the amino acid charge
When pH> pI, amino acid has negative charge and when pH < pI , amino acid is positive.

This is how I think of it. Does anyone have a better rationale?
If solution's pH > pI, the amino acid is used to being neutral in a solution with more free H+ ions. So it will donate some H+ to the solution to simulate its comfortable, neutral environment. B/c the amino acid loses its protons, it carries a negative charge.

If the solution's pH < pI, the amino acid is used to being neutral in a solution with less free H+ ions floating in solution. So it "sequesters" those H+ ions in a bid to move closer toward its more basic, neutral environment. Because it adopts free H+ ions, its carries a positive charge.

 

[BerkReviewTeach]

Oh in regard to pI. Yea I have absolutely 0 understanding of isoelectric point. Do you know of any online sources that have a decent explanation? TBR didn't talk much about it.

Start with pages 182-189 of the second BR organic chemistry book. It's probably the best explanation of isoelectric points you'll find anywhere. It shows some shortcuts and gives applications.

 

[MedPR]

Start with pages 182-189 of the second BR organic chemistry book. It's probably the best explanation of isoelectric points you'll find anywhere. It shows some shortcuts and gives applications.

I love you right now. I've been reading various stuff online about it but none of it is making sense. Can you guys start making prep books for COMPLEX and Step 1? I'll need those in about 2 and a half years (hopefully). Thanks 🙂