BR pI calculations

[brrenters]

---

Members don't see this ad.

Hi SDN,

was wondering if any of you had experience with the BR isoelectric point questions. I know the ballpark values for the pka of the C and N terminals, and the ballpark values for every side chain, but the practice examples seem to test to the pinpoint values for the terminals. For example, in asking for the pI of tyrosine, I had to pick between 5.6 and 6.2. I guesstimated around 6, and wrongly picked 6.2.

Will the actual MCAT test to this level of scrutiny?

Thanks!

 

[shaboobly]

Hi SDN,

was wondering if any of you had experience with the BR isoelectric point questions. I know the ballpark values for the pka of the C and N terminals, and the ballpark values for every side chain, but the practice examples seem to test to the pinpoint values for the terminals. For example, in asking for the pI of tyrosine, I had to pick between 5.6 and 6.2. I guesstimated around 6, and wrongly picked 6.2.

Will the actual MCAT test to this level of scrutiny?

Thanks!

you estimated too much. you need to take the mean of the pKa of the carboxyl and amino groups which comes to be around 5.7 =/= 6.2 and thus 5.6 is closer and the best answer.

 

[Sandstorm]

Hi SDN,

was wondering if any of you had experience with the BR isoelectric point questions. I know the ballpark values for the pka of the C and N terminals, and the ballpark values for every side chain, but the practice examples seem to test to the pinpoint values for the terminals. For example, in asking for the pI of tyrosine, I had to pick between 5.6 and 6.2. I guesstimated around 6, and wrongly picked 6.2.

Will the actual MCAT test to this level of scrutiny?

Thanks!

Feel free to correct this, but I've never had to memorize the pka values for side chains for the mcat. Know which order they get protonated/de protonated, yes, but not memorize the actual numbers. This might be TBR over-doing it.

Source: I've been teaching and tutoring mcat prep for 3 years

 

[shaboobly]

Feel free to correct this, but I've never had to memorize the pka values for side chains for the mcat. Know which order they get protonated/de protonated, yes, but not memorize the actual numbers. This might be TBR over-doing it.

Source: I've been teaching and tutoring mcat prep for 3 years

you need to know what happens to amino acids when they are in acidic, basic solutions and how to determine the pI. you should probably know what happens at the 2 pKas for an amino acid, where the terminals are.. there's really not much else. all else you would prob need would be most likely be given in the passage or question stem.

 

[brrenters]

you estimated too much. you need to take the mean of the pKa of the carboxyl and amino groups which comes to be around 5.7 =/= 6.2 and thus 5.6 is closer and the best answer.

I guess what I'm asking is, do I really need to know that the pka of the amino group is 8.8 is asparagine vs. 9.4 in serine vs. 10.01 in aspartic acid? I understand that I have to take the avg of the pka of the amino and carboxy group, but when the pka range varies to that extent, am I wrong to think that deciding between 5.7 and 6.2 as the pI is unreasonable?

 

[V5RED]

Feel free to correct this, but I've never had to memorize the pka values for side chains for the mcat. Know which order they get protonated/de protonated, yes, but not memorize the actual numbers. This might be TBR over-doing it.

Source: I've been teaching and tutoring mcat prep for 3 years

I would tend to agree with this. The MCAT topic lists have some numbers that they expect you to know, but pKa's of amino acid sidechains are not among them. TBR goes too far in a lot of places.

 

[sciencebooks]

I would tend to agree with this. The MCAT topic lists have some numbers that they expect you to know, but pKa's of amino acid sidechains are not among them. TBR goes too far in a lot of places.

As would I. I've never seen anything on the MCAT test specific numbers like that. Heck, they hardly expect you to memorize the gas law constant (I've seen it given so many times) that I doubt this would be expected.

 

[flashlightpen]

Feel free to correct this, but I've never had to memorize the pka values for side chains for the mcat. Know which order they get protonated/de protonated, yes, but not memorize the actual numbers. This might be TBR over-doing it.

Source: I've been teaching and tutoring mcat prep for 3 years

Can you explain a little bit more about that? My TPR book touched on that a little bit, but it didn't make much sense to me.

 

[Sandstorm]

Can you explain a little bit more about that? My TPR book touched on that a little bit, but it didn't make much sense to me.

If you start with your amino acid in acidic solution, then titrate with a strong base like naoh, first to be deprotonated will be your cooh group, then your r group (if it's an acidic or basic side chain), then your amino group.

 

[flashlightpen]

If you start with your amino acid in acidic solution, then titrate with a strong base like naoh, first to be deprotonated will be your cooh group, then your r group (if it's an acidic or basic side chain), then your amino group.

Cool, so in that particular example, it will deprotonate the lowest pka (most acidic, lowest pH) values first (and go increasingly). Correct?

order of deprotonations when titrated with BASE!:
01. COOH = pka ~2
02. R = pka < ~9
03. NH3 = pka ~9

 

[Sandstorm]

Yes