Amino Acids (Part 4 of 5) - Titrations and pI (Isoelectric pH)
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- Опубліковано 8 вер 2024
- Moof's Medical Biochemistry Video Course: moof-university...
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Here, I discuss amino acid titration curves and how to find / calculate / determine the isoelectric point (the isoelectric pH), otherwise known as the pI, which is the pH at which an amino acid will most likely have an overall charge of zero.
I chose to use the basic amino acid histidine as the example for how to find isoelectric pH (pI) because it has three titratable groups: 1) the alpha carboxylic acid group, 2) the alpha amino group, and 3) the side chain's imidazole group. Most amino acids have only two titratable (or ionizable) groups, the alpha carboxylic acid group and the alpha amino group. So, finding the pI for most amino acids is super easy - you just have to take the average of the pKa values of the two ionizable groups, the alpha carboxylic acid group and the alpha amino group. For amino acids with three ionizable groups [the acidic amino acids (aspartic acid and glutamic acid) and the basic amino acids (histidine, arginine, and lysine)], solving for the pI is slightly more complex. You still must average out two pKa values, but which two? The answer is: the two pKa values that are on either side of the zero-charged species.
So, this video is a walkthrough for the titration of histidine specifically and how to determine which two pKa values must be averaged out to determine the pI and WHY that is.
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I've been struggling with this concept all semester and you just explained it so much more clearly better than my professor did. Thank you!!
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Finally, someone who made titration so crystal clear. Thank you!! :D
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Thank you! My titration confidence just went through the roof :)
by far one of the most helpful videos I've watched! I could NOT understand this concept and now I do!!! Thank you!
dilbar09naz Sweet! Thank you for the kind words! I'm happy to have helped! You're welcome!
Ι am sorry i didnt quite understand why we need the overall charge of the molecules! ??
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Well, at least I get why it's happening.
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“Right?”
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This is very helpful also to understand the netcharge thank you
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Had this histidine on my biochem exam and didn't get it right. However, as I go back to review it, this makes sense!
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Me: 4:39 “As we add 🅱️ A S S”
Davie504
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Thank you. I understood everything except how to label IP and EP. I had no examples to help me!
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Thank you, this makes so much sense!!
so the amino group is more acidic than the imidazole because it has a lower pKa and will donate first?
it is very helpful thank you so much
The GOAT
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Very helpful thank you for the review!
I love your content! Thank you so much for devoting your time. I watch your videos all the time. Can you explain where the equivalence point (or points?) are and what this means? I am confused if there is only one equivalence point or more than one for these amino acids that have an ionizable R group in addition to the 2 backbone ionizable groups.
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MrRNGLDR Sure thing!
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this was very helpful! Thank you for making this video. I was just wondering how you picked the equivalence points to begin with based on looking at the curves in the graph. Thank you again!
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In the Alanine example you gave the pKa for the functional groups at a net charge of +1. Are you missing a step in calculating the Isoelectric pH? For example, wouldn't the pKa for the COOH need to be doubled to represent the pKa of COO-, giving an overall charge = 0. Perhaps, the pKa values are NOT proportional, but my point is simply that we need to find the pKa at a net charge = 0. From there, you could plug into the equation correctly.
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Bella Lol. Sure thing!
guys for the last part of the video, the Alanine part i think that there's some mistakes (or maybe me?)
if we see protonated Alanine has charge of +1, which comes from COOH and NH3+.
The pKa of the carboxyl group is 2,34 and the amine group is 9,64.
If the curve of titrations begins to rise until pH of 2,34, the carboxyl group lose all of its proton so they become COO- and the Amino Acids has overall charge of zero.
so if we want to take the pI for Alanine, dont we just do 2,34/2 since the charge in the beginning is 1 then its rise till 0 and we do the pH= pKa + log[base]/[acid] then we got pH = pKa that equal as pKa of COOH/2?
omg.. im sorry my fault now i do understand your alanine was right! thanks a lot
Great explanation!
Thanks!
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Your histidine structure is not quite that. It's the Nitrogen on the right that should have a double bond.
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Can you explain how you got the pka values like 1.82, 6.0 and 9.17 please. Im a little confused
they are given
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גל פ No prob.
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Thanks
Just wondering; how are you supposed to know the pH for each group (amino, carboxyl) on each AA? Just learn them by heart? If our prof was half as good as you are - life would be so much easier!
I think you mean pKa. The pH is subjected to both groups. pKa's are derived constants from testing. You should be given them in the problem. It would be cruel to require you to memorize them.
Daniel Barragan yeah pKa that's what I meant! We are given an pH and asked to solve the pKa and pI for it, that's why I was wondering :/
Ida Ettanen I'm a little late on this one, but yeah -- what Daniel said. They're constant values that should be provided to you, should they be necessary.
How do you know what gets hydrogenated and what does not? for example why wasnt the second NH in the R group not protonated?
thanks very much!
Would the X axis represent the reading on the burette at the time the pH is taken?
I loved it except I got lost in the beginning as to how you found out where on the graph that the inclines and plateaus were :/.... well here goes watching it again, must have missed something.
Thank you so much! Very helpful video!
The buffer region would also correspond to the half equvalence/pka then, correct?