How to Calculate Peptide Charge and Isoelectric Point MCAT Trick
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- Опубликовано: 27 июл 2024
- leah4sci.com/aminoacids presents: How to calculate peptide charge and Isoelectric Point (pI) without a calculator for the MCAT
📺Watch Next: Peptide Isoelectric Point with pI Shortcut • Peptide Isoelectric Po...
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This is video 7 in the MCAT amino acids tutorial video series. Learn how to quickly find the charge and isoelectric point of any given dipeptide, tripeptide or even polypeptide without wasting time on useless trial and error calculations.
Referenced in this video:
Amino Acid pKa table in this tutorial: leah4sci.com/amino-acid-charg...
MCAT Math series: leah4sci.com/mcatmath
Catch this entire video series along with my amino acid cheat sheet, tutorials and practice quiz on my website:
leah4sci.com/aminoacids
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Thank u so much Leah.....I am planning to start my post doc career in protein chemistry and thought of brushing up the things I read long back...came across your videos and i wish if i could get back my college days again..u r a damn gud teacher..i must say...all the very best..keep going and uploading such wonderful tutorials..
Aww thanks for your kind words! Happy to help!
Spring 2019, final quarter in the enduring Ochem series. You have enlightened my knowledge and your tutorials are a great supplement to my lecture notes. Thank you so much Leah it's been a great year following your channel. BIOCHEM here I come!
AWESOME! Glad that the videos have been helpful
Can't believe how clear and understandable you can explain something. You make it all so much easier. Thank you!
You're very welcome! Glad to help
@@Leah4sciMCATwhere’s the fent pack?
Thank you so much for this!!! You are a life-saver and amazing. I also used your videos for my o-chem class last year and now for biochem! Thanks Leah!
You're very welcome! I'm glad I have been able to help you!
This has truly helped with my molecular cell biology class. You make something that is complicated in class and simplify it and make it easy to understand. I really appreciate it!
I'm so glad to help! Thanks for the compliments. :)
Thank you! I was struggling with this concept and this video helped me so much! highly appreciated
You're so welcome! I'm glad to hear that it helped you understand!
By far the best videos for biochem! Your explanations are the best, they always flow perfectly with the content, and the visuals are on point. Thank you so much for creating this content, it truly is very valuable
Wow, thank you! I'm so glad to here how much my resources are helping you!
This is extremely helpful! Also your voice is very soothing and relaxing.
Thank you and happy to help!
You make everything so easy! The shortcut is amazing thank you Leah
You're so welcome!
Thank you so much!!! Tomorrow I have a test about PI and I didn't understand it but now I do!!!! Thank you so much!❤
You're very welcome!
u r a DIVA! u just rock it... thank u for all the help u r giving me thru ur lectures...
Oh wow, thanks so much!
Hi! I'm a medical college student in Korea and I'm currently studying biochemistry. I've been struggling with my studies and just happened to click this video. It was extremely helpful -better than my lectures actually :) and thank you sooo much for your wonderful work!!
You're so welcome, I'm glad it helped!
Thank you for the thorough explanation!
You're so welcome!
Thank you so much for these videos they're super helpful!
Totally worth the sub :)
You're very welcome, and thanks for the sub!
Thanks for this helpful video! Your explanations are great
You're very welcome!
I was really stressing bc I wasn't understanding much in my biochemistry course, goshhh did this helppp so so much! thank you lots !
You're so welcome, glad it helped you understand!
This is so helpful. Thank you so much!
You're welcome! :)
This is so extremely helpful. Thank you so much!
You're so welcome!
THIS WAS SO HELPFUL.THANK YOU SO MUCH!
You're welcome!
Incredibly clear teaching style, makes learning this a breeze.
Glad it was helpful!
You carried me through organic chemistry, and now you are carrying me through biochem. Thank you, you're the best!
Thank you for your wonderful feedback! You're very much welcome.! :)
This is sooooo helpful! best video in this field. Thank you!
You're welcome! Glad it was helpful!
THANK YOU SO MUCH LEAH THIS WAS REALLY HELPFUL
You're very welcome! Glad I could help!
Oh my gosh Leah you're such an angel. Thank you so much. I give you my salute. I hope you can help me in my future struggles.
Aww, thank you for your kind words!
This video is very helpful. More explanation than what I learned from my lecture. Thank you so much.
You're welcome! :)
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Glad to help!
ABSOLUTELY AMAZING! THE BEST TEACHER EVER!!!
Thanks for your kind words :)
great! Thank you so much for the trick!!!
Glad I could help :)
thank you so much for these videos i’ve been struggling with this exact concept and simply watching your video on the bus has cleared it up. thank you so much !!
Glad it helped clear things up for you, you're welcome!
Thank yo so much, this makes sense to me and I actually feel comfortable doing this now! Here's to hoping I do well on the exam
I'm so glad to hear it! You're welcome.
thank you! you explained really well!!!
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Your videos are amazing. Thank you so much for these tips.
You're welcome!
Thank you so much 🙏🏻 really very helpful video for me. Much needed. 🤗
Glad it was helpful!
I've been struggling to understand this for a while, but you explained it in a very clear and didactic way...Thank you so much!!! :)
You're so welcome, glad it was helpful!
@@Leah4sciMCAT whats the difference between the formal and net charge?
Formal charge is the charge on each individual atom, net charge is the total. For example glycine at pH = 7 has a net charge of zero, but a formal charge of +1 on the amino N and -1 on the carboxy C
Great explanations, you're a lifesaver!
Glad to help!
I wish you the best! Thank you!
Thank you!
Thank you Leah! You're the best. :)
You're very welcome!
I rarely comment on these videos but I had to. I've been struggling with this topic in my biochem class and this one video just solved it! Glad to know you're so clear because I'm starting to study for the MCAT. Thanks soooo much :) Wish you the best!
Thanks to your video I can do it in my head! (I use my fingers and it's very helpful actually)
You're very welcome! Glad to help
Can't thank you enough!! you are so awesome
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This video is so helpful! thank u!
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Glad I could help!
Hey leah thank you so much..... You make learning so easy 😘
So glad to hear that!
Omg this was the best explanation ever. It saved my ass on the exam. Thank you so much
You're welcome. Glad the video helped :)
wow, it's amazing what a skill you are having in teaching.
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your way of teaching is very clear!
Glad to hear that, and you're very welcome!
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You're so welcome! Thank you for watching. :)
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Thankyou Ms. Leah bc of u i survived our org chem and got a high grade
You're welcome and CONGRATULATIONS! That's awesome :)
thank you!!! i finally get it!!
You're welcome! Glad to help! :)
I really learned more from this video the day of my exam than I have all semester
I'm glad it helped!
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Thank you for your kind words :)
THANK YOU :D
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Hi Leah , thanks for the Video ❤️ ..its really helpful ..
i have a question ... If we have Gln , Ser, Thr, Asn in the peptid .. Why didnˋt you write the Pka for their side Chains ?
You're very welcome, glad you like it!
I'm sorry, but I don't offer tutoring through RUclips comments. For help with questions like this and more, I recommend joining the MCAT Study Hall. For more details visit join.mcatstudyhall.com/ or contact me through my website leah4sci.com/contact/
Thank you so much :)
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So helpful.
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You saved my biochemistry exam 😍❤️
I'm glad I could help!
in all its essence your voice is angelic.
Wow, thanks!
Hey Leah , are you sure we have to memorized the ph level of amino acids? and also memorized the side chain? I'm reading my Examcrackers book and it side that not required...
They're wrong! Every student I interview after EVERY exam says you have to know them. Read about it here: leah4sci.com/tag/mcat-feedback/
Hello Leah, how do you work a problem that is asking for the pI of a peptide without given any pH value and just the N terminus and C terminus pKa's?
You have to memorize pKa values
pH is not required for pI
Thank you Mam
You're welcome :)
You are awesome!
Thank you! You are too!
We owe you money for this video thank you🙌
You're very welcome!
Could you please explain why the deprotonated carboxyl group of Glu is -1 and not -2? Doesn't COO have a -1 charge initially?
I'll be happy to! When protonated, the carboxyl group is neutral and appears with its hydrogen as -COOH. When deprotonated, it has a charge of -1 and appears without its hydrogen as -COO. As there is only one proton to remove on the carboxyl group, it would not be able to hold a -2 charge.
Thanks for putting this into English! My prof doesn't seem to know how to teach this well
LOL! I'm sorry you're having trouble with your professor, but I'm glad I could help!
Thanks @LEAH4SCIMCAT
You're very much welcome :)
At timestamp 17:20, why is the R side chain of glu -1/2 and not +1/2 at PH=4?
Since the pH of 4 is equal to the pKa of the Glutamic acid side chain, we find that about half of them are protonated in solution and half of them are deprotonated. Remember that a protonated carboxy group carries a neutral charge of zero, while a deprotonated carboxy group carries a -1 charge. Halfway between 0 and -1 is -1/2.
Hello, thank you for another very helpful video. You did saythat we have to memorize the pKa for every amino acid side chain, but you made a comment on your website that we do not. Which one is it? Here is the comment for reference: "I would not. I’d know a general amino side chain, general carboxyl side-chain, acid/base and Histidine since it’s so close to physiological pH and therefor exists as a 10:1 ratio in many cases".
Yes and no, the key is to memorize a few key values as I explain in detail here: leah4sci.com/amino-acid-charge-in-zwitterions-and-isoelectric-point-mcat-tutorial/
Hello Leah. Thanks much. Are these values still valid to retain for MCAT in 2020? Thanks much.
Thanks for asking! Of course! Amino acids are the most commonly tested topic on the current MCAT.
How can I know what is the predominant form of the peptide at certain pH?
It is this exact question that the video strives to answer. The short answer is that you must compare the pKa of each ionizable group to the pH of the solution to determine its individual charge, and eventually sum these charges for an overall charge of the peptide. Is there a specific point in the video you have a question about?
Great!
Thank you
Recently found your channel. Are answers available for the extra practice questions on your site?
Yes, there are answers to most of the online quizzes.
why we choose -1/2 over +1/2 at 17:15 min? I am not clear on how to choose one over other. Could anyone help clarify. thanks.
Since the pH of the solution is equal to the pKa of the Glutamic acid side chain, we find about half of them protonated in solution and half of them deprotonated. Remember that a protonated carboxy group carries a neutral charge of zero, while a deprotonated carboxy group carries a -1 charge. Halfway between 0 and -1 is -1/2.
Doesn't arginine's side chain stay positive in whatever ph solution because of its resonance structure that makes it planar? In addition to the MO theory? I'm confused about that and how you have only one amine.
At which point in the video?
am i wrong if i say that the pKa charge for amine group(lys) is 9...i just look at the list of pka..it show the pKa of lys is 9
No, you’re not wrong. I was using a generic pKa of ~10 for the terminal amino group. The alpha amino groups of the various amino acids have pKa’s that range anywhere from 9-11. See my chart on the MCAT Amino Acids Practice Quiz for more specific values: leah4sci.com/amino-acids-practice-quiz
How is it that (Glu) and the Alpha-Carboxyl group is neutral (0) at 16:55 but (ASP) and the Alpha-Carboxyl Group are (-1) at 3:03? Can you explain why is it like this in this case?
The charge of the carboxyl group changes, depending on the pH of the solution it is in. If pH > pKa of the carboxy, the group is deprotonated with a -1 charge. (As is the case with Asp at 3:03.) If the pH < pKa of the carboxy, the group is protonated with a neutral charge. In the case of Glu at 16:55, pH = pKa, so the charge is in between -1 and 0. You'll see the charge of Glu is -1/2 at this point in the video.
If there are three pka which two to consider and which to omit, can someone help me ok this?
I have a shortcut for this! I'm going to challenge you to watch around 11:20 in the following video: ruclips.net/video/HL1qx8bTDnA/видео.html
What if we have 2 cysteines in the peptide and we want to calculate its net charge above 8.3 pH (pKa of cysteine). will we have -2 charge contributed by the cysteines or will they form a disulfide bond and contribute no charge?
Depends on the reactive conditions, if they are in a disulfide bridge then they are uncharged
Why is the sulfur neutral when protonated?
Charge has nothing to do with protonating status. Instead, do a quick formal charge calculation as I teach here leah4sci.com/formal-charge-formula-and-shortcut/
Why does the Carboxy Group on Asp and the R-Group on Asp become (-1) when Deprotonated, but Neutral when they are Protonated? I'm very confused by this. Shouldn't the Carboxy Group on the C-Terminus have a -1 charge to begin with????
See my formal charge video to understand what's happening here
Thank you so much for making this video! Very good tips to memorize and easy to understand
biochemistry. Please make more videos like this. I really appreciated your hard work.
You're very welcome, I'm so glad my resources have helped!
can someone pls explain me why when it's positive it represents a basic amino acid and when it's negative it represents an acidic amino acid and in louis defenition of acids and bases it's totally the opposite i mean the acid should be positive and able to have electrons and the base should be negative and able to donate electrons ?
This is a tricky concept. the charge comes from how you see the side chain at physiological pH, however if you look at each R group in it's neutral state you'll see how they clearly follow the rules for acids and bases.
For example: Gluatamic Acid has an acidic H+ when neutral, but a pH = 7 it has acted as an acid does and donated the H+ giving you a negative conjugate base. This is the deprotonated form OF the acid
So just an update! Idk if someone else has mentioned this: the MCAT now provides you with either pI or the needed side-chain pKas. You don't need to memorize them.
Hey! Can you link the source that says this/ where you heard it from?
@@essencie5753 Kaplan MCAT biochemistry book states that you will "not be expected to know exact values for isoelectric points or side chain pKa values. The MCAT will either provide you with numerical data to use in answering questions or test clear cut distinctions that can be made without additional information."-6th edition, 2019. Kaplan is credited to be a reliable source for MCAT prep (rivaled only by Princeton).
Obviously, you will need to know what the properties of the AA are (acid/base/NP/polar), and from that, you can estimate where pKas and pIs are (Generally, acid pIs are less than 7 while basic group pIs are above 7).
I hope this helps! If you feel you don't want to take any chances then ofc go ahead and memorize but it should be intuitive :)
Kaplan has caused quite a few of my students to miss questions because of this statement. You do, in fact, need to know approximate values as I teach here: leah4sci.com/amino-acid-charge-in-zwitterions-and-isoelectric-point-mcat-tutorial/
Okay so the way I learned this was that you take the average of the pKas where the amino acid (or peptide) has a net charge of +1/2 and -1/2 to determine the pI. Would this not make it more accurate than taking the avg of pKas at a net charge of +1 and -1?
The approach is very similar, however you can't always count on having a pKa value AT the pH (for the + and - 1/2) additionally, how do you know which is the second pKa value to use with this approach?
How do you determine the charge?
The charge on the overall molecule is determined by adding together the charges of the individual side chains, as well as the charges on the carboxy and amino groups. If looking to determine the charge on any of these individual groups, visit my tutorial at leah4sci.com/amino-acid-charge