Non-Ionizable Vs. Ionizable R-Groups - Video Tutorials & Practice Problems
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Non-Ionizable Vs. Ionizable R-Groups
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In this video, we're gonna distinguish between non ionizable and ionizable r groups. So non ionizable r groups are basically just r groups that are uncharged. And so recall that an ion is just an atom or molecule with a charge. And so if you are non ionizable, what that means is that you are not capable of gaining a charge, and that's because these r groups are lacking acid base reactions or proton trans transfers. They lack those, and that's why they are uncharged and non ionizable. And so, amino acids that have non ionizable r groups, they only have 2 p k a values, one for the amino group and one for the carboxyl group, but they do not have a third p k a value for their R group. So their R group does not have a pKa. And so really, there are actually 13 amino acids that have non ionizable R groups, and we'll see those in our example below. Now, really quickly, let's talk about ionizable R groups. So ionizable R groups are essentially the opposite of non ionizable R groups. So ionizable R groups are charged groups that, result from acid base reactions or H+ or proton transfer, so they do have these reactions. And so because, ionizable R groups are capable of ionizing and gaining a charge, they, amino acids that have ionizable r groups have 3 p k a values because the R group does have a pKa. And so, the ionizable R groups are all capable of forming ionic bonds, and that's a really big distinct distinction between amino acids with ionizable r groups and amino acids with non ionizable R groups because amino acids with non ionizable R groups are not able to form ionic bonds, and that's 13, the vast majority of them, that are not able to do that. And so in our example, what we're gonna do is we're going to total the total number of amino acids that, fall into each of the indicated ionization groups. So we've got all of these groups in the color code, shown in the right here. And so in this, chart, what you'll see is that the pink, all of the pink colored ones are positively charged amino acids at physiological pH, a pH of 7. And so recall from our mnemonic, dragons eat knights riding horses, that the knights riding horses are the positively charged, basic amino acids. And so there are 3 of them, so arginine so these are in alphabetical order, so we have arginine, histidine, and lysine, which are these 3 here. So there are 3 amino acids acids that are positively charged at pH 7. And with the negatively charged at pH 7, we know that the dragon's eating, those, or the dragon's eating, those, are the d and the e, those are the negatively charged acidic amino acids and so they are negatively charged at pH 7. And, of course, those include aspartic acid and glutamic acid or aspartate and glutamate. And so those are in yellow and you can see those here, aspartic acid and glutamic acid. And so, there are 2 amino acids that fall into that group. And then finally, there are some amino acids that are still ionizable, but they're actually uncharged at physiological pH, which is why we don't have them grouped up here with our charged amino acids at physiological pH. And those 2 amino acids that can become charged, but at a different pH, other than physiological pH, are cysteine, which is found here, and, tyrosine. And so there are 2 amino acids here that fall into that group. So there are a total when you add these, all up from each of these groups, there are a total of 7 amino acids that are ionizable and that means, of course, that the the rest of them, the rest of the 20 are going to be non ionizable. So again, there are 13 amino acids that are non ionizable and not capable of gaining a charge. Now, what I wanna do before we move on is to take a look at these ionizable, but mostly uncharged at physiological pH, and those are cysteine and, tyrosine. And so with cysteine, recall it's, 3 letter code is CYS, and it has a sulfhydryl group just like this. This is its R group. And so notice that cysteine, its R group pKa, which is shown in this column over here, its R group pKa is 8.3, so we can put that in over here. And so when the pH is above 8.3, what will happen is, cysteine's r group is going to donate its hydrogen and gain a negative charge, just like we see here. And so, it's important to be able to recognize that, how cysteine ionizes, since we already know how the other 5 ionize, because we memorize their structures when they were ionized. It's important to memorize specifically how these guys ionize since we already know how the positive and negatively charged ones ionize. And so the other amino acid that falls into this group is Tyrosine, and notice Tyrosine's, pKa is 10.1. So when the pH is above 10.1, tyrosine's r group will donate its hydrogen and gain a negative charge as well. So it's important to recognize how these 2 ionize as we move forward in our practice problems and our next topics. And so this concludes our lesson on the distinction between non ionizable and ionizable R groups, and I'll see you guys in our practice videos.
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Problem
Problem
Which pair of amino acids could form an ionic bond between the R-groups?
A
Asp & Glu
B
Gly & Leu
C
His & Arg
D
Cys & Lys
3
concept
Memorizing Amino Acids With Ionizable R-Groups
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2m
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Video transcript
So now that we know that there are only 7 amino acids with ionizable R groups, you might be looking for a strategy to not only memorize the 7 amino acids, but also the nature of the ionizations for those R groups themselves in terms of will they gain a positive charge when they're ionized or will they gain a negative charge when they're ionized? And so, the good thing is that of these 7 amino acids, it actually includes all of the charged amino acids that we already memorized in our mnemonic, Dragons Eat Nights Riding Horses. So it includes those 5 in addition to the 2 that we talked about in our previous lesson, tyrosine or y and cysteine or c. And so in our example, we have the mnemonic to help us memorize all 7 of those amino acids and the nature of their ionizations. And so the mnemonic is just an extension to what we already know, and it's just yucky crazy dragons eat knights riding horses. And so, basically, what we've added here is Tyrosine and Cysteine with yucky and, crazy. And so yucky and crazy are both negative words. You can't really think of something positive when it comes to yucky crazy dragons trying to eat you. And so, notice that these all have negative charges, so there's a negative charge here as well. So the only ones that have a positive charge are the ones, with the knights riding horses because they again, they're noble, they're coming to save the day, and they're positive. But there's nothing positive about yucky crazy dragons trying to eat you, so they're all negative. And again, there's a distinction between the yucky and the crazy, the tyrosine and the cysteine, from the dragons eating. Even though they're both negatively charged, there's a distinction, and that's shown by the blue shade and the yellow shade. And so with the blue shade, the difference is that these, tyrosine and cysteine, are uncharged at pH 7. At physiological pH, they're uncharged. And that's why we didn't group them originally with the charged amino acids, which are all charged around physiological pH. But even though tyrosine and cysteine, again, are uncharged at physiological pH, they are likely to be ionized and gain a charge at other pHs. And so that's why we've got this mnemonic to help us memorize them. And so hopefully, this will help you guys with a memory tool and, we'll be able to utilize it as we move along through our course. So this concludes this lesson, and I'll see you guys in our next video.