Draw the predominant structure of Arg at pH 6.5? (pK ...

Question

Draw the predominant structure of Arg at pH 6.5? (pK _a1 = 9.04, pK_a2 = 2.17, pK_a3 = 12.48).

Accepted Answer

So this practice problem wants us to draw the predominant structure of arginine specifically at a pH of 6.5. And then it goes and it gives us the 3 pKas for arginine's ionizable groups. And so recall that arginine's one letter code is r, and, the mnemonic that helps us memorize arginine structure is dragons eat knights riding horses. And so the r here is arginine. And so arginine's r group has components of both lysine and histidine, and we'll get to its r group after we start drawing the predominant structure. So we know that when we're drawing an amino acid that there's gonna be an amino group. So let's put the amino group in a dotted box here because we'll get to it shortly. And then we know that there's going to also be an alpha carbon and a carboxyl group. So we know there's gonna be a carboxyl group. Here's the alpha carbon. And then this over here is gonna be the, protonated protonated carboxyl group. So we know that in here is gonna be either an Oh or an O minus group. And so with this alpha carbon here, remember that the r group has to be going down and dash. So remember that for the r group, when you're drawing an l amino acid, so we're trying to draw an l amino acid, that the r group must go, down and dash, so r d d. So it has to be going down and dashed. And so, let's get that out of here. And so now that we know that, we have to have a dash going down for our R group to get an L amino acid, this is going to be the overall structure of our R group. So now remember that just like, arginine's r group, has 3 pointy ends on it, so it has one pointy end here, one over here, one over here because it has features of a sword. It has 3 pointy ends that you could really try to poke somebody with. Its r group is gonna have a 3 carbon start to the chain, so it's gonna have a c h 2, another c h 2, another c h 2. And then it's gonna have a triangular structure, so the three points also helps you remember of a triangular nitrogen structure. So the way this structure works is there's a nitrogen, a carbon, and another nitrogen, and this carbon is double bonded to another nitrogen. And so then you just fill in the hydrogens. This one's gonna have 1, this one will have 2, and this one is also going to, well, we're not sure how many it's gonna have because we're trying to figure out if it's charged or not. So it's either gonna have 1 or 2, so for now we'll just leave it as an h until we figure out if it's going to be charged or not. So now that we have arginine's R group mostly filled in, and we know that the amino group is going to go over here, So we know the amino group will go here and the carboxyl group will go over here. Now, we can start to compare the pH of the solution, which is given to us as 6.5 to each of the individual r groups. I mean, each of the individual pKas. And so what you'll see here is that pka 1 here must be for the amino group because pka 1 has an art has a value of about 9, and we know that the alpha amino groups, which are over here, have to have a pKa of between 910.5. So pKa 1 must be this guy over here, so we'll put pKa 1 over here. The carboxyl group, which is this one over here, must be pKa 2 because pKa 2, we know that, alpha carboxyl groups have to have a pKa of about 2, so that has to be this one. So we'll put that in green right next to it, pKa 2 is this one. And then the pKa 3, of course, has to be the one for red. So this has to be for the r group, which is down here. So we'll go ahead and put pKa 3 over here. So now we just again compare the p k pH to each of the pKas. We We can start with the blue over here. So the pH is, 6.5, and a pH of 6.5 is less than a pka 1 which is equal to 9.04. And when the pH is less than the pka, what that means is that the conjugate acid is gonna predominate and the conjugate acid form of the amino group is an NH 3 plus. And so we that's what we can put into this group over here. We can put in NH 3 and we'll put the plus in red just to make it a little easier to see. Now we can do the same and compare the carboxyl group, pKa to the pH. So the pH is still 6.5, so pH is 6.5, is greater than pKa 2, which is equal to 2.1 7, right here. And so again, when the pH is greater than pKA, what that means is that the conjugate acid is gonna be lower in concentration and the conjugate base is gonna predominate. And the conjugate base of the carboxyl group here, so the conjugate base is going to be a deprotonated carboxyl group, so it's just gonna be an O minus. So in here, we can just put in an O minus and we'll put that there. So now, the last thing that we need to do is look at the, compare the pKa 3 to the pH. So again, the pH is gonna be still equal to 6 oops. It's gonna be equal to 6.5. And the pH of 6.5 is actually less than pKa 3, which is equal to 12.48, shown up here. And so when you have, again, the pH is less than the pKa, what that means is that, the conjugate acid is gonna predominate. And so the conjugate acid form is gonna be protonated. And so that means that this, nitrogen here is gonna have indeed 2 hydrogens on it and it's going to be protonated. And so now we have all 3 of our groups in the appropriate charges, so we have a positive charge on the amino group, a negative charge on the carboxyl group, and a positive charge on the r group. So 2 positive charges and one negative charge comes out to a net charge of just positive 1. And so that concludes this practice problem. We've drawn the structures, and, we will see you in the next practice problem.

Struggling with Biochemistry?

Draw the predominant structure of Arg at pH 6.5? (pK _a1 = 9.04, pK_a2 = 2.17, pK_a3 = 12.48).

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Zwitterion practice set

Struggling with Biochemistry?

Draw the predominant structure of Arg at pH 6.5? (pK a1 = 9.04, pKa2 = 2.17, pKa3 = 12.48).

Watch next

Zwitterion practice set

Draw the predominant structure of Arg at pH 6.5? (pK _a1 = 9.04, pK_a2 = 2.17, pK_a3 = 12.48).