Threonine has two chiral centers. Draw L-threonine and indicate which carbon atoms are chiral. Which carbon atom is responsible for D and L configuration?

Question

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Hi, everybody. Here's our next problem. It says isoleucine is an amino acid that has two chiral centers provide the structure of L isoleucine and indicate each chiral carbon identify the carbon atom that causes the D and L configuration. So of course, when we have a chiral molecule, we have an anti emer non superimposable mirror images. And in the case of amino acids, we label them as L or D configuration. And we recall that in nature only the L configuration is used. So first, let's start with the basic structure of isol lucine. We know that we have our amino acid backbone with its H three N plus our alpha carbon, which is CH and then our carboxylate group C double bonded to an O with an O minus. And then our, our group comes off of that alpha carbon. So we can look at the structure of isoleucine and in that case, it's a three carbon chain. So we have CH and then C two C three and then a methyl branch coming off of that top carbon. That was just CH. So that is our, our group. I'll go ahead and highlight that in blue So we need to think about the structure of L isoleucine. Well, for the purposes of keeping straight are L and D and Antar, we have a standard way of writing the amino acids that's to line up all the carbons in a row with the carboxyl group on the top. So we'll draw, here's our carbon carbon and then we draw the double bond to an O sort of off to make sort of a Y shape on the left and then our O minus on the right. So there's our carboxylate group. So next will be the alpha carbon. And then we know our alpha carbon has a hydrogen and an NH three plus as well as the R group. Well, we draw the R group on the bottom. So our group will extend down from the carbon, but we have to decide which side to put our hydrogen and which side to put our nitrogen on. Well, here's where that L and D comes in because the Cairo center here is this alpha carbon, which is carbon number two as I go down. And it's the one that causes the L and D configuration because when NH three plus is on the left, this is the L configuration. So you think L for left, the nitrogen should be on the left and then the hydrogen is on the right, if they are reversed, when NH three plus is on the right, that is the D configuration. So we've arranged our configuration, we've identified the first chiral carbon and the one that is responsible for the LD difference. And now we just need to put our, our group on. So we know next, we have a three carbon chain. So we have C and then for the purpose of identifying chiral center, we'll go ahead and draw out all the bonds. So our carbon has a hydrogen and a methyl group, we know our methyl group won't be Cairo because of those three hydrogens, then we have a carbon with two hydrogens and the final terminal methyl group. So neither of our methyl groups will have a chiral carbon, obviously, nor will our second to the last carbon. So 1234 in the downward order of the chain, it has two hydrogen. So it's not chiral. But when we look at the first carbon in our R group, I'm going to go ahead and highlight the R group to keep straight or the carbons in the R group here just to keep straight compared to the other structure that I drew. When we look at that first carbon in the R group, we see it has a hydrogen, it has a methyl group, it has an ethyl group C two C three and then it has the alpha carbon with its rest of the amino acid backbone. So this first carbon in the R group is also a chiral center as it has four different atoms bonded to it in a tetrahedral arrangement. So we need to see, have we done everything our problem as of us said, provide the structure of L isoleucine. So we did that here. This is the L arrangement with the nitrogen on the left of the alpha carbon indicate each chiral carbon. So our alpha carbon is chiral and our first carbon in the R group is chiral and then identify the carbon that causes the L and D configuration. And that would be the alpha carbon. So we'll just put an arrow there, say alpha carbon causes LD configuration as we discussed. That's because if the nitrogen is on the left of that alpha carbon, when you draw it in this format with the carb carb box slate group on the top and the R group on the bottom, then it is L if the nitrogen is on the right, it's the D arrangement. So we've answered everything our question asks of us. We'll see you in the next video.

Threonine has two chiral centers. Draw L-threonine and indicate which carbon atoms are chiral. Which carbon atom is responsible for D and L configuration?

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Key Concepts

Chirality

Stereochemistry

D and L Configuration