Threonine, an amino acid, has four stereoisomers. The stereois...

Question

Threonine, an amino acid, has four stereoisomers. The stereoisomer found in nature is (2S,3R)-threonine. Which of the following structures represents the naturally occurring amino acid?

Structures of threonine stereoisomers, highlighting the naturally occurring (2S,3R)-threonine.

Accepted Answer

Hello everyone. Let's do this problem and says the naturally occurring stereoisomer of amino acid isoleucine is two S3 S. Isoleucine identify the structure which represents this stereoisomer. And we're given four structures. ABC and D. They all followed the structure of isoleucine which isoleucine if we write out the traditional Iupac nomenclature name has the name two amino three methyl Pantano acid. So all of these structures follow this name, but we're simply changing the configuration on carbons two and three in each structure. So answer choice A has the methyl group and amino group wedge on carbons two and three. Answer choice B has the methyl group wedge and the amino group dash. Answer choice C has the methyl group dash and the amino group wedge and answer choice D has both the methyl group and the amino group dash. So how do we determine which configuration is two S and three S? We are going to use the K OL pre log nomenclature rules to review how to determine R or S configuration. So on these chiral carbons, they have four different groups, right? So our first task is to assign the priorities of these four different groups. And we do so by atomic mass. So the higher the atomic mass of the first atom in the group, the first atom connected to that chiral center. Then the higher the priority that group is OK. What if we have a tie between two atoms? Say we have two carbon groups? Well, then we look at the adjacent atoms and simply use the same atomic mass rule to compare the adjacent atoms connected to that initial atom. All right, what if we have a double bond or a triple bond, then we simply count those atoms multiple times. So for example, if I have a carbon double bonded to an oxygen, we equate that to a carbon being bonded to two oxygen atoms. OK. So now once we've assigned the priorities, we're almost ready to assign R R S configuration. But we have to look at the orientation of our lowest priority group group number four. Is it wedged or is it dashed? Because this determines which rubric we use to assign R R S configuration? So we do so by connecting groups 12 and three in a circle, and that circle is either going to go counterclockwise or clockwise. So if group number four is wedge and groups 12 and three connect counterclockwise, that is our configuration. If the circle goes clockwise, that is S configuration. And the opposite is true for when group number four is dashed. If group number four is dashed and groups 12 and three connect counterclockwise that is now s orientation. And if they connect clockwise, when group four is dashed, that is now our orientation. OK. So let's use this rulebook To determine the configuration at Carbons two and 3 for all four of these answer choices. And the one with S configuration at both carbons two and three is our correct answer. So we'll start with carbon three, the carbon with the methyl group and the groups are not changing for all four of these answer choices. So once we figure out the priorities for answer choice. A that will be easy to apply them to all answer choices. So looking at carbon three, the carbon with the methyl, we have a hydrogen, which is always the lowest priority. And then we have three carbon groups. OK. Well, our methyl group has three hydrogens. Then to the left, we have an ethyl group so that carbon has two hydrogens and another carbon. And the carbon to the right has a carbon, a nitrogen and a hydrogen. Well, the carbon with the nitrogen is going to be the highest priority because that nitrogen has a higher atomic mass than the carbon atoms in the other two groups. So that is priority group number one. And then the ethyl group is going to be priority number two because that first carbon has another carbon bonded to it. Whereas the methyl group just simply has three hydrogen atoms bonded to that carbon. So that will be third priority. So we've assigned our priorities. 2nd step was the Orientation of Group # four. It is dashed and connecting groups 12 and three, our circle is going clockwise. So this is going to be our orientation. OK. Well, we already know this is an incorrect answer choice because it has our orientation and our correct answer should have S and S orientation. So I'll quickly show carbon # two and we'll show the groups assigning priorities. So hydrogen is the lowest priority. Then the amino group is going to be first priority because of that nitrogen, whereas the other two groups are carbon groups. So to the right, we have a carboxylic acid group, that's a carbon with three oxygens, right? So that's going to be second priority. And the other carbon group has two carbons and a hydrogen. So that is third priority. And in this case group number four is dashed, our circle 123 goes counterclockwise. So this is S orientation. So we are looking for this wedged amino group to have S orientation. OK. Well, the only answer choice with a wedge amino group is answer choice C. So I know that answer choice C is going to be my correct answer. As long as carbon three has S orientation, which if we look at carbon three in this answer, choice answer choice A I see that the methyl group is wedged and an answer choice C it is dashed. So that means the configuration is changing. And since it was R configuration and answer choice A, that means it's going to be S configuration in answer choice C. So that's how we know that answer choice C has S S configuration and is our correct answer. I'll still go through each answer choice really quick if you wanna stick around and see how I determine the configuration of each one. So we already know the priorities of each group. So that will be easier. So looking at carbons three, I have priority one for the carbon with the amino priority two for the ethyl group, priority three for the methyl four for the hydrogen connecting groups 12 and three that goes clockwise. And Group # four is dashed. So that is our configuration just like the first answer choice. And then on carbon number two, we have hydrogen fourth priority amino group, first priority carboxylic acid. Second other carbon chain. Third priority group number four is wedge here and the circle goes 123 counter clockwise. So that is going to be our configuration. So notice how the direction of the circle is not going to change in any of these answer choices. So it simply depends on the Orientation of Group # four. All right, looking at c our correct answer on carbon three, we have hydrogen fourth priority wedge, methyl group, third priority methyl group. Second priority carbon with the amino group. First priority group number four is wedge, like we said In the circle, 123 goes clockwise. So that is going to be s orientation. On Carbon two, we have hydrogen. Fourth priority amino, first priority carboxylic acid, second priority carbon chain. Third priority group number four is dashed here and circle goes 123, counter clockwise. So that is going to be S orientation. OK. And lastly answer choice D on carbon three, in order of 1234 priority groups we have first, the carbon with the amino second, the ethyl group. Third, the methyl group. Fourth, the hydrogen group number four is wedge here and we know the circle is going clockwise. So that is S orientation. Carbon number two, we have first priority amino group, second priority carboxylic acid group. Third priority carbon chain. Fourth priority hydrogen Group # four is dashed here or sorry wedge. The Circle 123 goes counter clockwise. So that is our orientation. So D is incorrect. B and A are incorrect. And that shows C to be our correct answer. I'll redraw the structure here just to make it nice and clear. A five carbon chain with a carboxylic acid group on carbon one wedge amino group on carbon two and a wedge hydrogen dashed methyl on carbon three. So that makes c the correct answer for this problem with two S3 S configuration. That's it for this one. I hope this video was helpful. And thanks for watching.

Threonine, an amino acid, has four stereoisomers. The stereoisomer found in nature is (2S,3R)-threonine. Which of the following structures represents the naturally occurring amino acid?

Key Concepts

Stereoisomerism

Chirality and Chiral Centers

Cahn-Ingold-Prelog Priority Rules

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