A note on sugar nomenclature: biochemists use a special system to refer to the stereochemistry of sugar molecules, employing names of historical origin in addition to the designators ' D ' and ' L '. You will learn about this system if you take a biochemistry class. As you can see, D -erythrose is a chiral molecule: C 2 and C 3 are stereocenters, both of which have the R configuration.
In addition, you should make a model to convince yourself that it is impossible to find a plane of symmetry through the molecule, regardless of the conformation. Does D-erythrose have an enantiomer? Of course it does — if it is a chiral molecule, it must. The enantiomer of erythrose is its mirror image, and is named L-erythrose once again, you should use models to convince yourself that these mirror images of erythrose are not superimposable.
Notice that both chiral centers in L-erythrose both have the S configuration. To avoid confusion, we will simply refer to the different stereoisomers by capital letters. Look first at compound A below. Both chiral centers in have the R configuration you should confirm this for yourself! The mirror image of Compound A is compound B, which has the S configuration at both chiral centers.
If we were to pick up compound A, flip it over and put it next to compound B, we would see that they are not superimposable again, confirm this for yourself with your models!
A and B are nonsuperimposable mirror images: in other words, enantiomers. Now, look at compound C, in which the configuration is S at chiral center 1 and R at chiral center 2. Compounds A and C are stereoisomers: they have the same molecular formula and the same bond connectivity, but a different arrangement of atoms in space recall that this is the definition of the term 'stereoisomer.
However, they are not mirror images of each other confirm this with your models! By definition, they are diastereomers of each other. The structure of the amino acid D-threonine, drawn without stereochemistry, is shown below. D-threonine has the S configuration at both of its chiral centers.
Draw D-threonine, it's enantiomer, and its two diastereomers. D-glucose and D-fructose are not stereoisomers, because they have different bonding connectivity: glucose has an aldehyde group, while fructose has a ketone. The two sugars do, however, have the same molecular formula, so by definition they are constitutional isomers.
D-glucose and D-ribose are not isomers of any kind, because they have different molecular formulas. Exercise 5 : Identify the relationship between each pair of structures. Your choices are: not isomers, constitutional isomers, diastereomers but not epimers, epimers, enantiomers, or same molecule. Exercise 6 : Identify the relationship between each pair of structures. Hint - figure out the configuration of each chiral center.
Learning Objective interpret the stereoisomerism of compounds with three or more chiral centers. It works well for sugars since they can be built up so systematically like the binary system. Of course, sugars are not always so helpfully drawn in Fischer projections — they form rings. Note 1. The same is true for — -erythrose, which returns a completely identical compound. From this it can be deduced that the structure of the new compound must be such that the molecule has an internal mirror plane i.
A compound with equal and opposite optical rotation is formed by performing the same operation on — -threose. These two compounds are enantiomers. In order for that to be true, the relative orientation of the hydroxyl groups in threose must be anti. The same reasoning can be used in the opposite direction reduction. Nowadays we tend to use syn and anti instead]. Great post. It reminds us that all tetrahedral stereochemistry must be defined relative to something. How will we name it giving priority to -OH or -NH2?
A carbon connected to both -OH and -NH2 would not be very stable an aminal. Question, what if you flip the Fischer projection of the molecule, phenylalanine for instance. The rule is you must have the most oxidized carbon at the top of the Fischer. So the Fischer must be drawn with the carboxylic acid at the top. D- and L- still applies for phenylalanine. May I know if my understanding below correct? I assign priority 1 to NH2 and 4 to H. I do not sum up the Ar of the atoms when assigning priority.
For the amino acid threonine, would the D or L be determined by the position of the hydroxyl group rather than the amino group on the second chiral center farther from the carboxylate?
Your email address will not be published. Save my name, email, and website in this browser for the next time I comment. Notify me via e-mail if anyone answers my comment. This site uses Akismet to reduce spam. Learn how your comment data is processed. Next What is Mutarotation? The enantiomer , L-glucose can still be prepared synthetically:. Notice that the absolute configuration of all the chiral centers are inverted and therefore, these isomers are enantiomers.
Remember — D and L isomers are enantiomers! All the chiral centers are inverted when switching from D to L configuration and vice versa. There are hundreds of amino acids, however, we will discuss the stereochemistry of only 20 of them.
And it is because these 20 amino acids can be found in peptides and proteins of humans and other mammals. Amino acids are also characterized by the D and L notation and just like there is a trend of carbohydrates naturally occurring in D form, amino acids also have preferred stereochemistry. Except for glycine, which is achiral, all of them are L amino acids.
Interestingly, 18 out of these 19 amino acids have an S configuration and only Cysteine , being an L amino acid, happens to have an R configuration :.
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