There are thousands of amino acids, however, us will discuss the stereochemistry of only 20 of them. And also it is due to the fact that these 20 amino acids can be discovered in peptides and proteins the humans and also other mammals.

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Amino mountain are also characterized by the D and also L notation and just favor there is a tendency of carbohydrates naturally developing in D form, amino acids also have wanted stereochemistry. Except for glycine, i beg your pardon is achiral, every one of them space L amino acids.




Interestingly, 18 the end of this 19 amino acids have an S configuration and only Cysteine, gift an l amino acid, happens to have actually an R configuration:



The factor for this exception is the truth that in Cysteine, there is a sulfur connected to the carbon on the stereogenic center, and also because the its greater atomic number, it bring away the priority end the COOH group which go not occur in various other amino acids. 

 Are D and also L always Enantiomers?

I perform want to carry up second note about the relationship that D and also L isomers. Yes, we declared that D and also L isomers room enantiomers due to the fact that all the chiral centers have actually opposite configuration. This is true, however, when managing cyclic develops of sugars, you have to keep in mental epimers which room diastereomers that differ in the configuration of just one chiral center. And if these diastereomers are cyclic hemiacetals favor sugars are, climate they space classified as anomers. One example is the relationship in between ɑ-D-glucose and β-D-glucose:




This, however, fits here much more as a side note and also we will comment on the details around epimers and anomers in a separate post.



D and L vs (+) and also (-)

Even despite the “D” notation was initially used together an abbreviation come dextrorotatory (turning the airplane of polarized light clockwise) since D-glucose is in fact dextrorotatory, it was discovered later the not all D sugars space dextrorotatory. For, example, D-Erythrose rotates the aircraft of polarized irradiate counterclockwise and therefore, the is levorotatory.




Therefore, D and also L room not related to the optical rotation and also the direction of rotation is given by the (+) and also (-) signs or through the lowercase (d) and also (l).

So, remember:

Just like the (R) and (S) designations, the D and also L notation is no necessarily concerned the optical rotation. There deserve to be a street which is D-(+) and one that is D-(-). Exact same for the together isomers.Don’t confuse this through the lowercase d and also l notation together these do stand for dextrorotatory (+) and levorotatory (-).


Why carry out we use D and L rather of R and also S?

The main advantage of utilizing the D and L notation is the brevity. For example, using D-glucose is much easier than (2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanal and transforming each the the R and S designation because that the enantiomer is no an efficient approach either – L-glucose sounds better than (2S,3R,4S,5S)-2,3,4,5,6-pentahydroxyhexanal.

An alternative means of compressing the construction of every the chiral centers in one symbol is the (+) and also (-) indicators of the optical rotation. The problem, however, is that the optical rotation deserve to vary depending upon the temperature, solvent, and also the light source.

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Now, speaking of carbohydrates with multiple chiral centers: we will certainly go over the key examples, classify and also name lock in the following article “Aldoses and Ketoses” together there is as well much brand-new information there.