1
$\begingroup$

When papers or articles say that all proteinogenic amino acids are 'levorotatory' or 'L', they often make a point of saying that only half of them are truly, optically levorotatory. All of them (except glycine) are called 'L' because of their 'opposing' configuration to glyceraldehyde, the original molecule deemed to be dextrorotatory...

Well, all sugars used by living things are supposedly 'R', or so I keep reading....

Are they similar in configuration to glyceraldehyde, or actually dextrorotatory to light?

EDIT: What I really wanted to know, in posting this question, is whether less than half, or most, or all organic sugars and carbohydrates are ACTUALLY optically active, in the sense of lowercase d or +,....

Remember, I noted that only 10 of 20 proteinogenic amino acids are actually levorotatory to polarized light....

$\endgroup$
3
  • 2
    $\begingroup$ Levorotatory is lower case "l" or a minus sign, not "L". All natural amino acids, save, glycine, are of the Fischer/Rosanoff designation L. Dextrorotatory means "d or +". Most, if not all, natural sugars are of the Fischer/Rosanoff designation D. The carbon that gives this assignment happens to be R. D and L are configurations; d and l are optical rotations. Search "carbohydrate configurations" in Search on Chemistry at the top of the page. $\endgroup$
    – user55119
    Nov 16, 2020 at 22:51
  • 1
    $\begingroup$ So, the answer to your question is "yes". Dextrorotatory sugars are "d" or "+". But a sugar has no chirality. Chirality is not a property of a molecule. Only carbon atoms have a chirality, and this chirality is R or S. $\endgroup$
    – Maurice
    Nov 17, 2020 at 10:18
  • 2
    $\begingroup$ I would say chirality is a property of the molecule rather than just a specific center. A molecule is chiral if it has no improper rotation symmetry elements, which can be caused by point center of chirality like a carbon atom, but also from axial chirality (e.g. a helical structure). I guess you mean we can't assign R or S labels to a whole molecule, but I would refer to these labels as "configurations" rather than "chiralities". $\endgroup$
    – Tyberius
    Dec 3, 2021 at 17:42

1 Answer 1

7
$\begingroup$

Dextrorotatory = d = (+). Levorotatory = l = (-). These terms describe the absolute sense of rotation in an optically active substance.

D and L are older terms for the sense of chirality that can be traced back to dextro- or levorotation of glyeraldehyde. The actual compound may have the same or opposite sense of rotation, but the assignment is determined by analogy. (Cf. Fisher/Rosanoff).

R and S describe absolute stereochemistry of a center, so a specific molecule with multiple chiral centers cannot be sufficiently described with a single R or S, but require one for each stereocenter. This designation is part of the more modern (and widely accepted) Cahn-Ingold-Prelog system, which has largely been adopted by IUPAC.

Note that while D and L might be sufficient to describe a sugar, anything more complicated than a 3-carbon aldose or 4-carbon ketose is not likely to be properly described with a single R or S because there are multiple stereocenters.

$\endgroup$

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.