How to Understand the Relation between the R/S System and the D/L System

It may sometimes look like there exists a correlation between the R/S system and the D/L system.


However, the R/S system (absolute configuration) is NOT directly related to the D/L system (relative configuration).

Let’s recall that the optical rotation is NOT directly related to the absolute configuration either. How to understand the relation between the optical rotation and the absolute configuration? Click HERE!

The side-chain of Serine has a hydroxyl group, −OH. If a thiol group, −SH, would substitute it to form Cysteine, the D/L labeling would not be affected by the swapping. However, this replacement would invert the molecule's R/S labeling, because the CIP (Cahn-Ingold-Prelog) priority of CH2OH is lower than that of COOH but the CIP priority of CH2SH is higher than that of COOH. Therefore, D- or L- does not necessarily mean R- or S- respectively!

Since it is convenient to have the same label for the commonly occurring structures of a specific kind of molecule in higher organisms, the D/L system is generally used in certain areas of biochemistry, viz., amino acid and carbohydrate chemistry. In the D/L system, naturally occurring carbohydrates are nearly all D- while naturally occurring amino acids are nearly all L-. All proteinogenic amino acids are S-, except for Cysteine, which is R-.



Click/tap to enlarge the image

Dexter/Laevus versus Rectus/Sinister - Sounds sinister❓ Let Chemaficionado know in the comments below or at mychemistryhomework@gmail.com❗

References:

(1)    Wikipedia Contributors. Cahn–Ingold–Prelog priority rules. Wikipedia. https://en.wikipedia.org/wiki/Cahn%E2%80%93Ingold%E2%80%93Prelog_priority_rule

s.

(2)    Wikipedia Contributors. Absolute configuration. Wikipedia. https://en.wikipedia.org/wiki/Absolute_configuration.

(3)    Brown, C.; Ford, M. Higher Level Chemistry, 2nd ed.; Pearson Education: Harlow, Essex, 2014.

(4)    Ncert. Chemistry : Textbook for Class XI - Part.II; National Council Of Educational Research And Training: New Delhi.

(5)    Graham, T. W.; Fryhle, C. B.; Snyder, S. A. Organic Chemistry.; Hoboken, Nj John Wiley Et Sons, Inc, 2016.

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