# DL vs dl notation [closed]

As the L-isomer of glucose, it is the enantiomer of the more common D-glucose.

Source: Wikipedia

As far as I know enantiomers or Optical isomers are non superimposable mirror image structures with chiral centres and they are represented by d and l isomers, not D and L forms. Is there a difference between the capital and lower case designations?

• @DGS As D and L denote the configuration wrt the glyceraldehyde and not optical activity then why should they be considered ways for denoting enantiomers? – Tyto alba Apr 1 '16 at 14:45
• Because glyceraldehyde is chiral at the alpha-carbon, so if you branch out from there to form the amino acid or sugar you want, you'll get one or the other enantiomer. Diasteromers of glucose aren't called glucose. There is (+)- or (-)- glucose, and then there is (+)- and (-)-galactose, which is a diastereomer of glucose but not either form of glucose. – SendersReagent Apr 1 '16 at 14:54
• – Faded Giant Aug 31 '16 at 22:52

The notation d, l, and dl (for dextrorotatory and laevorotatory, respectively) is used to designate the sign of optical activity. According to Basic terminology of stereochemistry (IUPAC Recommendations 1996), this notation is obsolete and its usage is strongly discouraged. The recommended notation uses the prefixes (+), (−), and (±), respectively.

Since optical rotation is a distinguishing characteristic of enantiomers, the notation using d, l, and dl as well as the notation using (+), (−), and (±) can be used to distinguish the enantiomers of a chiral molecule. However, without further information, the absolute configuration (R or S) of the chiral molecule is indeterminable. Thus, it remains unknown whether the dextrorotatory enantiomer (d and (+), respectively) is the R enantiomer or the S enantiomer.

The notation D, L, and DL is based on the arbitrary convention according to which d-glyceraldehyde was named D-glyceraldehyde, l-glyceraldehyde was named L-glyceraldehyde, and the racemate dl-glyceraldehyde was named DL-glyceraldehyde. (Note that the stereodescriptors D, L, and DL shall actually be written in small capitals, which cannot be correctly typeset here.) The enantiomers were taken to have the absolute configuration represented by the Fischer projection.

Today, we know that D-(+)-glyceraldehyde actually is (2⁠R)-2,3-dihydroxypropanal.

D- and L- isomers vs d- and l- isomers: DL is newer, dl is old school. Both are ways of denoting enantiomers.

Sometimes they don't match up, though. Wikipedia says nine of the nineteen L-amino acids are dextrorotary, so they are "d", but their configuration is " L."

This is because lowercase deals with optical rotation (d- = (+); l- = (-) ), while D- and L- deal with configuration: "the version synthesized from naturally occurring (+)-glyceraldehyde being considered the D-form."

More here

D and L sugars are not the same as d and l sugars. These two abbreviations denote entirely different configurations. D and L denotes the position of hydroxyl group at the asymmetric carbon of a monosaccharide, whereas d and l denotes the rotation of plane polarized light i.e. d: Dextrorotatory l: levorotatory

• While not a bad answer, I don't see what it adds that isn't covered by Loong already. – Tyberius Nov 20 '17 at 18:35