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In the chapter on Biomolecules in my book, the following points are given regarding carbohydrates.

Numbering the C-chain with consecutive rank

The C-atom present in the aldehyde group of aldoses and the terminal C-atom nearest to the keto-group of the ketoses are numbered as the first carbon or C1.

D- and L- Configuration

i) The D- and L- symbols refer to the configuration of monosaccharides. These do not indicate the dextrorotatory or laevorotatory specific rotations.

ii) In case of a monosaccharide, if the asymmetric C-atom of the highest rank is similar to the asymmetric carbon of D-glyceraldehyde (H-atom on the right,-OH group on the left) then the monosaccharide is of D-configuration. If the H-atom is on the left and -OH group is on the right, then the monosaccharide is of L-configuration.

iii) As per convention, if in a monosaccharide unit (expressed by Fischer projection formula), the -OH group falls on the right of the highest ranked asymmetric carbon, then the monosaccharide has D-configuration and if the hydroxyl group falls on the left, it has L-configuration.

Use of + and - sign

D- and L- symbols are followed by '+' and '-' in brackets. These repesent the dextro- and laevo- specific rotation. There is no relation between optical activity and nature of rotation. Example: Both glucose and fructose have D-configuration, but their optical activities are positive (dextrorotatory) and negative (laevorotatory) respectively.

Points ii and iii seem to be contradictory to me. Can someone please shed some light on this. I noticed that point iii is talking about Fischer projection. May be point ii is talking about some about mode of representation. Can someone please explain the paragraph under Use of + and - sign

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It's really strange and contradictory info, I don't know what the ii paragraph is talking about. Referencing the source as “my book” makes is hard to verify and e.g. check if there is some illustratory graphics.

To the last paragraph: The cited information would lead to e.g. “D(+)-glucose”, which is not consistent with typographical details of IUPAC nomenclature of carbohydrates (or IUPAC nomenclature of organic chemistry). The correct name would be (+)-ᴅ-glucose.[1]

2-Carb-4.5.  Optical rotation

If the sign of the optical rotation under specified conditions is to be indicated, this is done by adding (+)- or (−)- before the configurational prefix. Racemic forms are indicated by (±)-.

Examples:

ᴅ-Glucose or (+)-ᴅ-glucose
ᴅ-Fructose or (−)-ᴅ-fructose
ᴅʟ-Glucose or (±)-glucose

Fructose is an example of laevorotatory ᴅ- sugar (therefore is also called levulose).

The stereochemical configuration (especially when it is of one of many chiral atoms, and in sugars, sometimes it's the one that forms the acetal group of the major cyclic form, in some sugars not) is not directly related to the physical optical rotation.

References:

  1. Nomenclature of Carbohydrates (Recommendations 1996), 4.5. Optical rotation
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  • $\begingroup$ The '+' or '-' sign is written after 'D-' or 'L-' as per my knowledge. All the articles I have read online follow this. $\endgroup$ – MrAP Jan 31 at 10:15
  • $\begingroup$ @MrAP Yes, the mentioned notation is used quite often, though it's not the recommended one. $\endgroup$ – mykhal Jan 31 at 10:20
  • $\begingroup$ In the last paragraph, I was facing problem in understanding this line: "There is no relation between optical activity and nature of rotation". $\endgroup$ – MrAP Jan 31 at 10:23
  • $\begingroup$ @MrAP I tried to address that – there are many chiral atoms in sugars, but the D/L notation is related to single one only. You did not mention if you are aware of "optical rotation" term, it's a physical unit, rotation of polarized light passing through the sample. $\endgroup$ – mykhal Jan 31 at 10:38
  • $\begingroup$ For background, chiral molecules affect plane-polarized light such that the angle of polarization increases or decreases from the reference point. Historically, this was described as rightward or leftward rotation. From the Latin words for right ("dexter") and left ("laevus") we get dextrorotary (D) and laevorotary (L). But when carbohyrate structures were first being elucidated, there was no way to tell which enantiomer went with which rotation, so conventions were established to designate enantiomers as D or L without knowing the actual rotation associated with each. $\endgroup$ – Andrew Jan 31 at 13:04

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