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I had just started to read about carbohydrates when I came across D-(+)-glucose. My book says that the '+' sign refers to its optical rotation measured through the polarimeter, and that it could either be + or −, but throughout the book D-glucose is always designated with the + sign.

Is it possible for there to be D-(−)-glucose?

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For a given configuration there is a corresponding and fixed rotatory effect. However, the actual rotatory effect - that seen by a polarimeter - can either be + or −, independently of the D- or L-configuration of the stereogenic centre that shows up in the compound name (in the case of sugars, it refers to the carbon furthest away from the =O group).

In brief, D-glucose is always D-(+)-glucose.

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  • $\begingroup$ Yes, infact in the case of glucose it's always D(+) and L(-), but for some other sugars like fructose it's D(-) and L(+). $\endgroup$ – Pritt Balagopal Jul 13 '17 at 15:03
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    $\begingroup$ The naming along D (or L) is convention, based only about this peculiar $\ce{OH}$ group adjacent to the $\ce{CH2OH}$ in the Fischer projection. The rotatory effect however depends on the whole molecule. $\endgroup$ – Buttonwood Jul 13 '17 at 15:29
  • $\begingroup$ In Emil Fischer's day, there was no D,L terminology. Configuration was designated as d,l and rotation as +,-. Today they are synonymous. The D,L nomenclature evolved from the Delta,Lambda configurations of Rosanoff (1906). See, ursula.chem.yale.edu/~chem220/chem220js/STUDYAIDS/history/… $\endgroup$ – user55119 Nov 27 '17 at 21:00
  • $\begingroup$ This is general not necessary limited to sugars. The label is given by priority rules. The verse must be measured (perhaps calculated with some quantomechanical method). $\endgroup$ – Alchimista Jan 7 '18 at 16:12

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