Continuing from where we left off introducing carbohydrates and kicking it off with MONOSACCHARIDES :glucose and fructose, we venture deeper into the realm of Carbohydrates.

NOMENCLATURE OF SUGARS

D vs L Designation : ENANTIOMERS

Monosaccharide sugars are further categorized depending on their structure, in which they display stereoisomerism.

Looking at the structures of glucose and fructose, D&L designation is based on the configuration about the chiral/asymmetric carbon .

D-and-L-configurations-of-glucose

if more than one chiral center exists, D& L designation refers to the chiral center ( carbon in this case) furthest from carbonyl group.
if the -OH group is on the left on the furthest chiral carbon – L-designation

if – OH group is on the right on furthest chiral carbon – R-designation

NOTE: in the image on the left, chiral carbons are denoted by * and -OH in question is labelled.

dl-fructose_num-svg

NOTE: – Observe, D & L designations are displays of stereoisomerism , the moleules are mirror images of each other, therefore

groups on the RHS of one molecule is switched to the LHS in the mirror image , and vice versa, not only the hydroxyl ( -OH) which determines D&L designation.

D & L designations are ENANTIOMERS of each other

******************************************************* D******D ********************

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**** ***************************************************************************************************************************************************After the sugars have been classified as D or L configuration, they differ once again as epimers, another form of optical isomerism********************

EPIMERS :

two sugars that differ only in configuration around one C atom. Simply put 🙂

Pentose and Ketose sugars can also cyclize from their straight chain to form Haworth projections.

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D-glucose cyclizes to give β-glucopyranose ring

D-glucose cyclizes : molecule bends so that C1 (anomeric carbon) reacts with C5, thus forming a ring structure
This results, forming a ring structure, for glucose. ——–>hemiacetal (in red) pyranose – 6 membered ring

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<———OR similarly for fructose, forming a hemiketal(show in red)furanose-5 membered ring

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