Sugar molecules chiral" pairs

Individual pairs of enantiomers are called threo or erythro. The threo form has substituents on opposite sides of the main backbone chain represented as Fischer projection, whereas the erythro form has them on the same side, respectively. These terms function correctly in case of linear sugar molecules, but in other cases there are often problems with determining what is actually the main chain [53]. In such cases, individual chiral carbon atoms can always be characterized using configurational notation (R and S). 

Sugar chemistry has an added complexity. The carbon backbone of a linear monosaccaride is made up of linked chiral centres. For example, hexoses have four chiral carbons resulting in eight D- and L-stereoisomers of which seven pairs are known in biology. Upon cyclization a further stereocentre is formed which leads to a and (-> anomeric forms as shown below. Although they are distinct stereochem-ically, if the hexose is cleaved and reformed the new ring may have the same or opposite stereochemistry. As this process occurs naturally and quite rapidly in water the chirality is therefore not a permanent feature of the molecule. 

If additional chiral centers exist in the molecule and remain unchanged during this conversion, the pair of compounds with a mixture of R and S configurations at a single site would be related as diastereomers (more specifically, epimers) rather than enantiomers, so such a process would be described as an epimerization rather than a racemization. For example, sugars can undergo epimerization at the anomeric carbon via a resonance-stabilized carbocation intermediate. 

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