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 .
- 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.
- 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
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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 : 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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6( or 5) carbon sugar ≠ 6 membered ring structure ; Note fructose and glucose are both hexose sugars but their rings have different a number of sides.
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Futher helpful links to these topics :
http://guweb2.gonzaga.edu/faculty/cronk/CHEM198pub/L07-index.cfm
http://www.rpi.edu/dept/bcbp/molbiochem/MBWeb/mb1/part2/sugar.htm#deriv