Drawing and recalling the structures of carbohydrates have been a nightmare for many biochemistry students and teachers. But another question is why do we acually need to draw then at all. The logic is simple, a cloth that we wear and the starch that we eat are made of same glucose molecule (C6H12O6), yet we can not exchange out choice, just because of a subtle difference in the position of H and OH across one carbon atom, so is the case with several other sugars. At biological level, the fate and role of different configurations of sugars are different and hence, it is important to know the correct structure to represent while studying or teaching biochemistry. This blog post highlights one of my recommendations recently published in official journal of international union of biochemistry and molecular biology. [Find Full Article Here]
Sigma Notation
The idea behind structrual notations of sugars is very simple. At present this is developed for hexoses but extrapolated to other sugars.Sigma notation is mainly a symbolic presentation or a shorthand to depict the position of H and OH across the assymetric carbon atoms.
Each notation consist of a letter sigma with a superscript of a number depicting the specific carbon atom across which OH is present on left (as per Fisher or Rosanoff projections), with an additional information like first letter of the sugar, number of carbon atoms and aldose or ketose nature.
This acts a simple memory aid to remember and draw all the structures of carbohydrates. Sigma notations are helpful in may ways like quickly recally the linear or cyclic structrues, determination of epimeric relationship or determination of RS nomenclature of a specific sugar. As hexose sugars can be mainly aldose or ketose, the two sugars have difference in the number of assymetric carbon atoms and hence steroisomers. Number of steroisomers of a sugar are represented by formula 2^n, where n is number of assymetric carbon atoms.
Recalling Aldose Structures
As Aldohexoses contain four stereocentres (carbon number, 2,3,4, and 5) hence, they have 2^4 = 16 possible stereoisomers, 8 L -isomers and 8-D-isomers. All D sugars have OH on the fifth carbon on right side while all L-sugars have OH on the left side. hence we are left with possible combinations of H and OH on carbon 2, 3 and 4 to produce eight variants. This include no OH on left (all on right) in D-allose, OH of carbon 2 in D-altrose, OH on carbon 3 on left in D-glucose, OH on carbon 4 on left on D-gulose, OH on carbon 2 and 3 in D-mannose, OH on carbon 2 and 4 on left in D-idose, OH on carbon 3 and 4 in D-galactose and OH on carbon 2, 3 and 4 in D-Tagatose.
The simple Mnemonic to remember is the increasing order of these numeric combinations which are placed over sigma as shown in figure below, and then connected with AA GG MIGT (All Alert Guys Get Mig Twenty); 0,2,3,4,23,24,34,234.
So you are ready to draw all the structures within no time. All L-sugars for each molecule will be corresponding mirror images of respective D form.
(A - Allose, A- Altrose, G- Glucose, G- Gulose, M- Mannose, I-Idose, G-Galactose, T - Tagatose)
Recalling Ketose Structures
Similarly, ketohexoses contain three stereocentres (carbon number 3,4, and 5) hence, they have 2^3 = 8 possible stereoisomers, 4 L -isomers and 4-D-isomers. All D sugars have OH on the fifth carbon on right side while all L-sugars have OH on the left side. hence we are left with possible combinations of H and OH on carbon 3 and 4 to produce four variants. This include no OH on left (all on right) in D-Psicose, OH of carbon 3 in D-fructose, OH on carbon 4 on left in D-Sorbose and OH on carbon 3 and 4 on left in D-Talose.
The simple Mnemonic to remember is the increasing order of these numeric combinations which are placed over sigma as shown in figure below, and then connected with PFST(Psycho Frankie Shot Tanya); 0,3,4,34.
So you are ready to draw all the structures within no time. All L-sugars for each molecule will be corresponding mirror images of respective D form.
In order to further draw the cyclic structures of the above molecules, all the OH placed left in Fisher projection would remain up in Haworth cyclic projection and those on right would be below the ring in Haworth projection.
Hope this notation would be useful for many students and Teachers across the globe. For using any of the content or images please seek permission from the publisher of the article.
You May Read the Complete Article and Recommendation Here .