More on sugar structural representations… Isn’t the internet wonderful?

These 2 comments were posted regarding the recent post on glycosidic bond representation.

Almost since the year dot sugar chemists have indulged themselves in bond representations with right angle bends in them and ever since I started teaching on the BSc Food science course at London South Bank University in the 1970s undergraduates have, at first, found them difficult. In the earlier editions of my food chemistry textbook* I experimented with bonds having a smooth bend but these are only a partial solution and very difficult to render neatly with packages such such as ChemDraw. The best solution is to move as quickly as possible from Tollens and Haworth structures to actual chairs. Purist organic chemists often insist that you can’t teach chairs (and boats and planes) until you’ve done many hours on the thermodynamics of cyclohexane derivatives etc. but this is nonsense. Sugar behaviour in food is very much easier to understand from a chair. “

Tom Coultate —

Dr Coultate is the Author of the excellent “* “Food: the Chemistry of its Components” 5th edition, publ. Royal Society of Chemistry, 2009”.

That comment prompted this response From SteveB – “Dr. Coultate is right on the mark when he makes the suggestion of moving quickly to the use of chair conformations to graphically represent sugar molecules. Not only do chair representations deal quite nicely with the graphically messy ‘right angle bond’, but they also better represent the steric and electronic interactions of whatever axial and equatorial functionality is present“.

I though it worthwhile then to show representations of typical chair conformations when I came across yet another fabulous and credible resource on the internet.

It is…

Structural Basis of Glycan Diversity by Carolyn R. Bertozzi and David Rabuka

in Essentials of Glycobiology 2nd Ed via The National Center for Biotechnology Information.

Editors – Richard D. CummingsJeffrey D. EskoHudson H. FreezePamela StanleyCarolyn R. BertozziGerald W. Hart& Marilynn E. Etzler.

Thankfully, as the information was created by or for the US government, the site is within the public domain, and so content is reproducible with appropriate attribution.

A great resource are the downloadable powerpoint teaching slides of all their diagrams !

So here is an example pertinent to the 2 comments reproduced above showing the conversion from the Haworth projection of β-D-glucose and in the chair conformations, also showing the predominance of the 4C1 chair where all -OH groups are equatorial and as far away from each other as possible.

chairs

“FIGURE 2.8. (a) β-D-Glucose in Haworth projection and in its 4C1 and 1C4 chair conformations; (b) envelope and twist conformations for a five-membered ring structure”.

We can now see the value of this in rendering the glycosidic bond fee of ambiguity in this example showing maltose and gentiobiose. Of course maltose is salient to our original discussion of the representation of D-glucose in starch, sharing the same α1→4 glycosidic linkage.

chairs maltose