A couple of posts ago Steve from the excellent Breadcetera ** site asked if the structures in the link…
http://www.cheng.cam.ac.uk/research/groups/polymer/RMP/nitin/Starchstructure.html were incorrect.
SB “If I’m not mistaken, the structures for the 1,4-glycosidic and 1,6-glycosidic linkages shown here are incorrect. There appear to be extraneous carbon atoms on either side of the oxygen atom of the linkage”.
The use of the projection used in the Cheng link, and below, to describe the structure of polysaccharides makes the interesting, and as a I think about more after Breadcetera’s prompting, rather glib, assumption that folks won’t think that the corner between C1 and the glycosidic O and then the corner between the O atom and the next C4 are not actually C atoms, as the corners in the ring structure infer just that; those corners ARE meant to represent C atoms.
Look at my freehand version of a section of starch acetate to see.
The representations here at http://www.chemistryexplained.com [ http://www.chemistryexplained.com/Co-Di/Disaccharides.html ] might clarify this issue. The best one to look at with respect to starch is the maltose molecule in Figure 2. In the chemistry explained version all of the H and O atoms are also indicated
So this changes the above shorthand representation to this.
Hope this clarifies – Cheers, Andrew
**Breadcetera has also been named as one of the 50 Best Blogs for a Complete Culinary Education. Where they note that “Steve is an organic chemist turned bread baker, so you know he gets it right“.
OSU
From Steve Brandt at Breadcetera… cleaning up my act. Of course Steve is correct, there should be -OH [hydroxyl] groups at the end of the bonds represented emanating form C2 and C3 [at the front of the projection]. To see the full structure the link at chemistryexplained shown in the body of the post is illustrative.
Steve’s edited message.
“Accurately representing a chemical structure can often be an exacting task, especially when one needs to deal with the intricacies of stereochemistry. Because accurate molecular representations are fundamental to what we as chemists attempt to communicate, I felt compelled to pick one additional nit. I greatly appreciate your indulgence.
In the starch acetate structures you drew in your latest post, the hydroxyl groups were omitted from C2 and C3 of some of the saccharide moieties. While I understand that those hydroxyl groups are meant to be implicit, those who might just be learning to read and understand structures of this type might infer that those are methyl groups rather than hydroxyl groups.
With best regards,
Steve”
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 theses 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.
* “Food: the Chemistry of its Components” 5th edition, publ. Royal Society of Chemistry, 2009. Perhaps not well known on your side of the pond
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.