ID #1011

If you did not know a linear structure was cellulose, how could you identify that it is mimicing a 3D rotation? What are the clues to know it is an alpha or beta?

Remember the conventions by which we draw and label monosaccharides. Even if you don't know it's glucose, if there is a #6 carbon that is not part of the ring structure, by convention we're going to draw this above the ring. So if one is drawn below the ring, that almost certainly means the molecule was drawn upside down to demonstrate a rotation in 3-D space. Now with this same molecule, if you're looking to see if all the glycosidic linkages are alpha or beta, you need to remember what you just learned about the monomers of this molecule -- some of them have been rotated 180 degrees. So what might initially appear to be an alpha-1,4-glycosidic linkage is actually a beta, because that hydrogen will be on the bottom of the ring structure when you (mentally) turn the glucose right side up.

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