Why does the fructose monomer in sucrose appear different from isolated fructose?

organic-chemistry molecular-structure carbohydrates

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Ah, a (fairly) common conundrum that assails us Chemistry students when we start Biochem. ;-)

At first glance, the fructose molecule in your first picture, and that in the second picture appear to have different conformations:

Believe me, they're the same. "How?" you ask? Stay with me here (it requires a little imagination):

Draw a vertical line/axis on your first diagram along the plane of the furanose ring, connecting the oxygen atom at the apex, right down to the middle of the C3-C4 bond. Then rotate the molecule 180 degrees counter-clockwise.

Now compare this with the fructose residue in sucrose.

(I lack the necessary Org. Chem. sketching tools to depict this... apologies)

If you weren't able to wrap your head around "rotation" bit, then, let's assign locant numbers to the carbon atoms in the two diagrams and then compare them:

_{I've drawn them tiny, my bad}

Must've clicked by now ;)

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Author by

rohit_r

Updated on December 02, 2020

Comments

rohit_r almost 3 years

I understood that the representation of fructose is:

But during the formation of sucrose:

How did fructose, whose representation is given in the first picture, turn out to be different in the second. I tried to visualize myself rotating the molecule but the position of oxygen changes and it doesn't match with the one in sucrose.
- paracetamol over 6 years
  
  Hint: Draw a vertical line/axis on your first diagram, connecting the oxygen to the middle of the C3-C4 bond. Then rotate the molecule 180 degrees counter clockwise. Now compare this with the fructose residue in sucrose ;)