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

Question

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.

Answer 1

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)

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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 ;)