Although in oligo- and polysaccharides, fructose is always in the furanose form

Question

Although in oligo- and polysaccharides, fructose is always in the furanose form and it is the furanose form that is usually drawn in textbooks, in aqueous solution the predominant form of monomeric fructose is actually the pyranose form. In fact, the fructose in honey is mainly D-fructopyranose, which is actually much sweeter than the furanose form.

Thus the two forms of fructose are: a-D-fructopyranose a-D-fructofuranose

Question: If the standard free energy change for the conversion of fructopyranose to fructofuranose is 1.7 kJ/mol, what fraction of the total fructose in solution is fructopyranose?

Explanation / Answer

G=-RTlnk

given G=1.7kj/mol

k=[product]/[substrate]=[fructofuranose]/[fructopyranose]

using this, and taking T=25 degree C=298 K, we get,

k=0.50357

[fructofuranose]/[fructopyranose]=0.50357

so, {[fructofuranose]/[fructopyranose]}+1=1.50357=[total fructose soln]/[fructopyranose]

fraction of fructopyranose=1/1.50357=0.665

hope this helps! :)

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