Why do sugars like to cyclize in solution, thermodynamically speaking?
December 12, 2007 6:31 AM   Subscribe

What is the correct thermodynamic explanation for the fact that all monosaccharides with 5 or more backbone carbon atoms occur predominately in their cyclic form in solution?
posted by perissodactyl to Science & Nature (5 answers total) 4 users marked this as a favorite
Is this a homework question?
posted by grouse at 7:02 AM on December 12, 2007

Response by poster: Nope. Reviewing for a final exam and I can't seem to dig up an answer I trust. It seems to me that one part of the answer is that free energy is going to be lower when there are more bonds (as in the cyclic form). But I'm wondering if there are other factors I'm not considering.
posted by perissodactyl at 7:14 AM on December 12, 2007

Response by poster: Specifically, I'm wondering how to explain this behavior in terms of entropy.

The five-carbon lower bound is easy to understand... fewer carbons and you've got too much ring strain for the cyclic form to be favored.
posted by perissodactyl at 7:18 AM on December 12, 2007

Well, entropy and free energy are usually (always?) in opposition and entropy usually loses. If the free energy of a ring is lower than the free energy of the linear form, it will beat the lower entropy of a constrained ring (fewer degrees of freedom to flap around = lower entropy).

Gibbs tells us that every system seeks to minimize its free energy, not necessarily maximize its entropy.

biochemist, not physical chemist - for more detail I refer you to your local P Chem department
posted by Quietgal at 7:31 AM on December 12, 2007

Water likes to be next to water. A ring creates the lowest entropy stable boundary between the polyS and the water that does not require continual conformational transactions to adapt. It's a minimum within the solution space of all possible configurations of that molecule within that environment. It's the same reason why hydrophobic residues in complex proteins will tend to seek the centre of a spheroid protein macromolecule.

This link goes into more details about solute interaction than I needed to know.
posted by meehawl at 12:37 PM on December 12, 2007

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