Abstract
The predominantly populated conformation of carbohydrates in solution does not necessarily represent the biologically active species; rather, any conformer accessible without too large an energy penalty may be present in a biological pathway. Thus, the conformational preferences of a naphthyl xyloside, which initiates in vivo synthesis of antiproliferative glycosaminoglycans, have been studied by using NMR spectroscopy in a variety of solvents. Equilibria comprising the conformations (4)C1, (2)SO and (1)C4 were found, with a strong dependence on the hydrogen bonding ability of the solvent. Studies of fluorinated analogues revealed a direct hydrogen bond from the hydroxyl group at C2 to the fluorine atom at C4 by a (1h)JF4,HO2 coupling. Hydrogen bond directionality was further established via comparisons of fluorinated levoglucosan molecules.
Highlights
In recent years the impact of carbohydrate ring flexibility on biological systems has become even more apparent
Quantum mechanical calculations by Rovira and co-workers describe free energy landscapes for the ring conformations of gluco-2 and mannopyranosides,[3] displaying local minima for conformers not detectable in solution, but found in enzyme–substrate complexes. An interpretation of this is that the predominantly populated conformation in solution does not necessarily represent the biologically active species; instead, a conformer accessible without too large an energy penalty can exist in a biological pathway
In solvents with a good capability of accepting hydrogen bonds, a slight positive slope of the amount of 4C1 versus the dielectric constant, εr, is observed with the 4C1 population decreasing from 95% to 86% when εr is lowered
Summary
In recent years the impact of carbohydrate ring flexibility on biological systems has become even more apparent. Quantum mechanical calculations by Rovira and co-workers describe free energy landscapes for the ring conformations of gluco-2 and mannopyranosides,[3] displaying local minima for conformers not detectable in solution, but found in enzyme–substrate complexes. An interpretation of this is that the predominantly populated conformation in solution does not necessarily represent the biologically active species; instead, a conformer accessible without too large an energy penalty can exist in a biological pathway. It was further established that the suggested conformers made the molecule at hand more apt to undergo reaction, e.g., increasing the tendency for aglycon dissociation by increasing the C1–O1 bond distance in mannose
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