SOLUTION CONFORMATION OF SUCROSE FROM OPTICAL-ROTATION

We have calculated the optical rotation of methyl beta-D-arabinofuranoside, methyl beta-D-fructofuranoside, and sucrose as a function of ring conformation and, in the case of sucrose, of linkage conformation. Comparison with experimentally observed optical rotations allows assignments of solution conformations; our results agree with some but not all of the previously reported assignments. We conclude that the furanose ring form that predominates in aqueous solution has the same phase angle of pseudorotation for all three compounds, slightly to the northeast of the canonical northern conformer; i.e., (3)4T-4E in methyl beta-D-arabinofuranoside, (4)5T-5E in methyl beta-D-fructofuranoside, and 4E-(5)4T-5E in the beta-D-fructofuranosyl moiety of sucrose. These assignments are shown to be consistent with NMR parameters in the literature. The optical rotation of sucrose is best accounted for in terms of an equilibrium mixture of two linkage conformers, the predominant one being similar to the crystalline structure in which the O1'-O2 intramolecular hydrogen bond persists and the other representing a secondary energy minimum in which the O1'-O2 hydrogen bond is replaced with an O3'-O2 intramolecular hydrogen bond.