NMR Studies of H2O:N-Phenyl (N-Phenyl-β-D-Glucopyranosylamine) Urokamide Interactions in Dimethyl Sulfoxide at two Fields

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Abstract
During the course of NMR structural investigations of the title compound (N-phenyl uronamide) we noticed the presence of a strong H2O activity-dependent NOE between the small amount of H2O associated with the carbohydrate's hydroxyl protons in solutions of DMSO-d6 . The -OH/H2O off-diagonal t1 slices displayed a NOESY τm-dependence similar to molecules (τc = 0.1–5 ns) experiencing slow exchange (1–10 s−1). From T1 data at two fields a τc for the title compound was calculated to be ca. 0.54 ns at 313 K. The -OH/H2O exchange rate constant, K, increased from 0.32 to 11.14 s−1 as the molar ratio of [H2O]:[N-phenyl uronamide] increased from ca. 4.5 to 5.2. The latter finding indicated that the -OH/H2O proton exchange process, which is strongly affected by the translational diffusion of H2O, diminished as [H2O] approached that which was inherently a complex in the crystalline structure (e.g., C18H20O5N2·4H2O) and was, presumably, tightly hydrogen-bound to the -OH/NH funtional groups. To test this, the title compound was recrystallized from ethanol/2, 2-dimethoxypropane whereupon the bound H2O was eliminated; the lack of H2O induced a significant upfield shift in the resonance frequencies of all the exchangable (-OH Δδ=86.33Hz; -NH Δδ=67.84Hz) functional groups relative to the methine protons (CH Δδ=0.25Hz).