Solid-state NMR spectroscopy uncovers internal dynamics of antiviral compounds.

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In this study we analyzed the internal dynamics of antiviral compounds such as amantadine and rimantadine and the model compound hexamethylbenzene (HMB) using solid-state 2H NMR spectroscopy. Amantadine and rimantadine have been successfully used to treat influenza and dyskinesia, and their antiviral activity can be characterized using structural and dynamical information from solid-state 2H NMR relaxation. The relaxation data collected for a range of temperatures can be fit using analytical models for the internal dynamics of these compounds to yield physical parameters such as activation energies and diffusion constants that quantify the different modes of motion. Here, we performed quadrupolar-echo and relaxation experiments on powdered samples of deuterated amantadine, rimantadine, and HMB for temperatures between −150⁰C and 60⁰C. HMB displayed two rotational modes about its internal symmetry axes with vastly different timescales. Quadrupolar-echo spectra showed that rimantadine exhibits methyl rotation about its 3-fold axis with no rotation about the C-C axis linked to the cage, while amantadine undergoes slow rotation about the cage axis. The order parameters calculated from the residual quadrupolar couplings for all three compounds were large (>0.8), indicating restricted off-axial dynamics. Spin-lattice (T1Z) and quadrupolar-order relaxation times (T1Q) were calculated for different orientations within the polycrystalline samples. The temperature and angular dependences of these relaxation times were fit using three models: anisotropic rotational diffusion [1,2], axial diffusion, and discrete jumps, among which the anisotropic rotational diffusion model displayed universal applicability. Application of NMR relaxometry to these drugs within the membrane channel will further enable us to understand their activity and the effect of the lipid environment on their structural dynamics.