Polymorphs of Theophylline Characterized by DNP Enhanced Solid-State NMR

Area of Science:

• Solid-state NMR spectroscopy
• Materials characterization
• Organic chemistry

Background:

• Polymorphs and solvates significantly impact the properties of organic solids.
• Characterizing these solid forms is crucial for drug development and material science.
• Dynamic Nuclear Polarization (DNP) offers enhanced sensitivity for solid-state NMR.

Purpose of the Study:

• To demonstrate the utility of DNP-enhanced solid-state NMR for characterizing organic solid polymorphs and solvates.
• To develop and present robust sample preparation protocols for DNP experiments that preserve the integrity of theophylline polymorphs.
• To enable advanced structural elucidation of theophylline forms using 2D NMR correlation experiments.

Main Methods:

• Application of Dynamic Nuclear Polarization (DNP) to solid-state Nuclear Magnetic Resonance (NMR) spectroscopy.
• Investigation of three polymorphs and one hydrated form of theophylline.
• Development of sample preparation techniques including cryogrinding, inert atmosphere grinding, and careful selection of impregnating liquids.
• Acquisition of 2D correlation NMR experiments, such as 1H-13C and 1H-15N HETCOR and 13C-13C INADEQUATE.

Main Results:

• DNP-enhanced solid-state NMR successfully characterized different theophylline polymorphs and its hydrated form.
• Standard sample preparation methods (grinding, impregnation) were found to induce undesired polymorphic transitions or desolvation.
• Optimized protocols involving cryogrinding, inert atmosphere grinding, and specific impregnating liquids prevented phase transitions.
• Advanced 2D NMR experiments were feasible at natural isotopic abundance with the optimized protocols.

Conclusions:

• DNP-enhanced solid-state NMR is a powerful technique for distinguishing and characterizing polymorphs and solvates of organic solids.
• Careful sample preparation is critical to avoid artifacts and ensure accurate structural analysis.
• The developed protocols facilitate detailed structural studies of complex organic solid systems, including pharmaceuticals.