Use of CP/MAS solid-state NMR for the characterization of solvate molecules within estradiol crystal forms

To investigate the solvate molecules within estradiol crystal forms, four crystal forms of estradiol (EM, ET, EP and EC) were obtained by recrystallization from various organic solvents such as methanol, ethanol, isopropanol and acetone, and their physicochemical properties were characterized using XRD, TGA, DSC and solid-state NMR. The XRD patterns showed that the estradiol crystal forms were composed of high crystallinity and their degrees of crystallinity were nearly identical. The solvent phases and solvate molecules within the estradiol crystal forms were detected by thermal analysis and the solvate molecular structures were identified by further observation using solid-state NMR. Estradiol typically tends to crystallize in form of its hemihydrate by the solvate molecules from the mother solvents used for recrystallization. The residual solvent might be incorporated into the estradiol molecules to change the local chemical environment and cause the observed 13C CP/MAS NMR spectral changes. These results suggest that the thermal analysis and CP/MAS solid-state NMR spectroscopy could be useful at the dried solid state to characterize the solvate molecule within the recrystallized estradiols.