Solid‐state nuclear magnetic resonance investigation of neurosteroid compounds and magnesium interactions

The neurosteroid trans‐dehydroandrosterone (DHEA) and its analogs with slightly different modifications in the side chain attached to C17, that is, (3S)‐acetoxypregn‐5‐en‐20‐one (1) and (3S,20R)‐acetoxypregn‐5‐en‐20‐ol (2), have been synthesized to investigate DHEA–cation interactions. In this study, we applied solid‐state 1H/13C cross‐polarization/magic‐angle spinning (CP/MAS) nuclear magnetic resonance (NMR) spectroscopy to a series of DHEA analog/Mg2+ mixtures at different Mg2+ concentrations. The high‐resolution 13C NMR spectra of 1/Mg2+ mixtures exhibit two distinct 13C spectral patterns, one attributable to 1 free from Mg2+, and the other attributable to 1 with bound Mg2+. For 2, the 13C NMR spectra exhibit three distinct spectral patterns; besides that of the free form, the other two can be assigned to Mg2+‐bound forms. Based on the analysis of the chemical shift deviations (CSDs), we conclude that both 1 and 2 might be subject to a cation–π interaction via the C5–C6 double bond, in contrast to that observed previously for DHEA. As demonstrated, DHEA possesses two Mg2+ binding sites, that is, C17–O and C5–C6 double bond, in which the binding affinity of the former is at least three times stronger than that of the latter. The solid‐state 13C NMR investigation allows better understanding of the underlying cation binding effects of neurosteroid molecules in vitro. Using solid‐state nuclear magnetic resonance spectroscopy we show that DHEA analogs are able to interact with Mg2+. The interaction with the cation induces a conformational change in the DHEA analogs. It is suggested that Mg2+ might be interacting with DHEA through the oxygen atom attached to C17 involving the cation/lone pair of electrons and the double bond π electron arising from C5–C6.