Molecular dynamics in polycrystalline testosterone studied by proton NMR

Area of Science:

• Solid-state chemistry
• Molecular dynamics
• Nuclear Magnetic Resonance (NMR) spectroscopy

Background:

• Testosterone (17 beta-hydroxy-4-androsten-3-one) is a crucial androgen hormone.
• Understanding its solid-state properties is essential for pharmaceutical and biochemical applications.
• Polycrystalline forms can exhibit unique molecular behaviors.

Purpose of the Study:

• To investigate the molecular dynamics of polycrystalline testosterone.
• To characterize the reorientational motions of methyl groups within the testosterone molecule.
• To determine the activation energies and pre-exponential factors associated with these motions.

Main Methods:

• Proton Nuclear Magnetic Resonance (NMR) spectroscopy was employed.
• Measurements were conducted over a wide temperature range (70 K to 428 K).
• Analysis focused on dipolar second moment and spin-lattice relaxation times at 25 MHz.

Main Results:

• A reduction in the dipolar second moment indicated molecular motion.
• Two distinct minima in spin-lattice relaxation time were observed.
• These features were attributed to the reorientation of the two methyl groups in testosterone.
• Activation energies (Ea) were determined as 6.1 ± 0.5 kJ/mol and 11.9 ± 0.9 kJ/mol.
• Pre-exponential factors (τ0) were found to be (2.3 ± 0.1) x 10^-13 s and (2.85 ± 0.2) x 10^-13 s.

Conclusions:

• Polycrystalline testosterone exhibits methyl group reorientation.
• The study quantifies the energy barriers and timescales for these molecular motions.
• NMR is a powerful tool for probing solid-state dynamics of steroid molecules.