PHASE TRANSITION AND CH3-ROTATION IN SOLID CHOLESTEROL

The behaviour of solid cholesterol over a wide temperature range has been studied by NMR, calorimetry and dilatometry. Between - 190 and - 20°C the temperature dependence of the spin-lattice relaxation time is fully determined by rotation of the methyl groups. The activation energy for this rotation, calculated from the measurements, is 2.0 kcal/mole, which is somewhat lower than the corresponding values found for alkanes. In the temperature range between + 35 and 40°C a reversible endothermic transition (already reported in the literature) occurs which is characterized by a low transition enthalpy (0.7 kcal/mole), a decrease in the molar volume (5 ml/mole) and an increase in the spin-lattice relaxation time (13%). NMR measurements of cholesterol adsorbed on silica indicate that these effects rely on a change in the crystal structure and not on intra-molecular transitions (conformation isomerism). It is suggested that the polymorphous transition is related to a change in the packing of the terminal CH3-groups of the aliphatic side chain. © 1968.