alpha tocopherol; deuterium; phosphatidylcholine; artificial membrane; biological model; conformation; methodology; nuclear magnetic resonance spectroscopy; solution and solubility; Deuterium; Magnetic Resonance Spectroscopy; Membranes, Artificial; Models, Biological; Molecular Conformation; Phosphatidylcholines; Support, Non-U.S. Gov't; Vitamin E
Nuclear magnetic resonance has been applied to study the conformational dynamics of α-tocopherol (vitamin E) in solution and in model membranes. In nonviscous solution, 1H nuclear magnetic resonance (NMR) showed that α-tocopherol is in rapid equilibrium between two or more puckered conformers of its heterocyclic ring. The most likely conformers to be so involved are the two half-chair forms. Deuterium NMR spectra of specifically deuteriated α-tocopherol in multilamellar dispersions of egg phosphatidylcholine, measured in the liquid-crystalline state, were characteristic of axially symmetric motional averaging. The orientation of the rotational axis within the molecular framework was determined. Studies on oriented multilamellar membranes revealed that this axis is perpendicular to the surface of the membrane. The profile of quadrupolar splittings along the hydrophobic tail does not have a plateau, in contrast to that of the fatty acyl chains of the membrane lipids. Longitudinal relaxtion times (T1) were short. The presence of a minimum in their temperature dependence shows that molecular motion with an effective correlation time Teff ≈ 3 × 10-9 s is responsible for relaxation. However, the temperatures and absolute values of the minima depend on the position of the deuterium in the molecule, demonstrating that Teff represents a complex blend of motions.