Structure and dynamics of alpha-tocopherol in model membranes and in solution: a broad-line and high-resolution NMR study.

Abstract

Nuclear magnetic resonance has been applied to study the conformational dynamics of alpha-tocopherol (vitamin E) in solution and in model membranes. In nonviscous solution, 1H nuclear magnetic resonance (NMR) showed that alpha-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 alpha-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 relaxation times (T1) were short. The presence of a minimum in their temperature dependence shows that molecular motion with an effective correlation time tau eff approximately equal to 3 X 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 tau eff represents a complex blend of motions.