ABSTRACT
Membranes are a highly anisotropic medium. This anisotropy stems from the regular packing of constituent molecules, each with their own anisotropy. The lipid molecules are usually elongated amphiphiles with a polar headgroup and hydrophobic tail. Clearly, any property will depend upon whether it is measured along or perpendicular to the molecular axis. This is the reason why the properties of lipid membranes are expressed as tensors rather than as single isotropic values. Phosphatidylcholines adopt in water gel- and fluid-like phases. Recently, a new metastable gel phase of dipalmitoylphosphatidylcholine (DPPC) was discovered upon prolonged incubation at low temperature. X-ray analysis revealed the formation of three-dimensional crystals of stacked bilayers in which lipid molecules are arranged in an orthorhombic lattice. Spin-label partitioning was also utilized in studies of the phase transition of the charged lipids PS and PG. The basis for observing angular rotation of the spin labels lies in the modulation of the anisotropy of magnetic interactions.
