Electron diffraction study on single crystals of l-type and dl-type lecithins

Abstract

An electron diffraction study was carried out on thin single micro-crystals of l-type and dl-type dipalmitoyl lecithins grown in xylene suspensions and fine net patterns were obtained and the mechanism of the thermotropic phase transitions of them was clarified. From the apparent structure of diffraction patterns in low temperature, it is confirmed that the two dimensional lattices have p mm symmetry in l-type and in dl-type lecithins. Lattice parameters from the [ 001] projection are d 100 = 9.9 A ̊ and d 010 = 8.8 A ̊ in l-type, and d 100 = 17.2 A ̊ and d 010 = 8.9 A ̊ in dl-type. With anisotropic variation of dimensions along a and b axes, i.e. contraction for a and expansion for b, induced by temperature rise by electron irradiation during the observation, these diffraction patterns of the lattices of l-type and dl-type were transformed into those characterized by the six diffraction spots having nearly the same spacings. Four of them are observed on slightly outer and two are slightly inner positions as compared with their mean spacings of about (4.1 Å) −1 in l-type and about (4.2 Å) −1 in dl-type. The changes in the patterns observed indicate that at low temperatures the hydrocarbon chains are nearly perpendicular to the layer in dl-type lipid, and tilted with a more complicated packing in l-type ones. The dimension along a in dl-type is twice as large as that in l-type.