Studies in light field of the mesomorphic state of some biological membrane models

The paper presents studies under light beams of some systems which model the biological membrane (BM), namely fatty acids (FAs) and mixtures of fatty acids with cholesterol (Ch), and surfactant systems. Under a weak external electric field, the systems behave as weak dielectrics, with a hysteretic evolution of the current versus the applied voltage, and a negative resistance, which suggest a liquid crystal state. The microscopic aspect of both the systems in water showed a self-assembly of the molecules as a double layer, similar with the membranes structure. The hydrophobic part presents a lyotropic liquid crystal texture that changes in terms of the concentration. Some of the samples exhibit also a thermotropic mesomorphism, that is much influenced by adding cholesterol. Maintained under integral and polarized light, the textures and their colors change in a few seconds. In a low power cw laser beam, the light spreading has been evidenced and some non-linear optical effects were observed. The results seem to lead to the necessity to study both the lyotropic and thermotropic mesomorphism together and in interaction with different external stimuli, among them the light being an important tool for biological investigations.