Analysis and application of linear dichroism on membranes. Description of a linear-dichroism spectrometer
Linear dichroism (LD) refers to anisotropic absorbance of a system of macroscopically ordered chromophores. LD measurements based on light-modulation techniques are more sensitive and accurate than the conventional two-spectra technique. The set-up and an analysis of an LD spectrometer are reported. A method for the calibration of the instrumental LD scale has been developed. The method is based on the difference in Fresnel reflections of a tilted quartz plate for different linear polarizations of light. Lyotropic liquid-crystalline phases are frequently used as models for biological membranes. Many lamellar liquid crystals align spontaneously in thin layers between glass or quartz plates and form a macroscopically uniaxial system. LD of chromophores solubilized in such systems can be measured if the plates are tilted with respect to the analysing light beam. The interpretation of LD data on these systems may be complicated by the birefringence of the anisotropic phase and by the reflections at the various interfaces. A model taking these complications into account has been derived and is compared with a simplified model commonly used; both these models are tested experimentally.