Linear dichroism of bimolecular chlorophyll-lipid membranes. The role of anisotropy and dispersion.

Polarised absorption and reflection spectra of chlorophyll-containing bimolecular lipid membranes were obtained in the spectral range of 590--710 nm. The spectra were analysed using the formalism of the complex dielectric tensor which characterizes the optical anisotropy of the membrane and the light absorption therein. The maxima of the absorption spectra recorded at a 45 degree angle of incidence are located at 665 and 670 nm for light in which the electric vector is oriented parallel and perpendicular, respectively, to the plane of incidence. The analysis of these spectra shows that the spectral shift is wholly due to the dispersion of the real part of the dielectric tensor. The angle between the dipole transition moment in the red and the normal to the membrane was estimated to be 42.3--45.3 degrees. On the basis of these results, a model absorption spectrum, simulating the dichroic properties of oriented chloroplasts, was calculated for a system of parallel membranes. Some of the possible artifacts introduced into the dichroic spectra of chloroplasts due to anisotropy and dispersion are discussed.