Polarised absorption spectroscopy of chlorophyll-lipid membranes

Abstract

A technique has been developed for measuring visible absorption spectra of chlorophyll in lipid membranes. An expression is derived which enables the directions of the transition moments of the different absorption bands to be determined from polarisation data. It is found that the transition moments of the principal blue and red absorption bands of chlorophyll a make angles of 26° and 36.5° respectively with the plane of the membrane. On the assumption that these two transitions lie in the plane of the porphyrin ring and are mutually perpendicular, it may be deduced that the plane of the porphyrin ring is tilted at approx. 48° to the membrane surface. For chlorophyll b the transition moments of the blue and red bands are found to make angles of 29.5° and 36.5° with the plane of the bilayer, giving an angle of tilt of the porphyrin ring of approx. 51°. These results are compared with measurements of dichroism in vivo.