Orientation of the porphyrin ring in artificial chlorophyll membranes.

A sensitive photometric method is described by which the dichroism of lipid bilayer membranes in aqueous phase can be measured. The method is applied to black films with incorporated chlorophylla andb. With chlorophylla a relatively large dichroism is found in the Soret band and a much weaker dichroism in the red band. From the experimental data, the angles β B and β R between the "blue" and "red" transition moments and the membrane can be obtained. β B and β R are then used to calculate the angle γ of the porphyrin ring with respect to the membrane surface. For chlorophylla and three different lipids, values of γ between 44 and 49° are found.