Optical properties of artificial chlorophyll membranes

Summary

The composition and structure of lipid bilayer membranes containing chlorophylla have been studied with photometric and fluorometric methods. A sensitive double-beam spectrophotometer is described by which the pigment concentration in the bilayer can be determined. Up to 3×10¹³ chlorophyll molecules per cm² can be incorporated into the membrane, corresponding to a mean distance of 20 Å between the porphyrin rings. At high chlorophyll concentrations, the absorption peaks are shifted toward longer wavelengths, indicating an interaction between porphyrin rings in the film. Parallel to the spectral shifts, a large decrease in the fluorescence quantum yield and a depolarization of the fluorescence are observed. These findings suggest that transfer of excitation energy takes place between neighboring chlorophyll molecules in the membrane. When an oxidating agent (K₂S₂O₈) is added toone external phase, exactly half of the chlorophyll in the film is destroyed. This observation suggests that the chlorophyll molecules are localized in the membrane surfaces with the phytyl chains inserted into the hydrocarbon core of the membrane and the porphyrin rings facing the aqueous solution.