On the orientation of chlorophyll- aI in the functional membrane of photosynthesis

Abstract

Light induced redox reactions in photosystem I in photosynthesis of green plants are indicated by absorp tion changes with maxima at 430 nm and 705 nm. These negative going absorption changes have been attributed to the rapid photooxidation of a special chlorophyll namedP 700 [l] or chlorophyll-u1 [2] . The extremely rapid oxidation of this pigment [3] contributes to the extremely rapid generation of an electric potential difference across the functional membrane of photosynthesis [4-61. Thus the reaction path of the oxidation is likely to be vectorial in the membrane. We studied the orientation of the porphyrin ring of chlorophyll-a1 in the membrane, hoping for a better understanding of the structural conditions of the electric potential generation. Several models for the location and the orientation of this pigment have been discussed in the literature, However, it is obvious that they could not be substantiated by the present resolving power of electron microscopy and X-ray scattering for photosynthetic membranes. Moreover, model studies on the orientation of chlorophylls in bimolecular lipid membranes [7, 8 J cannot be used for conclusions by analogy as to the orientation of chlorophyll-a*, which is distinguished from the bulk chlorophylls by its reactivity and its spectroscopic properties. In principle it should be possible to obtain the desired information from studies on the linear dichroism of this pigment in membranes which are oriented with respect to the lab system. Various attempts to orient the submicroscopic inner membranes of chloroplasts have been reported. The most recent ones were based