Chloroplasts from the pea (Pisum sativum L.) suspended in iso-osmotic buffered medium were oriented by flow in specially constructed cuvettes and examined for circular dichroism (c.d.). In one cuvette the flow was transverse to the direction of the light-path, but the other cuvette was designed so that flow and the light-path were coaxial. The induced orientation is such that the chloroplasts appear to move edgewise. C.d. was maximum when the light-path lay in the plane of the chloroplast disc. The intense c.d. of intact chloroplasts ascribed by Gregory & Raps [Biochem. J. (1974) 142, 193-201] to bulk chlorophyll a was found to contain two components, one the split-exciton type centred at 682nm and the other a simple maximum at 700-710 nm. The chlorophyll a-710 form was distinguished by its greater dependence on chloroplast orientation. The preferred direction of the transition moment in both chlorophyll forms was at right angles to the plane of the chloroplast, that is, at right angles to the plane of the thylakoids. This is in conflict with several reports based on polarization of fluorescence. It is suggested that the present effect is due to thylakoid-thylakoid interaction. Evidence for this is the reversible diminution in the c.d. signal caused by illumination in the presence of electron-transport reagents. It is argued that the c.d. is an indicator of chlorophyll movement, or changes in the thylakoid-thylakoid distance, possibly related to ion movement, affecting energy transfer between photosynthetic units.

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