Circular polarization of fluorescence of chlorophyll in solution and in native structures

Chlorophyll dimers in solution, subchloroplast particles and chloroplasts were investigated by their circular dichroism and circular polarization of their fluorescence, which reflect their optical rotatory power in the ground state and electronically excited state, respectively. The chlorophyll dimers in fluid solution lose their optical activity upon electronic excitation, reflecting a marked concomitant change in the structure of the dimers. This change is arrested in a solution of very high viscosity. The pronounced difference between the circular polarization of the dimers in fluid media and that of subchloroplast particles and chloroplasts indicates that the former are not suitable models for associated chlorophyll in native structures in electronically excited states. Impairment of the photochemical activity of chloroplasts by heat treatment is accompanied by a reduction of the circular polarization of the fluorescence, which probably reflects a disorganization in structure. The same extent of circular polarization was observed in the fluorescence of chloroplasts regardless whether the reaction centers are open or closed; thus either the same molecules are emitting in the two cases or, if different molecules emit, they are packed in a similar way.