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Circular polarization of luminescence (CPL; Steinberg IZ (1978) Annu Rev Biophys Bioeng 7: 113–137) was applied to study pea chloroplasts in different structural states. The structural changes of chloroplasts were induced by variation of osmotic pressure, concentration of magnesium-ions or photoinhibition. Both large CPL and psi-type circular dichroism (psi, polymerization and salt induced) signals appeared in the presence of granal macrostructure and were sensitive to structural changes of the grana. The relation was studied between the amount of CPL expressed as an emission anisotropy factor gem and amplitudes of the red psi-type CD bands. The positive psi-type CD band was not directly correlated with gem possibly due to a large contribution of circular intensity differential scattering to the measured CD spectra. However, a linear correlation between the amplitude of the negative psi-type CD band and gem was found. The CPL signal of pea chloroplasts was attributed to a psi-type origin, which is observed in macroaggregates with densely packed chromophores with a long-range chiral order, and directly depends on the level of macroorganization. With the use of CPL-based microscopy, the long-range packing of LHC II particles can be studied in individual chloroplasts in future. In addition, the CPL method in general allows the study of the macro-organization of grana in green leaves, where conventional light-transmission methods fail. |
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