Direct observation of large chiral domains in chloroplast thylakoid membranes by differential polarization microscopy

Long-range chiral organization of the pigment-protein complexes in mature granal chloroplasts has been established by differential polarization imaging and local circular dichroism spectra. Linear and circular dichroism images of oriented chloroplasts were obtained in a confocal differential polarization microscope. The circular dichroism images display signals of opposite signs emerging from discrete regions with local dichroic values much larger than anticipated, indicating domains in the thylakoid membranes having long-range chiral organization. These domains are associated with positive and negative circular dichroism bands obtained at specific locations on the chloroplasts. Surprisingly, the local circular dichroism spectra do not display the excitonic shape of spectra obtained for macroscopic suspensions, but the latter can be produced by superposition of two local spectra of opposite sign. These data are evidence for the existence of long-range chiral order of the pigment-protein complexes in thylakoid membranes. The possible role of the long-range chiral domains in the efficiency of energy delocalization through the thylakoid membranes is discussed.