Macromolecular organization of chlorophyll a in aggregated chlorophyll a/b protein complex as shown by circular dichroism at room and cryogenic temperatures.

This report concerns the circular dichroic (CD) signal of intact chloroplasts of higher plants. The CD spectra of chloroplasts are compared with the aggregated form of the light-harvesting chlorophyll a/b complex at 25 degrees C and -250 degrees C. The light-harvesting chlorophyll aggregate has a CD of magnitude equal to or greater than chloroplasts, but of opposite sign, and it is not related to the CD of the unaggregated form, and hence its arrangement is an artefact compared to the arrangement in the chloroplast. We suggest that this preparation, which has pseudo-lamellar structure, is a clear example of a large CD signal being generated by macromolecular association. The asymmetry of organization in the chloroplast has an opposite sense to that of the aggregate, but affects only chlorophyll a, not chlorophyll b.