Organization of Chlorophyll a in the Light-Harvesting Chlorophyll a/b Protein Complex as Shown by Circular Dichroism

The development of thylakoid stacking, accumulation of the light-harvesting chlorophyll a/b protein complex (LHCP), and the changes of circular dichroism (CD) which reflect the organization of chlorophyll molecules in greening thylakoids of bean Phaseolus vulgaris cv Red Kidney leaves were investigated.Chloroplasts formed under intermittent light contained large double sheets of membrane with extensive appression in addition to separate lamellae. Thylakoids of such chloroplasts were devoid of LHCP and exhibited a relatively small CD in the chlorophyll absorption region. Upon continuous illumination, the rearrangement of membranes to characteristic grana and the accumulation of the LHCP was accompanied by the gradual appearance of the very intense CD signal with peaks at 682 to 684 (+) and 665 to 672 nanometers (-). The magnitude of differential absorption was approximately 100 times larger than that of the chlorophyll a in solution. This suggests a superhelical liquid crystal-like organization for LHCP, a texture which can be altered by changes of the electric field in the photosynthetic membranes.