Supramolecular structure of chlorophyll-protein complexes in relation to the chlorophyll a fluorescence of chloroplasts at room or liquid nitrogen temperature.

To investigate further the possibility that changes in the organization of the thylakoid pigment-protein complexes are monitored by the chlorophyll a fluorescence yield changes, or by changes in the F685/F730 ratio in the 77 degrees K fluorescence, as earlier proposed (A. Castorinis, G. Akoyunoglou, and J.H. Argyroudi-Akoyunoglou (1982) Photobiochem. Photobiophys. 4, 283-291; J.H. Argyroudi-Akoyunoglou, A. Castorinis, and G. Akoyunoglou (1982) Photobiochem. Photobiophys. 4, 201-210), the effect of pH, Cd2+ or Zn2+ addition, and trypsinization on all parameters was studied. We found that (a) the pH of the medium affects the ratio of oligomeric to monomeric structures of the complexes (monomers predominate at low pH), as well as the ratio F685/F730 of thylakoids or of the isolated CPIa complex (the F685/F730 ratio is high at low pH, and low at high). (b) Zn2+ or Cd2+ addition to thylakoids, suspended in Tricine (N-[2-hydroxy-1,1-bis(hydroxymethyl)ethyl]glycine), has no effect on the chlorophyll a fluorescence yield nor on the F685/F730 ratio at 77 degrees K; similarly, no effect can be noticed on the F685/F730 ratio at 77 degrees K of the isolated CPIa complex. These cations, contrary to Mg2+, do not affect the oligomer to monomer dissociation, but they instead reverse the cation effect and enhance the oligomeric structures. (c) Trypsinized thylakoids in Tricine do not show the Mg2+ effect on the chlorophyll a fluorescence yield, have a reduced F730/F685 ratio at 77 degrees K, and are deficient in the CPIa complex, which is mainly responsible for the F685/F730 ratio changes in the presence of cations. The results support the proposal that cation- or pH-induced changes in the chlorophyll a fluorescence yield at room temperature, or in the F685/F730 ratio at 77 degrees K, are irrelevant to the excitation energy distribution between the two photosystems. They instead seem to reflect changes in the organization of the supramolecular structure of the pigment-protein complexes in the photosystem I and photosystem II units.