EFFECT OF THE pH ON THE ABSORPTION SPECTRUM OF THE ISOLATED D1-D2-CYTOCHROME b559 COMPLEX OF PHOTOSYSTEM II

The effect of pH on the Qy absorption band was studied in the isolated D1-D2-cytochrome b559 complex. The pH-treatments were done on an ionexchange chromatographic column. The absorption spectra at 77K of the complex treated with acidic pH showed irreversible loss of absorbance at both the blue and the red sides of the Qy absorption band with minima at 664.5 and 683.5 nm, respectively. These absorption changes were not accompanied by modifications on the Qx absorption region characteristic of pheophytin pigments. Furthermore, the pigment composition of the D1-D2-cytochrome b559 complex remained unchanged after this treatment. The effects of basic pH effects were similar to that of the acidic pH, but somewhat more pronounced. These results suggest that chlorophyll pigments absorbing at 664.5 and 683.5 nm are located on or close to the surface of the complex. Freezing/thawing cycle treatment firstly affected the band absorbing at 683.6 nm indicating that it corresponds to the chlorophyll most exposed to the medium in the D1-D2-cytochrome b559 complex. At lower pH than 5 a small reversible change at 672.5 nm was measured that correlated with a reversible change at 542 nm, indicating that inactive pheophytin a would absorb at this wavelength.