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 Q(y) absorption band has been studied in the isolated D1-D2-cytochrome b559 complex. The pH treatments are done on an ion-exchange chromatographic column. The absorption spectra at 77 K of the complex treated with acidic pH show irreversible loss of absorbance at both the blue and the red sides of the Q(y) absorption band, with minima at 664.5 and 683.5 nm, respectively. These absorption changes are not accompanied by modifications in the Q(x) absorption region characteristic of pheophytin pigments. Furthermore, the pigment composition of the D1-D2-cytochrome b559 complex remains unchanged after this treatment. The effects of basic pH are similar to those of acidic pH, but somewhat more pronounced. These results suggest that chlorophyll pigments absorbing at 664.5 and 683.5 nm are located on or erase to the surface of the complex. Freezing/thawing cycle treatment first affects 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 pH < 5 a small reversible change at 672.5 nm is measured that correlates with a reversible change at 542 nm, indicating that inactive pheophytin a will absorb at this wavelength. (C) 1999 Elsevier Science S.A. All rights reserved.