Spectroscopic studies of the chlorophyll d containing photosystem I from the cyanobacterium, Acaryochloris marina.

Absorbance difference spectroscopy and redox titrations have been applied to investigate the properties of photosystem I from the chlorophyll d containing cyanobacterium Acaryochloris marina. At room temperature, the (P740(+)-P740) and (F(A/B)(-)-F(A/B)) absorbance difference spectra were recorded in the range between 300 and 1000 nm while at cryogenic temperatures, (P740(+)A(1)(-)-P740A(1)) and ((3)P740-P740) absorbance difference spectra have been measured. Spectroscopic and kinetic evidence is presented that the cofactors involved in the electron transfer from the reduced secondary electron acceptor, phylloquinone (A(1)(-)), to the terminal electron acceptor and their structural arrangement are virtually identical to those of chlorophyll a containing photosystem I. The oxidation potential of the primary electron donor P740 of photosystem I has been reinvestigated. We find a midpoint potential of 450+/-10 mV in photosystem I-enriched membrane fractions as well as in thylakoids which is very similar to that found for P700 in chlorophyll a dominated organisms. In addition, the extinction difference coefficient for the oxidation of the primary donor has been determined and a value of 45,000+/-4000 M(-1) cm(-1) at 740 nm was obtained. Based on this value the ratio of P740 to chlorophyll is calculated to be 1 : to approximately 200 chlorophyll d in thylakoid membranes. The consequences of our findings for the energetics in photosystem I of A. marina are discussed as well as the pigment stoichiometry and spectral characteristics of P740.