X-RAY-ABSORPTION SPECTROSCOPY OF CALCIUM-SUBSTITUTED DERIVATIVES OF THE OXYGEN-EVOLVING COMPLEX OF PHOSTOSYSTEM-II

X-ray absorption spectroscopy (XAS) has been used to characterize the structural consequences of Ca2+ replacement in the reaction center com plex of the photosynthetic oxygen-evolving complex (OEC). EPR and acti vity measurements demonstrate that, in the absence of the 17 and 23 kD a extrinsic polypeptides, it is nor necessary to use either low pH or Ca chelators to effect complete replacement of the active site Ca2+ by Sr2+, DY3+, or La3+. The extended X-ray absorption fine structure (EX AFS) spectra for the OEC show evidence for a Mn ... M interaction at c a. 3.3 Angstrom that could arise either from Mn ... Mn scattering with in the Mn cluster or Mn ... Ca scattering between the Mn cluster and t he inorganic Ca2+ cofactor. There is no significant change in the eith er the amplitude or the phase of this feature when Ca2+ is replaced by Sr2+ or DY3+, thus demonstrating that there is no EXAFS-detectable Mn ... Ca contribution at ca. 3.3 Angstrom in these samples. The only si gnificant consequence of Ca2+ replacement is a small change in the ca. 2.7 Angstrom Mn ... Mn distance. The average Mn ... Mn distance decre ases 0.014 Angstrom when Ca2+ is replaced by Sr2+ and increases 0.012 Angstrom when Ca2+ is replaced by Dy3+. A structural model which can a ccount both for the variation in Mn ... Mn distance and for the known properties of Ca2+-substituted samples is one in which there is a hydr ogen bond between a Ca2+-bound water and a Mn-2(mu-O)(2) unit. This sc heme suggests that an important role for the Ca2+ may be to modulate t he protonation stare, and thus the redox potential, of the Mn cluster.