ja9504505_si_002.pdf (153.1 kB)
X-ray Absorption Spectroscopy of Calcium-Substituted Derivatives of the Oxygen-Evolving Complex of Phostosytem II
journal contribution
posted on 1996-03-13, 00:00 authored by Pamela J. Riggs-Gelasco, Rui Mei, Demetrios F. Ghanotakis, Charles F. Yocum, James E. Penner-HahnX-ray absorption spectroscopy (XAS) has been used to
characterize the structural consequences of
Ca2+
replacement in the reaction center complex of the photosynthetic
oxygen-evolving complex (OEC). EPR and activity
measurements demonstrate that, in the absence of the 17 and 23 kDa
extrinsic polypeptides, it is not 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 (EXAFS) spectra for the OEC
show evidence for a Mn···M interaction at
ca. 3.3 Å that could arise either from Mn···Mn scattering
within the Mn cluster or Mn···Ca scattering between
the
Mn cluster and the inorganic Ca2+ cofactor. There is
no significant change in the either 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 Å in these samples.
The only significant consequence of Ca2+ replacement
is a
small change in the ca. 2.7 Å Mn···Mn distance. The
average Mn···Mn distance decreases 0.014 Å when
Ca2+ is
replaced by Sr2+ and increases 0.012 Å when
Ca2+ is replaced by Dy3+. A structural
model which can account both
for the variation in Mn···Mn distance and for the known
properties of Ca2+-substituted samples is one in
which
there is a hydrogen bond between a Ca2+-bound water and a
Mn2(μ-O)2 unit. This scheme
suggests that an important
role for the Ca2+ may be to modulate the protonation
state, and thus the redox potential, of the Mn cluster.