Ca K‑Edge XAS as a Probe of Calcium Centers in Complex Systems

Herein, Ca K-edge X-ray absorption
spectroscopy (XAS) is developed as a means to characterize the local
environment of calcium centers. The spectra for six, seven, and eight
coordinate inorganic and molecular calcium complexes were analyzed
and determined to be primarily influenced by the coordination environment
and site symmetry at the calcium center. The experimental results
are closely correlated to time-dependent density functional theory
(TD-DFT) calculations of the XAS spectra. The applicability of this
methodology to complex systems was investigated using structural mimics
of the oxygen-evolving complex (OEC) of PSII. It was found that Ca
K-edge XAS is a sensitive probe for structural changes occurring in
the cubane heterometallic cluster due to Mn oxidation. Future applications
to the OEC are discussed.