posted on 2015-09-29, 00:00authored byMikhail Askerka, David J. Vinyard, Gary W. Brudvig, Victor S. Batista
Ammonia binds directly to the oxygen-evolving
complex of photosystem
II (PSII) upon formation of the S2 intermediate, as evidenced
by electron paramagnetic resonance spectroscopy. We explore the binding
mode by using quantum mechanics/molecular mechanics methods and simulations
of extended X-ray absorption fine structure spectra. We find that
NH3 binds as an additional terminal ligand to the dangling
Mn4, instead of exchanging with terminal water. Because water and
ammonia are electronic and structural analogues, these findings suggest
that water binds analogously during the S2 → S3 transition, leading to rearrangement of ligands in a carrousel
around Mn4.