# Ammonia Binding to the Oxygen-Evolving Complex Probed by High-Energy Resolution Fluorescence Detected X-Ray Absorption Spectroscopy

## TL;DR

This study uses advanced X-ray techniques to investigate how ammonia binds to a key part of the photosystem involved in water oxidation.

## Contribution

The study provides new insights into ammonia binding modes using HERFD XAS and quantum chemical calculations.

## Key findings

- Ammonia likely substitutes the terminal water ligand W2 on Mn4 in the OEC.
- Mn4 is identified as the strongest or most accessible binding site for ammonia.
- Ammonia coordination on Mn1 as a sixth ligand is also a plausible binding mode.

## Abstract

The insertion pathways
and binding sites of substrate
water molecules
at the catalytic Mn4CaO5 cluster of the oxygen-evolving
complex (OEC) in photosystem II (PSII) remain a fundamentally unresolved
question toward understanding biological water oxidation. To address
this question, small molecules have been employed as “water
analogues” to probe substrate binding to the OEC. In this context,
the binding of ammonia has been extensively investigated and discussed
using spectroscopic, structural, and quantum chemical methods, but
a definitive answer regarding the ammonia binding site has not yet
been achieved. Herein, we present high-energy resolution fluorescence
detected (HERFD) Mn K-edge X-ray absorption spectroscopy (XAS) in
ammonia-treated S2 state samples of the OEC. Pre-edge features
were correlated with possible structural models with the aid of quantum
chemical calculations. The comparison of calculated and experimental
difference spectra between the native and ammonia-treated samples
allows us to evaluate different modes of ammonia interaction with
the OEC. The combined spectroscopic and theoretical investigation
suggests the substitution of the terminal water ligand W2 on Mn4 as
the most plausible ammonia binding mode, followed closely by the substitution
of the second terminal water ligand (W1), and the coordination of
ammonia on Mn1 as a sixth ligand. Our results are in line with the
leading interpretations of other spectroscopic and kinetic studies,
converging on the conclusion that the Mn4 ion is either the most accessible
or the strongest binding site for substrate analogues.