Electrostatic Fields Induce Accelerated Proton Coupled Electron Transfer Rates in Chlorophyll Model Compounds

TL;DR

This study shows how electrostatic fields can influence redox reactions in chlorophyll-like compounds, mimicking processes in photosynthesis.

Contribution

The novel synthesis of Mg-porphyrin complexes reveals electrostatic field effects on redox potentials and PCET rates.

Findings

Cation binding increases redox potentials linearly with total cationic charge.

π-cation radical complexes efficiently perform PCET reactions with charge-dependent rate enhancements.

Marcus theory explains the electrostatic field's effect on reaction rates.

Abstract

Chlorophyll-based pigments are crucial mediators of redox
processes
in photosynthesis, serving as the primary electron donors in photosystems
I and II. Despite their structural similarities, these pigments exhibit
a wide range of redox potentials (0.5–1.3 V vs SHE), and little
experimental insight into the origins of this variation is available.
To address this deficit, we have synthesized two crown ether-appended
Mg-porphyrin complexes as chlorophyll model compounds and demonstrated
their ability to bind redox-inactive metal cations. Cation binding
to the Mg-porphyrin complexes was found to increase their redox potentials
in a manner that depends linearly on the total cationic charge felt
by the complex, implicating a through-space electrostatic field effect.
The corresponding 1-electron oxidized π-cation radical complexes
were then prepared and characterized by UV–vis, FT-IR, and
EPR spectroscopies and ESI-MS. The π-cation radical species
were found to be competent for the PCET oxidation of a phenolic substrate,
mimicking the reaction between photo-oxidized chlorophyll and tyrosine
in photosystem II. Cation binding to the π-cation radical complexes
was found to increase the rates of their PCET and ET reactions in
a charge-dependent manner which could be rationalized using Marcus
theory. This work provides direct experimental evidence that electrostatic
fields can tune the redox potentials of chlorophyll model compounds,
leading to an increase in their oxidative reactivity.

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