Second-Sphere Protonation Imparts a Biologically Relevant Potential Shift in a Chlorophyll Model Compound

Herein, we report a chlorophyll model compound based on a Mg-porphyrin framework with a secondary coordination sphere designed to probe isolated electrostatic interactions with the porphyrin macrocycle.

Protonation of two second-sphere amine groups led to a significant increase in the redox potential of the porphyrin ring, drawing a strong analogy to the postulated influence of protonated amino acids on the properties of chlorophylls <i>in vivo</i>. Experimental techniques (electronic absorption spectroscopy, single-crystal X-ray diffraction, cyclic voltammetry) were complemented by density functional theory calculations to uncover the impact of the proximal charged substituents on the electronic structure of the Mg-porphyrin complex. 1-electron oxidation of the protonated complex yielded the corresponding π-cation radical complex, which mimicked the reaction between photo-oxidized chlorophyll and tyrosine during photosynthesis by rapidly oxidizing phenolic substrates to yield phenoxyl radicals via electron-transfer-limited proton-coupled electron transfer.