ATR-FTIR Spectroelectrochemical Study on the Mechanism of the pH Dependence of the Redox Potential of the Non-Heme Iron in Photosystem II

The non-heme iron that bridges the two plastoquinone electron acceptors, Q(A) and Q(B), in photosystem II (PSII) is known to have a redox potential (E-m) of similar to+400 mV with a pH dependence of similar to-60 mV/pH. However, titratable amino acid residues that are coupled to the redox reaction of the non-heme ion and responsible for its pH dependence remain unidentified. In this study, to clarify the mechanism of the pH dependent change of E-m(Fe2+/Fe3+), we investigated the protonation structures of amino acid residues correlated with the pH-induced E-m(Fe2+/Fe3+) changes using Fourier transform infrared (FTIR) spectroelectrochemistry combined with the attenuated total reflection (ATR) and lightinduced difference techniques. Flash-induced Fe2+/Fe3+ ATR-FTIR difference spectra obtained at different electrode potentials in the pH range of 5.0-8.5 showed a linear pH dependence of E-m(Fe2+/Fe3+) with a slope of similar to 52 mV/pH close to the theoretical value at 10 degrees C, the measurement temperature. The spectral features revealed that D1-H215, a ligand to the non-heme iron interacting with was deprotonated to an imidazolate anion at higher pH with a pK, of similar to 5.6 in the Fe3+ state, while carboxylate groups from Glu/Asp residues present on the stromal side of PSII were protonated at lower pH with a pK, of similar to 5.7 in the Fe2+ state. It is thus concluded that the deprotonation/protonation reactions of D1-H215 and Glu/Asp residues located near the non-heme iron cause the pH-dependent changes in E-m(Fe2+/Fe3+) at higher and lower pH regions, respectively, realizing a linear pH dependence over a wide pH range.

Automated summary

This study reveals that the deprotonation/protonation reactions of D1-H215 and Glu/Asp residues near the non-heme iron are responsible for the pH-dependent changes in E-m(Fe2+/Fe3+) at higher and lower pH regions, respectively, resulting in a linear pH dependence over a wide pH range.