Photosynthetic Electron and Proton Transport in Chloroplasts: EPR Study of ΔpH Generation, an Overview

This is a brief overview focused on the electron paramagnetic resonance applications to the study of the proton transport processes in chloroplasts. After brief description of structural and functional organization of the chloroplast electron transport chain, our attention is focused on the measurements of trans-thylakoid pH difference (ΔpH) with pH-sensitive spin-probes. The use of spin-probes is based either (i) on measuring the ΔpH-partitioning of spin-probes between the thylakoid lumen and external volume, or (ii) on monitoring changes in the electron paramagnetic resonance spectra of pH-sensitive nitroxide radicals located in the lumen. Along with the use of spin-probes, the intra-thylakoid pH (pH_in) can be determined by the "kinetic" method, which relies on the fact that the rate-limiting step in the chain of photosynthetic electron transfer (plastoquinol oxidation by the cytochrome b ₆ f complex) is controlled by pH_in. The results of ΔpH determinations in chloroplasts based on the use of pH-sensitive spin-probes and measurements of post-illumination reduction of photoreaction centers of Photosystem I are discussed in the context of the problem of energy coupling in laterally heterogeneous lamellar system of chloroplasts.