Abstract

In green plants oxygenic photosynthesis uses a manganese cluster to catalyze the oxidation of water molecule to molecular oxygen and protons. It is assumed that oxygenic photosynthetic organisms evolutionarily developed from their more ancient anoxygenic relatives. Until recently, the binding and utilization of manganese as a secondary electron donor in anoxygenic photosynthesis from Rhodobacter sphaeroides has only been achieved in genetically modified strains.
In our current work we revealed that the binding is facilitated by light-induced conformational changes. The conformationally-activated electron transfer from manganese to the oxidized bacteriochlorophyll dimer was found to be 50 % faster than that of detected in the dark-adapted conformation.
In order to characterize the accessibility of the binding site, the local dielectric constant was altered by incorporating hydrophobic molecules, such as detergents to the vicinity metal binding site. Tuning the dielectric properties of the binding site by incorporating detergent molecules diminished the observed differences between the electron transfers in the dark- and light-adapted conformations.
The EPR spectra of Cu-BTP complex at various BTP concentration and pH values were recorded. As the concentration of BTP increased, new spectral features emerged in Cu2+ spectra, indicating that the water molecules coordinating Cu2+ in the hexa-aquo complex are replaced by BTP molecules, forming a Cu-BTP structure. The pH dependency of Cu-BTP complex was studied by monitoring the EPR spectra of the complex from pH values of 4.0 to 9.5, and the effect of deprotonation of amine groups of BTP on coordination of copper ions was observed.