Abstract

Abstract
Manganese as secondary electron donor in native bacterial reaction centers
Matei-Alexandru Ivanescu

Catalytic water splitting by oxygenic photosynthetic organisms has provided a primary energy source for sustaining life for over 2 billion years. The incorporation of manganese ions by primitive photoautotrophs represents an evolutionary breakthrough in the transition from anoxygenic to oxygenic photosynthesis. This study presents the first time observation of electron donation from manganese to the oxidized dimer in native bacterial reaction centers. This has been achieved by simultaneously fulfilling three requirements: i) lowering the oxidation/reduction potential of the Mn2+ ions by coordination with bis-tris propane to as low as 332 mV ii) elevating the potential of the dimer by ~60 mV, and iii) increasing the lifetime of the charge-separated state. Kinetic analysis revealed that the first-order electron transfer proceeds with time constant of ~83 ms and the bound manganese must be about 22 Ǻ away from the dimer. Changes in dipole moments of the bacteriochlorophyll monomers and the change of the polarizability of the dimer upon manganese binding suggest that Mn2+ could bind in two solvent-accessible cavities near the two monomers. Measurements of electron transfer from cyt c22+ to P+ in an environment with high Mn2+ concentration reveal a disruption of the function of the natural secondary electron donor for the BRC which could have profound implications regarding the aforementioned evolutionary transition.