Electrostatic role of the non-heme iron complex in bacterial photosynthetic reaction center

To elucidate the role of the non-heme iron complex (Fe-complex) in the electron transfer (ET) events of bacterial photosynthetic reaction centers (bRC), we calculated redox potentials of primary/secondary quinones QA/B (Em(QA/B)) in the Fe-depleted bRC. Removing the Fe-complex, the calculated Em(QA/B) are downshifted by ∼220 mV/∼80 mV explaining both the 15-fold decrease in ET rate from bacteriopheophytin (HA-) to QA and triplet state occurrence in Fe-depleted bRC. The larger downshift in Em(QA) relative to Em(QB) increases the driving-energy for ET from QA to QB by 140 meV, in agreement with ∼100 meV increase derived from kinetic studies.