The role of the electronic structure of quinones in the charge stabilization in photosynthetic reaction centers

A specially organized quinone acceptor complex formed by the primary and se condary quinones, QA and QB, respectively, plays an essential role in charg e stabilization within the photosynthetic reaction centers, pRC. Semiempiri cal quantum chemical calculations (AM1 implemented in MOPAC93) for the role of the head group substituents in the redox functions of the quinones were carried out and demonstrated in this work. The ionization potential and th e atomic charges determined by Mulliken population analysis of ubiquinone s pecies substituted with different isoprene subunits, UQ(n=0-3) (2,3-methoxy -5-methyl-6-n-isoprenoyl-1,4-ubiquinone) and DQ (2,3,5,6-tetramethyl-1,4-ub iquinone) are presented here. It is concluded that the stabilization of lig ht separated charges is determined either by steric and/or electronic prope rties of quinone species bound to the pRC protein. The methoxy and isoprene chain groups of ubiquinone species play important roles in the steric and electronic stabilization of the molecules and in the mechanism of the elect ron transport. (C) 2001 Elsevier Science B.V. All rights reserved.