Contribution of bacteriochlorophyll conformation to the distribution of site-energies in the FMO protein.

he structural data for the Fenna–Matthews–Olson (FMO) protein indicate that the bacteriochlorophylls (BChls)display a significant degree of conformational heterogeneity of their peripheral substituents and the protein-induced nonplanar skeletal deformations of the tetrapyrrole macrocycle. As electronic properties of chromo-phores are altered by such differences, a conformational effect may influence the site-energies of specificpig-ments and thus play a role in mediating the excitation energy transfer dynamics, but this has not yet beenestablished. The difficulty of assessing this question is shown to be partly the result of the inability of the sequen-tial truncation approach usually employed to account for interactions between the conformations of themacrocycle and its substituents and an alternative approach is suggested. By assigning the BChl atoms to mean-ingful atom groups and performing all possible permutations of partial optimizations in a full-factorial design,where each group is either frozen in the crystal geometry or optimizedin vacuo, followed by excited state calcu-lations on each resulting structure (PM6//ZIndo/S), the specific effects of the conformations of each BChl compo-nent as well as mutual interactions between the molecular fragments on the site-energy can be delineated. Thisfactorial relaxationprocedure gives different estimates of the macrocycle conformational perturbation than theapproach of sequentially truncating the BChl periphery. The results were evaluated in the context of publishedsite-energies for the FMO pigments from three species to identify how conformational effects contribute totheir distribution and instances of cross-species conservation and functional divergence of the BChl nonplanarityconformational contribution are described