Energy and Charge Transfer Excitations in a Bacterial Reaction Center from First Principles

In purple bacteria, the fundamental charge separation step that drives the conversion of radiation energy into chemical energy, proceeds along one branch of a heterodimeric pigment-protein complex, the reaction center. An atomistic explanation of the directionality of this charge transfer process has not been provided yet. Here, we use first principles (time-dependent) density functional theory and Green's function-based many-body perturbation theory to investigate the electronic and excited state structure of the primary four bacteriochlorophyll pigments in the reaction center. We systematically study the effect of the protein environment and molecular vibrations on energy and charge transfer excitations. We show a pronounced effect of a subset of amino acids in the vicinity of the primary photoactive pigments on both types of excitations. However, analysis of transition and difference densities reveals transfer of charges along both reaction center branches, and both forward and backward charge transfer. A vibrational mode decomposition allows us to unravel the coupling between molecular vibrations and excited states and provides an intuitive understanding of vibronic coupling in these complex multichromophoric systems.