Recent experimental data point to an asymmetric ground-state electronic distribution in the special pair (P) of purple bacterial reaction centers, which acts as the primary electron donor in photosynthesis. We have performed a density functional theory investigation on an extended model including the bacteriochlorophyll dimer and a few relevant surrounding residues to explore the origin of this asymmetry. We find strong evidence that the ground-state electron density in P is intrinsically asymmetric due to protein-induced distortions of the porphyrin rings, with excess electron charge on the PMbacteriochlorophyll cofactor. Moreover, the electron charge asymmetry is strongly modulated by the specific orientation of the C31 acetyl group, which is hydrogen bonded to His168. The electronic excitation has a significant charge transfer character inducing a displacement of electron charge from PL to PM, in agreement with experimental data in the excited state. These results are relevant for the understanding of the unidirectional electron transfer path in photosynthesis.
