Quantitative evaluation of the near-C₂ symmetry of the bacterial photosynthetic reaction center

The conversion of solar light by the reaction center (RC) of purple bacteria proceeds through a construct of two identical pigment branches and a ferrous ion. The imperfect C₂ symmetry of the two branches has been a major issue in trying to understand the preference of only one branch for the energy migration process. In view of the significance of this imperfection, it is desirable to express this structural property on a quantitative scale. For this purpose we have applied on six different RC structures the continuous symmetry measure methodology, an approach which allows one to measure the degree of any symmetry group in any structure. For each structure, the C₂ content of the whole RC was calculated, as well as of the top special pair where it is believed that the initial charge separation occurs. Full analysis for all pairs was carried out for one of the RCs. It was found that the top pair of cofactors is the most symmetric and that the second pair is of the highest deviation.