A comparative study of conserved protein interactions of the primary electron donor in photosynthetic purple bacterial reaction centers

The pigment–protein interactions within the binding site of the bacteriochlorophyll (BChl) dimer constituting the primary electron donor (P) in several, native, photosynthetic bacterial reaction centers have been determined using Fourier transform Raman spectroscopy. For reaction centers whose primary sequence data are available, and assuming a structural analogy with the Rb. sphaeroides RC whose high-resolution three-dimensional structure is known, amino acid residues donating hydrogen bonds to P are proposed. Consequently, one may propose the microenvironment structure of the primary donors studied and correlate this deduced structure with the known absorption and redox properties of the primary donors. In this mini-review of our past work, we group and classify the primary donors with respect to their specific H-bonding interactions with the protein. This classification reveals trends in the H-bonding and certain physicochemical properties such as the P°/P+ redox midpoint potential, the positive charge distribution over the dimeric primary donors in their oxidized radical cation state P+, and the absorption maxima of the lower exciton Q_y absorption band of P.