Influence of the Protein Binding Site on the Excited States of Bacteriochlorophyll: DFT Calculations of B800 in LH2

Abstract

Effects of hydrogen bonding and the axial ligand interaction on the B800 band in two LH2 complexes Rhodopseudomonas (Rps.) acidophila and Rhodospirillum (Rs.) molischianum have been theo retically investigated by using density functional theory. The local electrostatic environment of the B800 bacteriochlorophyll is simulated as an atomic charge field consisting of the pigments in the protomer unit. Despite the fact that the B800 binding sites in two structures are very different, their absorption spectra are almost identical. Our calculations indicate that the charged axial ligand in Rs. molischianum and the hydrogen bonding in Rps. acidophila lead to similar red shifts, possibly explaining the above controversy. We also found (i) additional B800 bacteriochlorophyll transitions located between the Q and Soret regions for both LH2 complexes and (ii) the ligand to the B800 charge-transfer excited states in the long-wavelength region for the B800-alphaAsp(6) complex in,the Rs. molischianum LH2 system.