Fourier transform infrared spectroscopy of primary electron donors in type I photosynthetic reaction centers.

The vibrational properties of the primary electron donors (P) of type I photosynthetic reaction centers, as investigated by Fourier transform infrared (FTIR) difference spectroscopy in the last 15 years, are briefly reviewed. The results obtained on the microenvironment of the chlorophyll molecules in P700 of photosystem I and of the bacteriochlorophyll molecules in P840 of the green bacteria (Chlorobium) and in P798 of heliobacteria are presented and discussed with special attention to the bonding interactions with the protein of the carbonyl groups and of the central Mg atom of the pigments. The observation of broad electronic transitions in the mid-IR for the cationic state of all the primary donors investigated provides evidence for charge repartition over two (B)Chl molecules. In the green sulfur bacteria and the heliobacteria, the assignments proposed for the carbonyl groups of P and P(+) are still very tentative. In contrast, the axial ligands of P700 in photosystem I have been identified and the vibrational properties of the chlorophyll (Chl) molecules involved in P700, P700(+), and (3)P700 are well described in terms of two molecules, denoted P(1) and P(2), with very different hydrogen bonding patterns. While P(1) has hydrogen bonds to both the 9-keto and the 10a-ester C=O groups and bears all the triplet character in (3)P700, the carbonyl groups of P(2) are free from hydrogen bonding. The positive charge in P700(+) is shared between the two Chl molecules with a ratio ranging from 1:1 to 2:1, in favor of P(2), depending on the temperature and the species. The localization of the triplet in (3)P700 and of the unpaired electron in P700(+) deduced from FTIR spectroscopy is in sharp contrast with that resulting from the analysis of the magnetic resonance experiments. However, the FTIR results are in excellent agreement with the most recent structural model derived from X-ray crystallography of photosystem I at 2.5 A resolution that reveals the hydrogen bonds to the carbonyl groups of the Chl in P700 as well as the histidine ligands of the central Mg atoms predicted from the FTIR data.