Time-resolved step-scan FTIR investigation on the primary donor of the reaction center from the green sulfur bacterium Chlorobium tepidum.

The vibrational properties of the primary donor P(840) in the reaction center (RC) of the green sulfur bacterium Chlorobium tepidum and its interactions with the surrounding protein environment have been investigated by Fourier transform infrared (FTIR) difference spectroscopy at cryogenic temperatures. By using the step-scan technique with a time resolution of 5 mus on RCs that had been depleted of the iron-sulfur electron acceptors, the formation and decay of the triplet state (3)P(840) have been followed in infrared for the first time. The (3)P(840)/P(840) FTIR difference spectrum is compared to the P(840) (+)/P(840) FTIR difference spectrum measured under identical conditions on untreated RCs and recorded with the same step-scan set-up. The latter P(840) (+)/P(840) difference spectrum is essentially the same as those measured under steady-state conditions using the more conventional continuous illumination method. Comparison of the (3)P(840)/P(840) and P(840) (+)/P(840) spectra provides unambiguous assignment of the vibration of the 9-keto C=O group(s) of P(840) at 1684 cm(-1) as the only common negative band in the two spectra. This frequency corresponds to carbonyl group(s) free from hydrogen bonding interactions. The obtained results are discussed in the framework of the structure and photochemistry of the primary donor P(840).