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 P840 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 μs on RCs that had been depleted of the iron–sulfur electron acceptors, the formation and decay of the triplet state 3P840 have been followed in infrared for the first time. The 3P840/P840 FTIR difference spectrum is compared to the P840+/P840 FTIR difference spectrum measured under identical conditions on untreated RCs and recorded with the same step-scan set-up. The latter P840+/P840 difference spectrum is essentially the same as those measured under steady-state conditions using the more conventional continuous illumination method. Comparison of the 3P840/P840 and P840+/P840 spectra provides unambiguous assignment of the vibration of the 9-keto C=O group(s) of P840 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 P840.
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