Femtosecond infrared spectroscopy of reaction centers from Rhodobacter sphaeroides between 1000 and 1800 cm-1.

Time-resolved pump-and-probe experiments of reaction centers of the purple bacterium Rhodobacter sphaeroides (R26) in the mid-IR region between 1000 and 1800 cm-1 are recorded with a time resolution of 300-400 fs. The difference spectra of the states P*, P+HA-, and P+QA- with respect to the ground state P predominantly reflect changes of the special pair. They show positive and negative bands due to changes of distinct vibrational modes superimposed on a broad background of enhanced absorption. A number of certain bands can be assigned to the special pair P, to the bacteriopheophytin HA, and to the quinone QA. The temporal evolution of the IR absorbance changes is well described by the time constants known from femtosecond spectroscopy of the electronic states. Differences occur only at very early times, which are indicative of fast vibrational relaxation with a time constant of a few hundred femtoseconds.