S0-]T1 INFRARED DIFFERENCE SPECTRUM OF THE TRIPLET-STATE OF THE PRIMARY ELECTRON-DONOR IN RHODOBACTER-SPHAEROIDES PHOTOSYNTHETIC BACTERIAL REACTION CENTERS

Using steady-state light-induced FTIR difference spectroscopy, the vibrational spectrum of the triplet state of the primary electron donor (P) in Rb. sphaeroides R-26 reaction centers has been characterized for the first time. Comparison with the difference spectrum of the oxidation of P shows that the 9-keto C=O vibrations of the two bacteriochlorophyll molecules constituting P absorb at 1695 and 1682 cm-1 while the 10a ester carbonyls absorb at 1749 and around 1740 cm-1. Bands at 1648 and 1619 cm-1 are assigned to the free and hydrogen-bonded 2a acetyl C=O of P, respectively. All these bands are downshifted upon P-3 formation. These data demonstrate that the triplet state is mostly delocalized over the two bacteriochlorophyll molecules of P.