ULTRAFAST INTERNAL-CONVERSION IN 13(2)-OH-NI-BACTERIOCHLOROPHYLL IN REACTION CENTERS OF RHODOBACTER-SPHAEROIDES R26

The excited state lifetime of 13(2)-OH-Ni-bacteriochlorophyll (Ni-BChl ) is investigated by fluorescence quenching studies after incorporatin g the Ni pigment into the photosynthetic reaction center of Rhodobacte r sphaeroides R26, replacing there the two monomeric bacteriochlorophy lls BA and B-B. In contrast to the native reaction center, the excitat ion spectrum of the primary donor fluorescence shows a drastic reducti on of excitation energy transfer from the two Ni-BChls at the B-A,B-B sites to P. This effect is attributed to ultrafast internal conversion within the manifold of vibronic states of Ni-BChl starting from its l owest excited pi-pi singlet state. Based on ultrafast (100 fs) excita tion energy transfer from this excited state to the primary donor, the lifetime of the singlet excited Ni-BChl is estimated to be shorter th an 40 fs, favoured by the energetic proximity of a lower lying excited singlet state B-1(1g) of the central metal. Due to the large depressi on of the (1)Q(y)(Ni-BChl) band in the fluorescence excitation spectru m, the upper exciton component of the special pair can be unambiguousl y located at 814 nm, while in absorption spectra this weak transition is buried under the strong bands of B-A and B-B.