FS-study on energy relaxation in light-harvesting (LH1) complexes from Rhodospirillum rubrum with carotenoids of different conjugation length

This contribution comprises of a comparative fs-spectroscopic study on light-harvesting (LH1) complexes from mutants of Rhodospirillum rubrum. These complexes contain carotenoids of different conjugation lengths (n), Spirilloxanthin (Spx, n=13) as found in wild-type or tetra-hydro-Spirilloxanthin (thSpx, n=11), but no reaction center. After selective carotenoid (Car)-excitation we are able to separate the characteristic Car-signals in transient-absorption spectra, which are similar in both cases, but overall blue-shifted for thSpx. Besides the characteristic ground-state absorption bleaching, two excited state absorption components are observed. These are separated by about 1050 cm−1 and decay on different time scales. From comparison with an LH1 complex lacking Car, direct evidence is given for Car to bacteriochlorophyll energy transfer.