PARENT SESSION
Posters P3A Bacteriochlorophyll based antenna systems. Abstracts (219-238)

Excited state dynamics in the Rhodospirillum rubrum light-harvesting 1 protein reconstituted with non-native carotenoids. Joseph Shih¹, Seth Brietbart¹, Robert Niederman^*,1, ¹ Department of Molecular Biology and Biochemistry, Piscataway, NJ, USA

ABSTRACT- The recent discovery of several new S2 decay channels has generated considerable interest in the excited state dynamics of carotenoids. The light-harvesting (LH) 1 complex of Rs. rubrum exhibits a relaxation pathway, designated as S*, that promotes direct, ultrafast carotenoid triplet formation, in high yield, by an apparent fission of the S* state into a pair of triplets (Gradinaru et al., Proc. Natl. Acad. Sci. USA 98:2364-2369, 2001). Since this process appears to be governed by protein-induced polyene backbone distortions, Rs. rubrum LH1 is an ideal model for examining the role played by the protein scaffold in establishing the photophysical properties of associated carotenoids. We have begun reconstituting the Rs. rubrum LH1 protein with carotenoids of shorter conjugation length and higher excited state energies than the native spirilloxanthin (N = 13), to assess the effects of carotenoid chain conformation and unsaturation on the ability to form S* and generate carotenoid triplets. When carotenoidless LH1 was reconstituted with neurosporene (N = 9), this carotenoid was found to occupy most of the potential binding sites and showed a characteristic absorption redshift. The strength of the carotenoid association was confirmed in a photoprotection assay (t1/2 of BChl QY-absorption band of neurosporene-reconstituted LH1 = 210 min, vs. 238 min in the wild-type and 17.5 min in the carotenoidless LH1 proteins upon exposure to white light (1200 Wm-2)). Preliminary ultrafast transient absorption measurements showed that the reconstituted complex also generated a triplet state, but unlike the native complex, some energy from the S1 state appeared to be transferred to BChl.

KEY WORDS: Carotenoids, Rhodospirillum ribrum, Light-Harvesting, Reconstitution