A Near-Infrared Transient Absorption Study of the Excited-State Dynamics of the Carotenoid Spirilloxanthin in Solution and in the LH1 Complex of Rhodospirillum rubrum.

摘要

The spectroscopic properties of spirilloxanthin in an nhexane solution and bound to the core light-harvesting LH1) complex of Rhodospirillum rubrum were studied by near infrared ultrafast transient absorption spectroscopy. Global analysis of the kinetic traces measured after excitation of spirilloxanthin to the S2 (1Bu +) state enabled us to estimate the species-associated difference spectra that correspond to the excited-state absorption signals originating from the S1 (2Ag-) and S2 states. Analysis of the absorption originating from the S2 state has provided further insight into the characterization of the spirilloxanthin excited states, while by analyzing the profile of the S1-S2 transition, we place the energy of the S1 state of all-trans-spirilloxanthin at 11 500 cm-1, both in solution and in the LH1 complex. This low value excludes excitation energy transfer from the S1 state of spirilloxanthin to bacteriochlorophyll in the LH1 complex of Rs. rubrum and explains the observed low energy transfer efficiency from spirilloxanthin to bacteriochlorophyll in that complex. Our results indicate that the S* state of spirilloxanthin, which was recently found both in solution and in the LH1 complex (Gradinaru, C. C., et al. Proc. Natl. Acad. Sci. U.S.A. 2001, 98, 2364), does not exhibit detectable spectral features in the near-infrared region.