Probing the Carotenoid in Its Binding Site in a Reconstituted LH1 Complex from the Photosynthetic Bacterium Rhodospirillum rubrum with Electroabsorption Spectroscopy

Abstract

Stark spectroscopy is a powerful technique to investigate the electrostatic interactions between pigments as well as between the pigments and the proteins in photosynthetic pigment-protein complexes. In this study Stark spectroscopy has been used to determine two nonlinear optical parameters (polarizabilty change Tr(Δα) and static dipole-moment change |Δμ| upon photoexcitation) of the purified complete and reconstituted LH1 complexes from the purple photosynthetic bacterium, Rhodospirillum (Rs.) rubrum. The complete LH1 complex was prepared from Rs. rubrum S1, while the reconstituted complex was assembled by the addition of purified carotenoid (all-trans- spirilloxanthin) to the monomeric subunit of LH1 from Rs. rubrum S1. The reconstituted LH1 complex has BChl a Qy absorption maximum at 878 nm. This is shifted to the blue by 3 nm in comparison to the purified complete LH1 complex. Based on the differences in the values of Tr(Δα) and |Δμ| between these two preparations we can calculate the change in the electric field around the BChl a molecules in the two situations to be E Δ ′ 3.4 × 105 [V/cm]. This change can explain the 3 nm wavelength shift of the Qy absorption band in the reconstituted LH1 complex.