Stark Hole-Burning Studies of Three Photosynthetic Complexes

Abstract

Stark hole-burning spectroscopy at 1.8 K was used to determine the dipole moment changes fΔμ (f, the local field correction factor) for the B800 absorption band of the light harvesting 2 (LH2) complex of Rhodobacter sphaeroides, Rhodopseudomonas acidophila (strain 10050), and Rhodospirillum molischianum. Hole-burning values of fΔμ for the lowest energy exciton level (B870) associated with LH2's B850 band have recently been reported (Rätsep et al. Spectrochim. Acta, in press). Values for the lowest energy exciton level (B896) associated with the B875 band of the LH1 complex of Rb. sphaeroides (wild-type chromatophores and an LH1-only mutant) and the 825 nm band of the bacteriochlorophyll a (FMO) antenna complex of Chlorobium tepidum are also reported. For each band, fΔμ was determined for burn laser polarization parallel and perpendicular to the Stark field ES and several burn frequencies. The dependencies on laser polarization and burn frequency are typically quite weak. Importantly, fΔμ values for the above bands are small, falling in the range ∼0.5−1.2 D, with the lowest and highest values associated with the 825 nm band of the FMO complex and B800 band of the LH2 complex, respectively. For the B896 band of the LH1 complex, fΔμ ≈ 0.8 D. Such small values are consistent with the very weak linear electron−phonon coupling of antenna protein complexes as determined by hole-burning spectroscopy. Overall, the values for fΔμ from classical Stark modulation (CSM) studies (Gottfried et al. Biochim. Biophys. Acta 1991, 1059, 63; Beekman et al. J. Phys. Chem. B 1997, 101, 7293) are larger, in the cases of B850 and B875, by a factor of 3−4. (In CSM spectroscopy, one analyzes the response of the entire absorption band to the external field.) Discussion of the discrepancies between the two Stark techniques is given. It appears that difficulties inherent to the analysis procedure of CSM spectroscopy can lead to unreliable values for dipole moment and polarizability changes associated with absorption bands of photosynthetic complexes, especially when several excitonic levels contribute to the band, e.g., B850 and B875. An explanation for the small hole-burning values of fΔμ for the B870 and B896 levels associated with Cn cyclic arrays of strongly coupled BChl a dimers is given based on structural, symmetry, and energy disorder considerations. A key point is that, in the absence of energy disorder, the component of Δμj (j labeling the exciton level) perpendicular to the Cn axis is zero. Energy disorder, which destroys the Cn symmetry and leads to localization effects results in nonzero values which may depend on j when the protein-induced contribution to Δμj is taken into account.