STARK HOLE-BURNING STUDIES OF 3 PHOTOSYNTHETIC COMPLEXES

Stark hole-burning spectroscopy at 1.8 K was used to determine the dip ole moment changes f Delta mu (f, the local field correction factor) f or 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 f Delta mu for the lowest energy exciton level (B870) associated with LH2's B850 ban d have recently been reported (Ratsep et al. Spectrochim. Acta, in pre ss). Values for the lowest energy exciton level (B896) associated with the B875 band of the LH1 complex of Rb. sphaeroides (wild-type chroma tophores and an LH1-only mutant) and the 825 nm band of the bacterioch lorophyll a (FMO) antenna complex of Chlorobium tepidum are also repor ted. For each band, f Delta mu was determined for burn laser polarizat ion parallel and perpendicular to the Stark field E-S and several burn frequencies. The dependencies on laser polarization and burn frequenc y are typically quite weak. Importantly, f Delta mu values for the abo ve bands are small, falling in the range similar to 0.5-1.2 D, with th e 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, f Delta mu approximate to 0.8 D. Such small v alues are consistent with the very weak linear electron-phonon couplin g of antenna protein complexes as determined by hole-burning spectrosc opy. Overall, the values for f Delta mu from classical Stark modulatio n (CSM) studies (Gottfried et al. Biochim. Biophys. Acta 1991, 1059, 6 3; 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 f ield.) Discussion of the discrepancies between the two Stark technique s is given. It appears that difficulties inherent to the analysis proc edure of CSM spectroscopy can lead to unreliable values for dipole mom ent and polarizability changes associated with absorption bands of pho tosynthetic complexes, especially when several excitonic levels contri bute to the band, e.g., B850 and B875. An explanation for the small. h ole-burning values of f Delta mu for the B870 and B896 levels associat ed with C-n cyclic arrays of strongly coupled BChl a dimers is given b ased on structural, symmetry, and energy disorder considerations. A ke y point is that, in the absence of energy disorder, the component of D elta mu(j) (j labeling the exciton level) perpendicular to the C-n axi s is zero. Energy disorder, which destroys the C-n symmetry and leads to localization effects results in nonzero values which may depend on j when the protein-induced contribution to Delta mu(j) is taken into a ccount.