Stark effect spectroscopy of bacteriochlorophyll in light-harvesting complexes from photosynthetic bacteria

Abstract

The effects of an applied electric field on the Q y absorption and fluorescence spectra of three antenna complexes from photosynthetic bacteria have been measured at 77 K: the bacteriochlorophyll a protein (BCP) from Prosthecochloris aestuarii , the B800-850 light-harvesting complex from Rhodobacter sphaeroides and the B875 light-harvesting complex from Rhodobacter capsulatus . For BCP in a glycerol / buffer glass, the value of the change in dipole moment, |Δμ A |, for three of the resolved absorption bands was found to lie in the range 1.3–2.0 D / f (where f is the local field correction factor), and the Stark spectrum lineshape is well approximated by the second derivative of the absorption. This is similar to what is found for monomeric bacteriochlorophyll a (BChl a ) embedded in a polymer film and for the 800 nm monomer BChl a band of Rb. sphaeroides reaction centers; thus, the BChl a chromophores in BCP behave largely as a non-interacting set in an electric field. For B800-850 in a glycerol/buffer glass, the BChl a associated with the 800 nm band has a small |Δμ A | = 0.8–0.9 D/ f , while the BChl a of the 850 nm band is found to have |Δμ A | = 3.1–3.4 D/ f ; however, the Stark spectra of both components have anomalous lineshapes, which complicates the quantitative analysis. |Δμ A | for the 850 nm band is nearly unchanged upon attenuation of the 800 nm band by treatment with lithium dodecyl sulfate. The Stark spectrum of the B875 complex was obtained in whole chromatophore membranes. The lineshape for B875 is also unusual, and it appears that the electrochromism is not dominated by a dipole moment change. In all three antenna complexes, efficient energy transfer occurs from higher-energy states to the lowest-energy excited state, and, at 77 K, fluorescence occurs primarily from this lowest state. The electric field modulated fluorescence of BCP shows nearly the same quantitative effect as that obtained from absorption, with |Δμ F | = 1.6 D/ f . In contrast, the B800-850 complex shows an unprecedented, very large net decrease in fluorescence in an applied electric field. Possible mechanisms for this unusual result are discussed. B875 also shows an electric field induced fluorescence decrease; however, the Stark fluorescence spectrum is dominated by a first-derivative contribution, not unlike the electromodulated absorption spectrum. Explanations for this, and many of the results for strongly interacting chromophores, may lie in a breakdown of the standard treatments of electrochromism when strong intermolecular coupling dominates.