Dynamic infrared band–band spectroscopy of peripheral light-harvesting complexes from R. acidophila

Abstract

Infrared (IR) kinetic spectroscopy measurements have been employed to investigate and compare the electronic structure of B800 and B850 bacteriochlorophyll (BChl) pigment aggregates within peripheral (LH2) antenna complexes of photosynthetic purple bacteria. Following initial excitation of the B800 chromophores at 800 nm, a strong absorption band centred at ∼1.3 μm was observed in the transient spectrum which is characteristic of monomeric BChls. Evolution of a transient spectrum in the near-IR and IR regions was then tracked while the excitation energy migrated to the B850 chromophores on a ∼0.9 ps time scale. The B850 excited-state IR spectrum was found to differ substantially from that of B800, exhibiting a broader absorption band in the near-IR (centred at 1.1 μm) and new transient absorption features at 2.9 and 3.7 μm which are absent in B800. These results support a localised description of excitations for B800 but indicate the presence of considerably stronger inter-chromophore interactions between the B850 pigments. Differences between the excited-state spectra of B800 and B850 are interpreted as reflecting the formation of delocalized exciton and charge-resonance bands in B850.