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

Infrared (IR) kinetic spectroscopy measurements have been employed to inves tigate and compare the electronic structure of B800 and B850 bacteriochloro phyll (BChl) pigment aggregates within peripheral (LH2) antenna complexes o f photosynthetic purple bacteria. Following initial excitation of the B800 chromophores at 800 nm, a strong absorption band centred at similar to1.3 m um was observed in the transient spectrum which is characteristic of monome ric BChls. Evolution of a transient spectrum in the near-IR and IR regions was then tracked while the excitation energy migrated to the B850 chromopho res on a similar to0.9 ps time scale. The B850 excited-state IR spectrum wa s found to differ substantially from that of B800, exhibiting a broader abs orption band in the near-IR (centred at 1.1 mum) and new transient absorpti on features at 2.9 and 3.7 mum which are absent in B800. These results supp ort a localised description of excitations for B800 but indicate the presen ce of considerably stronger inter-chromophore interactions between the B850 pigments. Differences between the excited-state spectra of B800 and B850 a re interpreted as reflecting the formation of delocalized exciton and charg e-resonance bands in B850. (C) 2001 Elsevier Science B.V. All rights reserv ed.