Abstract

Chlorosomes are the principal light-harvesting bodies in green photosynthetic bacteria. These 100×30×12 nm ellipsoidal bodies contain ~10⁴ bacteriochlorophyll (BChl) c chromophores, as well as a BChl a pigment-protein complex that forms an interfacial baseplate between the chlorosome and the cytoplasmic membrane. The BChl c pigments in chlorosomes are organized into large oligomers, whose electronic and vibrational spectroscopy is remarkably similar to that of BChl c aggregates that form spontaneously from BChl c monomers in solution. This unique self-aggregating property has attracted wide attention because of its potential applications in artificial photosynthesis. The BChl c and BChl a antennae of chlorosomes from the green bacterium Chloroflexus aurantiacus exhibit broad Q_y (S₁←S₀) electronic absorption bands centered at ~740 and ~790 nm, respectively. Downhill BChl c → BChl a energy transfer occurs with ~10 ps kinetics in isolated chlorosomes [1,2]. In this work, we have focussed on the femtosecond internal energy transfer events within the BChl c antenna. It is currently believed [3] that this 740 nm antenna comprises several distinct BChl c spectral forms (c727, c744, c766 etc.) Equilibration among chlorophyll and bacteriochlorophyll spectral forms requires several hundred fs in most pigment-protein antenna complexes that have been studied to date [4].

© 1994 Optical Society of America