|
Photosynthesis is an extremely efficient converter of light into chemical energy, with an observed quantum yield for primary photochemistry approximately 90%. To achieve this the photosynthetic apparatus must be highly optimized, and some of the design principles that may be involved have been suggested. The role of delocalized exciton states of light-harvesting pigments in the energy transfer process has been considered by mathematical simulation of the light-harvesting process in model systems. Namely, it has been shown that aggregation of antenna pigments (allowing to consider each aggregate as a supermolecule) is biologically expedient, as an efficient strategy for light harvesting in photosynthesis. The question of whether this design principle is realized in a natural antenna has been examined for the 3D chlorosomal superantenna of green bacteria with the hole-burning spectroscopy. Spectral hole burning studies of intact cells of green bacteria Chlorobium phaeovibriodes. Chloroflexus aurantiacus and Chlorobium limicola have proven that the Qy- absorption system of antenna bacteriochlorophylls e or c (BChl e or BChl c) should be interpreted in terms of the delocalized exciton level structure of an aggregate. For the first time the 0-0 transition band of the lowest exciton state of BChl e and BChl c aggregates has been directly detected as the lowest energy inhomogeneously broadened band of the 1.8 K near-infrared excitation spectrum. These lowest energy bands have different spectral position of their maximums: approximately 739 nm in C.phaeovibriodes (BChl e band), approximately 752 nm in C.aurantiacus (BChl c band) and approximately 774 nm in C.limicola (BChl c band) cells. However, these bands display a number of fundamentally similar spectral features: (1) The magnitude of inhomogeneous broadening of these bands is 90 - 100 cm-1; (2) The width of each band is 2 - 3 times less than that of the monomeric BChl c (or BChl e) in vitro at 5 K; (3) Each band, being the lowest energy exciton band, manifests itself as the longest wavelength band in the circular dichroism spectrum; (4) At the wavelength of the maximum of each band for all the three species, the amplitude of the preburnt excitation spectrum makes up 20% of the maximum amplitude of the spectrum; (5) The weak exciton-phonon coupling of optical transitions corresponding to these bands is also a common feature. So, the hole spectra measured for C.aurantiacus, C.limicola and C.phaeovibrioides cells were shown to be consistent with the BChl c (and BChl e) organization in the chlorosomes as strongly exciton- coupled BChl c (or BChl e) aggregates, i.e. the delocalized excitons are in fact involved in the energy transfer process within these antennae. Thus, aggregation of the pigment molecules in natural light-harvesting antennae should be considered as one of the optimizing factors ensuring high efficiency of excitation energy transfer from antenna to reaction center.2362
|
