Comparative study of the energy transfer kinetics in artificial BChl e aggregates containing a BChl a acceptor and BChl e-containing chlorosomes of Chlorobium phaeobacteroides.

Chlorosomes are the light-harvesting organelles of green bacteria, containing mainly special bacteriochlorophylls (BChls) carrying a 3(1)-hydroxy side chain. Artificial aggregates of BChl c, d, and e have been shown to resemble the native chlorosomes in many respects. They are therefore seen as good model systems for understanding the spectroscopic properties of these antenna systems. We have investigated the excitation energy transfer in artificial aggregates of BChl e, containing small amounts of BChl a as an energy acceptor, using steady-state and time-resolved fluorescence. Global analysis of the kinetic data yields two lifetimes attributable to energy transfer: a fast one of 12-20 ps and a slower one of approximately 50 ps. For comparison, BChl e-containing native chlorosomes of Chlorobium phaeobacteroides and chlorosomes in which the energy acceptor had been degraded by alkaline treatment were also studied. A similar behavior is seen in both the artificial and the natural systems. The results suggest that the artificial aggregates of BChls have a potential as antenna systems in future artificial photonic devices.

Relevance of the diastereotopic ligation of magnesium atoms of chlorophylls in Photosystem I.

The central magnesium (Mg) atoms of natural occurring tetrapyrroles such as chlorophylls (Chls) and bacteriochlorophylls (BChls) are typically five-coordinated, a fact which leads to the formation of diastereoisomers if the Mg-ligand bond is stable on the time scale of the observation method. This possibility has only been briefly addressed before in a CD-study of BChl c aggregates [T.S. Balaban, A.R. Holzwarth, K. Schaffner, J. Mol. Struct. 349 (1995) 183]. On the basis of the chlorophyll-protein complex photosystem I (PSI), which has recently been characterized by single crystal crystallography [P. Jordan, P. Fromme, H.T. Witt, O. Klukas, W. Saenger, N. Krauss, Nature 411 (2001) 909], we find that chlorophyll a molecules are much more frequently bound by the protein matrix from one side (anti) than the other one (syn) in a ratio of 82:14, which corresponds to a significant DeltaDeltaG value of 4.3 kJ/mol. Syn and anti denote the orientation of the Mg-ligand with respect to the 17-propionic acid esterified by phytol. Furthermore, by parallel sequence analysis we find that the binding sites for both syn and anti chlorophylls have been strongly conserved during evolution-a fact which stresses the nonrandom manner in which chlorophylls are bound by the apoprotein in antenna complexes, in order to exert efficiently their light harvesting function and energy funnelling. Most remarkably, all the syn chlorophylls are part of the inner core antenna system. Results from semiempirical quantum mechanical and detailed exciton coupling calculations allow us to speculate on the functional relevance of the diasteretopicity for PSI functioning.