ON THE STRUCTURE OF BACTERIOCHLOROPHYLL MOLECULAR AGGREGATES IN THE CHLOROSOMES OF GREEN BACTERIA - A MOLECULAR MODELING STUDY

The supramolecular structure of methyl (3(1)R)-BChlide d aggregation has been explored by molecular modelling in order to elucidate the unusual structure of the BChl rods in the chlorosomal antennae of green bacteria. The aggregate construction progressed from a BChlide monomer in 5c coordination which was stepwise combined to form trimeric, pentameric and decameric chlorin stacks, all incorporating Mg....O-H as a basic interaction element which links two chlorins between the 3(1)-hydroxyl oxygen and the Mg. Up to the level of the trimer, the structures were optimized by both a semiempirical quantum chemical method (PM3) and a force field method, while larger structures were only modelled by the force field (MM+). Strong interactions were found by extended stacking of chlorins which are in van der Waals contact. Extended hydrogen bonding networks upon stack pairing brought about by OH....O = C bonds (bond length ca. 2.2 Angstrom, angle 139-153 degrees) between appropriately situated chlorin pairs and by electrostatic interactions lead to very large energy stabilizations. The structural features of a modelled 40mer BChl aggregate are in full accord with all spectroscopic and low-resolution structural information on the in-vitro and chlorosomal BChl aggregates. Most important, from the rotation angle between stacks of ca. 16 degrees and the stack-to-stack distance of 7.6 Angstrom a tubular structure can be extrapolated to form on further extension of the aggregate. It has a predicted diameter of about 5.4 nm (Mg-Mg distance), i.e. very similar to that found for the rod elements in the chlorosomes of Chloroflexus.