Synthetic zinc and magnesium chlorin aggregates as models for supramolecular antenna complexes in chlorosomes of green photosynthetic bacteria

A comparison of the spectra of in vitro (3-hydroxymethyl-13(1)-oxometallochlorin) and in vivo chlorosomal (bacteriochlorophyll-c) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacteriochlorophyll-c aggregates in the extramembranous antenna complexes (chlorosomes) of green photosynthetic bacteria. Synthetic zinc and magnesium chlorins have been found to aggregate in 1% (vol/vol) tetrahydrofuran and hexane solutions and in thin films to form oligomers with the Q(y) absorption bands shifted to longer wavelengths by about 1900 (Zn chlorins) and 2100 cm(-1) (Mg) relative to the corresponding monomer bands. Visible absorption and circular dichroism spectra of various zinc chlorins establish that a central metal, a 3(1)-hydroxy and a 13(1)-keto group are functional prerequisites for the aggregation. Vibrational bands measured by IR spectroscopy of solid films reveal two characteristic structural features of the oligomers: (1) a five-coordinated metallochlorin macrocycle with an axial ligand (bands at 1500-1630 cm(-1)), and (2) a hydrogen bond between the keto oxygen of one chlorin and the hydroxy group of a second chlorin, the oxygen of which is chelated to the metal atom of a third molecule, i.e. C=O ... H-O ... M (=Zn or Mg).