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-131-oxometallochlorin) and in vivo chlorosomal (bacterio-chlorophyll-c) aggregates suggests a similar supramolecular structure for the artificial oligomers and the bacte-riochlorophyll-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 Qy 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 31-hydroxy and a 131-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).