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 (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 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).