Abstract
Abstract— 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).
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