RESONANCE RAMAN-SPECTROSCOPIC STUDY OF METALLOCHLORIN AGGREGATES - IMPLICATIONS FOR THE SUPRAMOLECULAR STRUCTURE IN CHLOROSOMAL BCHL-C ANTENNAE OF GREEN BACTERIA

The resonance Raman spectra of several synthetic Zn and Mg chlorins were measured in nonpolar solvents in order to characterize the structural elements required for oligomerization of BChl c in chlorosomes of green bacteria. The stretching vibrations of the chlorin and the conjugated keto group were analyzed in terms of the coordination state of the metal ion and the inter- and intramolecular interactions of the C=O group, respectively. In the monomeric Zn chlorin the free carbonyl group gives rise to bands at ca. 1700 cm(-1). In the presence of hydrogen bond donors these bands shift down by about 30 cm(-1). Bands of covalently linked bis(metallochlorins) in the range 1640-1660 cm(-1) are assigned to a special hydrogen-bonded ketone, C=O...HO(Me)...M (M = Zn, Mg), formed by a methanol bridge between the ketone of one chlorin and the central metal of the other one. In aggregates constituted by Zn or Mg chlorins which possess a hydroxyl group at position C-3, these modes are observed in the same frequency range. In these aggregates, the hydroxyl substituent mediates the interactions between the metal ion and the keto group. These assignments of the artificial Zn and Mg chlorin aggregates also serve for the elucidation of the supramolecular structures of the native bacteriochlorophyll c aggregates in photosynthetic antenna complexes (chlorosomes) of green bacteria. It is shown that all these aggregates have C=O...HO(R)...M bonds and pentacoordinated central metals with one axial ligand as the most important structural element required for aggregation and self-organization.