Resonance Raman Spectroscopic Evidence for the Identity of the Bacteriochlorophyll c Organization in Protein-Free and Protein-Containing Chlorosomes from Chloroflexus auvantiacus

Abstract

Protein-free and protein-containing chlorosomes from Chloroflexus aurantiacus, strain Ok-70 fl, were studied by resonance Raman (RR) spectroscopy. Both preparations gave the same spectra of the bacteriochlorophyll c (BChl c) chromophores in the range of 1200-1750 cm^-1. This strongly corroborates previous evidence [Griebenow et al., Z. Naturforsch. 45c, 823-828 (1990), and references therein] that the three-dimensional structure of the antenna complexes is not determined by direct interaction with protein but rather is due to BChl c selforganization. The analysis of the coordination-sensitive marker bands of the chlorin macrocycle reveals a mixed six- and fivefold ligation of the Mg ion. Based on two C = O stretching vibrations originating from a free and a Mg-bound C-9 keto group, it is concluded that only in the six-coordinated state the keto group serves as an axial ligand to the Mg ion of a neighbouring chlorin. The second permanently bound axial ligand is attributed to the C-2a hydroxyl group.