Organization of the pigment molecules in the chlorophyll a/ c light-harvesting complex of Pleurochloris meiringensis (xanthophyceae). Characterization with circular dichroism and absorbance spectroscopy

Abstract

By the aid of circular dichroism (CD), absorbance and fluorescence spectroscopy, we studied the molecular organization of the pigment molecules in cells, isolated chloroplasts and the chlorophyll a/ c light-harvesting complex (LHC) associated with photosystem II of the chlorphyll c-containing alga, Pleurochloris meiringensis. In cells and chloroplasts, similarly to higher plant chloroplasts, a (+) 693 nm CD band accompanied by a tail outside the absorbance indicated a long-range chiral organization of the chlorophyll molecules. The LHCII of these algae exhibited an intense negative CD band at 679 nm. However, in contrast to the chlorophyll a/ b LHCII of higher plants, where the intense, non-conservative (−)684 nm band has been shown to be associated with long-range chiral organization of the macro-aggregates, the intense, non-conservative (−)679 nm band in the chlorophyl a/ c LHC originated from the non-aggregated form of the complexes. In sharp contrast to the trimers or monomers of the chlorophyll a/ b LHCII, in the chlorophyll a/ c LHC no split excitonic CD bands could be detected in the red spectral region, thus CD provided no indication for the occurence of excitonic interactions among the Q Y transition dipoles of the chlorophyll molecules. Gaussian analysis of the absorbance and CD bands showed that the (−)679 nm CD signal is given rise by a small number of long-wavelength absorbing chlorophyll a molecules. Data obtained with LHC treated with low concentrations of acetone or digitonin strongly suggest a specific binding site of chlorophyll a to the protein, which upon binding a chlorophyll a molecule induces a bathochromic shift and asymmetry in the electronic structure of the molecule. These results and literature data strongly suggest that the organization of the pigment molecules in chlorophyll a/ c antenna complexes is significantly different from the organization of the chlorophyll a/ b complexes of green algae and higher plants.