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RESEARCH PRODUCT

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

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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.