Organization of the pigment molecules in the thylakoids and the chlorophyll a/c light-harvesting complex of a xanthophyte alga, Pleurochloris meiringensis. A linear dichroism study

Abstract

We have studied the orientation of the absorption transition dipoles of the pigment molecules in thylakoid membranes, in the isolated core complex of photosystem I (CCI) and in the chlorophyll a/c light-harvesting complex of photosystem II (Chl a/c LHC) of the xanthophyte alga Pleurochloris meiringensis. In thylakoids, linear dichroism (LD) spectra reveal two distinct orientations of the Q y transition dipoles of the Chl a molecules: the long-wavelength Chl a molecules tend to lie in the plane of the thylakoid membrane, while those at shorter wavelengths tilt out of the membrane plane. Our data show that in CCl, the orientation of Q y dipoles of Chl a, and the overall orientation pattern of all transition dipoles in the visible region, are very similar to those in higher-plant photosystem I particles. The Chl a molecules with their Q y dipoles tilting out of the membrane plane are associated with the LHC, and constitute a substantial part of the Chl a population. The majority of the carotenoid molecules of LHC also tend to tilt out of the membrane plane. These data further corroborate our earlier conclusion that the molecular organization of thylakoid membranes and most particularly of the Chl a/c LHC differs significantly from the organization of higher-plant thylakoids and the main Chl a/b complexes, respectively.