Polarized fluorescence measurements on ordered photosynthetic antenna complexes: Chlorosomes of Chloroflexus aurantiacus and B800-B850 antenna complexes of Rhodobacter sphaeroides

Abstract

We have used a new and relatively easy approach to study the pigment-organization in chlorosomes from the photosynthetic bacterium Chloroflexus aurantiacus and in B800-850 antenna complexes of the photosynthetic purple bacterium Rhodobacter sphaeroides. These particles were embedded in compressed and uncompressed gels and the polarized fluorescence was determined in a 90 degrees setup. Assuming both a rotational symmetric distribution of the particles in the gel and of the transition dipole moments in the particles, the order parameters <P(2)> and <P(4)>, describing the orientation of the symmetry axis of the particles with respect to the direction of gel expansion can be determined. Moreover, the direction parameters, describing the orientation of the absorption and emission dipole moments with respect to the symmetry axis of the particles can be obtained.The value of <P(2)> is essential for quantitative interpretation of linear dichroism measurements and usually it is estimated from theoretical approaches, which may lead to incorrect results. For the rod-like chlorosomes the value of <P(2)> appears to be the same as predicted by the theoretical approach of Ganago, A. O., M. V. Fok, I. A. Abdourakhmanov, A. A. Solov'ev, and Yu. E. Erokhin (1980. Mol. Biol. [Mosc.]. 14:381-389). The agreement with linear dichroism results, analyzed with this theoretical approach shows that the transition dipole moments are indeed in good approximation distributed in a rotationally symmetric way around the long axis of the chlorosomes. Moreover, it appears those BChl c molecules, which fluoresce, are oriented in the same way with respect to the symmetry axis as the rest of these pigments, with the dipole moments close to parallel to the long axis.The B800-850 complexes appear to orient like discs, whereas the transition dipoles of the BChl a 800- and 850-nm bands are oriented almost perpendicular to the symmetry axis. These findings are in agreement with the minimal model for these complexes proposed by Kramer, H. J. M., R. van Grondelle, C. N. Hunter, W. H. J. Westerhuis, and J. Amesz (1984. Biochim. Biophys. Acta. 156-165).The amount of orientation of the particles appears to vary for different gels and it is lower than predicted by the theory of Ganago et al., showing that application of their approach for these particles leads to incorrect interpretations.The approach that is used in this study allows determination of orientations of those dipole moments, which transfer their excitation energy to the fluorescing species, in contrast to linear dichroism measurements, where the orientations of all absorbing dipole moments are studied. For the polarized fluorescence measurements, the amount of orientation of the particles is determined experimentally, whereas for linear dichroism this amount has to be estimated from theoretical models. The value of <P(2)> that can be determined from the fluorescence measurements can, however, also be used for a quantitative interpretation of the linear dichroism results.