Polarized fluorescence measurements on ordered photosynthetic antenna complexes

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.

Structural Information on the Light-Harvesting Complex II of Green Plants That Can Be Deciphered from Polarized Absorption Characteristics

“…The average reduced LD over the wavelength interval from λ 1 to λ 2 (LD r ) is then defined according to and its value can vary between -0.5 and 1.0. 13 It should be noted that the given definition of LD r differs in two ways from the generally used definition of the reduced LD. The latter is defined per wavelength and can be obtained from the above given definition by omitting the integration.…”

Photosynthetic antennae. Photosynthetic light‐harvesting

“…Since this transition is polarized perpendicular to the plane of the ring or membrane, the polarization of the emission upon excitation in the main band would be -0.2, in contrast to the observed value of 0.1 [5, 22,261. Also the low-temperature fluorescence of oriented LH2 is polarized in the plane of the ring [33]. Furthermore, the fluorescence quantum yield should decrease rapidly upon lowering the temperature, and should be close to zero at 4.2K.…”

Anisotropic distribution of emitting transition dipoles in chlorosome from Chlorobium tepidum: fluorescence polarization anisotropy study of single chlorosomes

“…aurantiacus could be accounted for by assuming a uniaxial orientation distribution of transition dipoles within a single chlorosome, in which the condition l 1 [ l 2 = l 3 (here l i : |l i |) is satisfied (Shibata et al 2007). According to the analysis, we evaluated the value l 3 /l 2 /l 1 = 0.4/0.4/1, which was not available by the linear dichroism (LD) measurements of aligned chlorosomes due to incomplete alignment (van Dorssen et al 1986;van Amerongen et al 1988van Amerongen et al , 1991Matsuura et al 1993). Since the experiment was conducted at cryogenic temperature, the orientation distribution for the lowest excited state of (BChl-c) n was evaluated.…”