Linear dichroism and circular dichroism in photosynthesis research

The efficiency of photosynthetic light energy conversion depends largely on the molecular architecture of the photosynthetic membranes. Linear- and circular-dichroism (LD and CD) studies have contributed significantly to our knowledge of the molecular organization of pigment systems at different levels of complexity, in pigment–protein complexes, supercomplexes, and their macroassemblies, as well as in entire membranes and membrane systems. Many examples show that LD and CD data are in good agreement with structural data; hence, these spectroscopic tools serve as the basis for linking the structure of photosynthetic pigment–protein complexes to steady-state and time-resolved spectroscopy. They are also indispensable for identifying conformations and interactions in native environments, and for monitoring reorganizations during photosynthetic functions, and are important in characterizing reconstituted and artificially constructed systems. This educational review explains, in simple terms, the basic physical principles, and theory and practice of LD and CD spectroscopies and of some related quantities in the areas of differential polarization spectroscopy and microscopy

Photothermal spectroscopy of bacteriochlorophyll-lipoprotein complexes

Photoacoustic spectra at room and 85 K temperatures as well as photothermal beam deflection spectra of bacteriochlorophyll-lipoprotein complexes from purple bacterium Chromatium minutissimum were measured. Spectra were compared and obtained differences were tentatively explained by various inertion of these two methods. Photothermal beam deflection method measure the heat which is generated in close surroundings of absorbing pigment molecule, whereas usage of more inert photoacoustic signal is averaged over contributions from various pigments located in a larger sample volume and therefore is similar to absorption spectra

Linear-dichroism measurements on the LH2 antenna complex of Rhodopseudomonas acidophila strain 10050 show that the transition dipole moment of the carotenoid rhodopin glucoside is not collinear with the long molecular axis

We have applied linear-dichroism experiments to determine the orientation of the transition dipole moment, corresponding to the main absorption band of the carotenoid, rhodopin glucoside, in the light-harvesting complex LH2 from Rhodopseudomonas acidophila strain 10050. The crystal structure of this LH2 complex is known and MO calculations have been performed that indicate that the transition dipole moment is 9.1degrees off axis from the extended pi-electron conjugated chain (Dolan, P. M.; Miller, D.; Cogdell, R. J.; Birge, R. R. Frank, H. A. J. Phys. Chem. B 2001, 105, 12134-12142). Our experimental results confirm that the transition dipole moment is not oriented along the long axis of the carotenoid molecule, and they are in excellent agreement with the MO calculations. However, they disagree with the linear-dichroism results of the same authors. Possible reasons for the discrepancy are discussed