Macromolecular organization of chlorophyll a in aggregated chlorophyll a/b protein complex as shown by circular dichroism at room and cryogenic temperatures

Self-assembly and structural-functional flexibility of oxygenic photosynthetic machineries: personal perspectives

This short review, with a bit of historical aspect and a strong personal bias and emphases on open questions, is focusing on the (macro-)organization and structural-functional flexibilities of the photosynthetic apparatus of oxygenic photosynthetic organisms at different levels of the structural complexity-selected problems that have attracted most my attention in the past years and decades. These include (i) the anisotropic organization of the pigment-protein complexes and photosynthetic membranes-a basic organizing principle of living matter, which can, and probably should be adopted to intelligent materials; (ii) the organization of protein complexes into chiral macrodomains, large self-assembling highly organized but structurally flexible entities with unique spectroscopic fingerprints-structures, where, important, high-level regulatory functions appear to 'reside'; (iii) a novel, dissipation-assisted mechanism of structural changes, based on a thermo-optic effect: ultrafast thermal transients in the close vicinity of dissipation of unused excitation energy, which is capable of inducing elementary structural changes; it makes plants capable of responding to excess excitation with reaction rates proportional to the overexcitation above the light-saturation of photosynthesis; (iv) the 3D ultrastructure of the granum-stroma thylakoid membrane assembly and other multilamellar membrane systems, and their remodelings-associated with regulatory mechanisms; (v) the molecular organization and structural-functional plasticity of the main light-harvesting complex of plants, in relation to their crystal structure and different in vivo and in vitro states; and (vi) the enigmatic role of non-bilayer lipids and lipid phases in the bilayer thylakoid membrane-warranting its high protein content and contributing to its structural flexibility.

Optical effects of sodium dodecyl sulfate treatment of the isolated light harvesting complex of higher plants

The light-harvesting complex (LHC) of higher plants isolated using Triton X-100 has been studied during its transformation into a monomeric form known as CPII. The change was accomplished by gradually increasing the concentration of the detergent, sodium dodecyl sulfate (SDS). Changes in the red spectral region of the absorption, circular dichroism (CD), and linear dichroism spectra occurring during this treatment have been observed at room temperature. According to a current hypothesis the main features of the visible region absorption and CD spectra of CPII can be explained reasonably successfully in terms of an exciton coupling among its chlorophyll (Chl) b molecules. We suggest that the spectral differences between the isolated LHC and the CPII may be understood basically in terms of an exciton coupling between the Chl b core of a given CPII unit and at least one of the Chla's of either the same or the adjacent CPII. We propose that this Chl a-Chl b coupling existing in LHC disappears upon segregation into CPII, probably as a result of a detergent-related overall rotation of the strongly coupled Chl b core which changes the relative orientations of the two types of pigments and thus the nature of their coupling.

The light-harvesting chlorophyll a/b protein acts as a torque aligning chloroplasts in a magnetic field

Displacement of particles from the purified light-harvesting chlorophyll a/b protein aggregate (LHC) was studied in magnetic fields of various strengths (0 to 1.6 T) by polarized fluorescence measurements. Macromolecular aggregates of LHC have a considerable magnetic susceptibility which enables the particles to rotate and align with their nematic axes parallel with H. As LHC is embedded in a transmembrane direction thylakoids should align perpendicular to H, the mode of alignment experimentally observed in thylakoids. The value of the magnetic susceptibility could be estimated by relating it to the integral susceptibility of the chlorophyll molecules in LHC. The fitting of this value with the field strength dependency of the fluorescence polarization ratio (FP) revealed a relationship between the LHC content of various photosynthetic membranes and their capacity for alignment, which suggested that LHC might be the torque ordering chloroplasts in a magnetic field.

Antenna chlorophyll a complexes in mutant and developing barley

The chlorophyll a antenna of photosystems I and II were each isolated after detergent treatment by gel electrophoresis or sucrose gradient centrifugation from a b-less mutant of barley grown in daylight and from wildtype barley developed in intermittent light. We identified each fraction by both its electrophoretic position and PS I activity (P700 content) in the case of the mutant, and by both PS I and PS II activity (DCIP reduction from DPC) in the light-limited plants. The proportion of Chl a in each photosystem was estimated from the amount in each gel or sucrose gradient band, and from addition of the areas under the absorption spectra (650-710 nm) of each fraction to match the spectrum of the solubilized thylakoids. The latter method was possible because the spectrum (77 K) of each fraction was unique; in the mutant about 70% of chlorophyll is associated with PS I and 30% with PS II. In the light-limited plants, the reverse is true with nearly 70% associated with PS II. RESOL analyses of both absorption and fluorescence emission spectra of all isolated fractions indicated an abnormal arrangement of antenna chlorophyll molecules in the light-limited, developing membranes even though their reaction centers are fully functional.

The detergent and salt effect on the light-harvesting chlorophyll a/b complex from green plants

The light harvesting accessory pigment-protein complex (LHC) with a chlorophyll (Chl) a/b ratio of 1.2 was isolated by treating pea chloroplasts with Triton X-100. The LHC was used to investigate the action of ionic (sodium dodecyl sulfate) and non-ionic (Triton X-100) detergents. By optical methods (absorption and fluorescence spectra, measurements of fluorescence yield, phi, and lifetime, tau) two successive stages of the process were demonstrated, namely (1) interaction between detergent monomers and proteins and (2) solubilization of pigments into detergent micelles, which is facilitated by the presence of salts. The concentration ranges, characteristic of these stages, differ by 1.5-2 orders of magnitude for SDS, but slightly overlap for Triton X-100. At the second stage, certain changes occur in LHC absorption and fluorescence spectra. Several stable states of the LHC were established: (1) an aggregated state formed the presence of 10 mM MgSO4 with tau approximately 0.6 ns; (2) the dialyzed LHC with tau approximately 0.9 ns; (3) the states of the LHC in detergent solution with tau approximately 2.3, 2.9, 3.4 ns; (4) a 30 kilodalton monomer obtained by SDS-polyacrylamide gel electrophoresis with tau approximately 4.1 ns. The fluorescence parameters of the LHC states were compared with those of Chl a in detergent micelles (for the micelles tau = 5.6-6.0 ns). The tau/phi ratio (the criterion for emission heterogeneity) for the LHC in the absence of a detergent was shown to be higher at least by a factor of 3.5 than that for Chl a in the presence of a detergent. Successive additions of the detergent to the LHC cause gradual decrease in the tau/phi ratio, and for the LHC monomer it reaches practically the same value as for Chl a in detergent micelles. The results are discussed on the basis of the data obtained previously, It is suggested that in vivo LHCs do not form such aggregates as in water solution without a detergent.

Spectral analysis of chlorophyll-protein complexes from higher plant chloroplasts

A spectral analysis of chlorophyll-protein complexes was carried out to gain information about the state of chlorophyll in vivo. A light-harvesting chlorophyll a/b protein complex and a Photosystem I complex were isolated from pea and from wheat chloroplasts by treatment with 0.5% Triton and centrifugation in a sucrose gradient. Resolution of absorption spectra (89K) of these fractions showed that their forms of chlorophyll were not altered by the isolation procedure. However, because the sum of the spectra of the fractions had a different shape from the chloroplast, it may be assumed that a third chlorophyll-protein complex was lost or changed in terms of the state of its chlorophyll. The spectrum of this missing chlorophyll was calculated and found to have a maximum near 683 nm. Circumstantial evidence indicates that this calculated spectrum may represent the native absorption of antenna chlorophyll a-protein of Photosystem II. The proportionality between the major absorbing forms of chlorophyll observed by curve analysis of different fractions suggests that the 660 and 678 nm forms may be the result of exciton interaction. The addition of a very small, narrow 675 nm band caused a very large improvement in fitting the spectrum of the antenna chlorophyll a/b protein with component bands, but not in Photosystem I spectra. A direct comparison of curve resolution with fourth derivative analysis shows the advantages of the former for studying the states of chlorophyll in vivo.