We report on an experimental study of the self-organization and phase behavior of hairy-rod -conjugated branched side-chain polyfluorene, poly͓9,9-bis͑2-ethylhexyl͒-fluorene-2,7-diyl͔-i.e., poly͓2,7-͑9,9-bis͑2-ethylhexyl͒fluorene͔ ͑PF2/6͒-as a function of molecular weight ͑M n ͒. The results have been compared to those of phenomenological theory. Samples for which M n = 3 -147 kg/ mol were used. First, the stiffness of PF2 / 6, the assumption of the theory, has been probed by small-angle neutron scattering in solution. Thermogravimetry has been used to show that PF2 / 6 is thermally stable over the conditions studied. Second, the existence of nematic and hexagonal phases has been phenomenologically identified for lower and higher M n ͑LMW, M n Ͻ M n * and HMW, M n Ͼ M n * ͒ regimes, respectively, based on free-energy argument of nematic and hexagonal hairy rods and found to correspond to the experimental x-ray diffraction ͑XRD͒ results for PF2 / 6. By using the lattice parameters of PF2 / 6 as an experimental input, the nematic-hexagonal transition has been predicted in the vicinity of glassification temperature ͑T g ͒ of PF2 / 6. Then, by taking the orientation parts of the free energies into account the nematic-hexagonal transition has been calculated as a function of temperature and M n and a phase diagram has been formed. Below T g of 80°C only ͑frozen͒ nematic phase is observed for M n Ͻ M n * =10 4 g / mol and crystalline hexagonal phase for M n Ͼ M n * . The nematic-hexagonal transition upon heating is observed for the HMW regime depending weakly on M n , being at 140-165°C for M n Ͼ M n * . Third, the phase behavior and structure formation as a function of M n have been probed using powder and fiber XRD and differential scanning calorimetry and reasonable semiquantitative agreement with theory has been found for M n ജ 3 kg/ mol. Fourth, structural characteristics are widely discussed. The nematic phase of LMW materials has been observed to be denser than high-temperature nematic phase of HMW compounds. The hexagonal phase has been found to be paracrystalline in the ͑ab0͒ plane but a genuine crystal meridionally. We also find that all these materials including the shortest 10-mer possess the formerly observed rigid five-helix hairy-rod molecular structure.

Chlorosomes of green photosynthetic bacteria constitute the most efficient light harvesting complexes found in nature. In addition, the chlorosome is the only known photosynthetic system where the majority of pigments (BChl) is not organized in pigment-protein complexes but instead is assembled into aggregates. Because of the unusual organization, the chlorosome structure has not been resolved and only models, in which BChl pigments were organized into large rods, were proposed on the basis of freeze-fracture electron microscopy and spectroscopic constraints. We have obtained the first high-resolution images of chlorosomes from the green sulfur bacterium Chlorobium tepidum by cryoelectron microscopy. Cryoelectron microscopy images revealed dense striations approximately 20 A apart. X-ray scattering from chlorosomes exhibited a feature with the same approximately 20 A spacing. No evidence for the rod models was obtained. The observed spacing and tilt-series cryoelectron microscopy projections are compatible with a lamellar model, in which BChl molecules aggregate into semicrystalline lateral arrays. The diffraction data further indicate that arrays are built from BChl dimers. The arrays form undulating lamellae, which, in turn, are held together by interdigitated esterifying alcohol tails, carotenoids, and lipids. The lamellar model is consistent with earlier spectroscopic data and provides insight into chlorosome self-assembly.

The structure of lyotropic liquid crystalline or gellike phases formed in cationic starch (CS)/ anionic surfactant/water systems in the temperature range 25-80 °C has been investigated by smallangle X-ray scattering. The surfactants were sodium dodecyl sulfate (SDS), sodium decanoate (NaDe), sodium dodecanoate (NaDod), sodium palmitate (NaPal), sodium oleate (NaOl), and sodium erucate (NaEr). The phases were in equilibrium with aqueous solutions at 60 °C and contained 15-25 wt % of CS and 10-30 wt % of surfactant, depending on the charge density of the CS and the chain length of the surfactant. The phases consist of CS/surfactant aggregates arranged in long-range structures similar to lyotropic mesophases formed by the pure surfactants alone, but they separate from aqueous solutions at much lower surfactant concentrations. When the charge density of the CS is low or the surfactant hydrocarbon chain is short, cubic or hexagonal phases are formed. As expected, the formation of lamellar phase is promoted by increasing these parameters. Temperature affects the stability of the phases and their structure. At high temperatures the long-range order breaks down, and the phases are akin to concentrated micellar CS/surfactant solutions.

The green filamentous bacterium Chloroflexus aurantiacus employs chlorosomes as photosynthetic antennae. Chlorosomes contain bacteriochlorophyll aggregates and are attached to the inner side of a plasma membrane via a protein baseplate. The structure of chlorosomes from C. aurantiacus was investigated by using a combination of cryo-electron microscopy and X-ray diffraction and compared with that of Chlorobi species. Cryo-electron tomography revealed thin chlorosomes for which a distinct crystalline baseplate lattice was visualized in high-resolution projections. The baseplate is present only on one side of the chlorosome, and the lattice dimensions suggest that a dimer of the CsmA protein is the building block. The bacteriochlorophyll aggregates inside the chlorosome are arranged in lamellae, but the spacing is much greater than that in Chlorobi species. A comparison of chlorosomes from different species suggested that the lamellar spacing is proportional to the chain length of the esterifying alcohols. C. aurantiacus chlorosomes accumulate larger quantities of carotenoids under high-light conditions, presumably to provide photoprotection. The wider lamellae allow accommodation of the additional carotenoids and lead to increased disorder within the lamellae.Chlorosomes (5, 13) are light-harvesting complexes found in three different phyla of photosynthetic bacteria. Chloroflexus aurantiacus belongs to the filamentous anoxygenic phototrophs (green nonsulfur bacteria) comprising members of the phylum Chloroflexi. All members of the green sulfur bacteria (phylum Chlorobi) contain chlorosomes. Very recently, a phototropic chlorosome-containing organism was found in the phylum Acidobacteria (9).Chlorosomes are oblong bodies attached to the inner side of the cytoplasmic membrane. A unique property of chlorosomes is that their main pigment, bacteriochlorophyll (BChl) c, d, or e, is organized in the form of an aggregate. A similar selfassembled aggregate can form in the absence of proteins and exhibits spectral and excitonic properties similar to those of pigments in the native chlorosomes (for a review, see reference 3). The BChl aggregates were suggested to form lamellar structures in chlorosomes of green sulfur bacteria with lamellar spacing between 2 and 3 nm, depending on the main BChl (BChl c or e) and the prevailing esterifying alcohol (38, 39). In this model, the lamellar layers are maintained by nonspecific hydrophobic interactions of the interdigitated esterifying alcohols, while the in-layer arrangement is mediated through specific interactions between the stacked chlorin rings. In BChl c-containing chlorosomes of Chlorobaculum tepidum (formerly Chlorobium tepidum), the lamellar system (spacing, ϳ2 nm) often remains parallel for the whole length of the chlorosome (33,38). In Chlorobaculum tepidum the lamellae exhibit considerable curvature, which was initially attributed to undulation (38), but recent end-on micrographs revealed a variety of curved lamellar structures, such as lamellar tubules or multilayered ...

Chlorosomes are the main light harvesting complexes of green photosynthetic bacteria. Recently, a lamellar model was proposed for the arrangement of pigment aggregates in Chlorobium tepidum chlorosomes, which contain bacteriochlorophyll (BChl) c as the main pigment. Here we demonstrate that the lamellar organization is also found in chlorosomes from two brown-colored species (Chl. phaeovibrioides and Chl. phaeobacteroides) containing BChl e as the main pigment. This suggests that the lamellar model is universal among green sulfur bacteria. In contrast to green-colored Chl. tepidum, chlorosomes from the brown-colored species often contain domains of lamellar aggregates that may help them to survive in extremely low light conditions. We suggest that carotenoids are localized between the lamellar planes and drive lamellar assembly by augmenting hydrophobic interactions. A model for chlorosome assembly, which accounts for the role of carotenoids and secondary BChl homologs, is presented.