Structural and Functional Hierarchy in Photosynthetic Energy Conversion—from Molecules to Nanostructures

Basic principles of structural and functional requirements of photosynthetic energy conversion in hierarchically organized machineries are reviewed. Blueprints of photosynthesis, the energetic basis of virtually all life on Earth, can serve the basis for constructing artificial light energy-converting molecular devices. In photosynthetic organisms, the conversion of light energy into chemical energy takes places in highly organized fine-tunable systems with structural and functional hierarchy. The incident photons are absorbed by light-harvesting complexes, which funnel the excitation energy into reaction centre (RC) protein complexes containing redox-active chlorophyll molecules; the primary charge separations in the RCs are followed by vectorial transport of charges (electrons and protons) in the photosynthetic membrane. RCs possess properties that make their use in solar energy-converting and integrated optoelectronic systems feasible. Therefore, there is a large interest in many laboratories and in the industry toward their use in molecular devices. RCs have been bound to different carrier matrices, with their photophysical and photochemical activities largely retained in the nano-systems and with electronic connection to conducting surfaces. We show examples of RCs bound to carbon-based materials (functionalized and non-functionalized single- and multiwalled carbon nanotubes), transitional metal oxides (ITO) and conducting polymers and porous silicon and characterize their photochemical activities. Recently, we adapted several physical and chemical methods for binding RCs to different nanomaterials. It is generally found that the P(+)(Q(A)Q(B))(−) charge pair, which is formed after single saturating light excitation is stabilized after the attachment of the RCs to the nanostructures, which is followed by slow reorganization of the protein structure. Measuring the electric conductivity in a direct contact mode or in electrochemical cell indicates that there is an electronic interaction between the protein and the inorganic carrier matrices. This can be a basis of sensing element of bio-hybrid device for biosensor and/or optoelectronic applications

Bacteriophytochromes in anoxygenic photosynthetic bacteria

Since the first discovery of a bacteriophytochrome in Rhodospirillum centenum, numerous bacteriophytochromes have been identified and characterized in other anoxygenic photosynthetic bacteria. This review is focused on the biochemical and biophysical properties of bacteriophytochromes with a special emphasis on their roles in the synthesis of the photosynthetic apparatus

Interactions between photosynthesis and respiration in facultative anoxygenic phototrophs

The respiratory and photosynthetic electron transport chains of the two facultative phototrophs Rhodobacter (Rb.) sphaeroides and Rb. capsulatus are arranged in such a way to be spatially segregated in separate regions of the internal membrane system (CM and ICM). The CM part contains the majority of the oxidative redox components which are therefore in redox non-equilibrium with most of the photochemical RCs; conversely, the major part of the photosynthetic carriers (including RCs, Cyt c2 or Cytcy and Cyt bc1 complex) are located in the ICM part of the membrane. This spatial level of organization is paralleled by an arrangement of these photosynthetic elements in supramolecular complexes in order to allow a fast and efficient cyclic electron transfer by limiting the diffusion of the reactants. However, these two levels of arrangement are not present in all types of photosynthetic bacteria. Indeed, species like Blastochloris viridis or Rubrivivax gelatinosus, contain a large excess of RCs over the Cyt bc1 complexes so that the formation of supercomplexes is stoichiometrically hindered. Further, in obligate aerobic phototrophs such as for example Roseobacter denitrificans, the Qa is fully reduced under anaerobic conditions and this might be due to the lack of both quinol oxidase and ICM system. The expression of photosynthetic and respiratory components is controlled by the oxygen tension and by the redox state of the system. This genetic coordination mechanism does not necessarily require a direct interaction of the two sets of components in line with their different spatial membrane location. The signals to which the system responds originate from either specific respiratory components, e.g. cbb3 oxidase, as in the case of oxygen sensing, or from redox carriers involved in both oxidative and photosynthetic ET, as for redox sensing. Although the genetic control of the supramolecular arrangement of the ETCs is, at present, largely undefined, the working scheme presented here, suggests a tentative framework of genetic regulatory connections in Rb. capsulatus and/or Rb. sphaeroides

Orientation of the hemes of high potential cytochromes relative to photosynthetic membranes, as shown by the linear dichroism of oriented preparations

The orientations of high potential cytochromes with respect to photosynthetic membranes was investigated in spinach chloroplasts and in Rhodopseudomonas viridis. The general approach consists of detection with polarized light of photoinduced absorbance changes related to the oxidation of the cytochromes. The orientation of cytochrome c/sub 558/ was measured at room temperature in chromatophores and whole cells of Rp. viridis, oriented on glass slides and in a magnetic field respectively. The orientation of cytochrome b/sub 559/ of green plants was detected at 77K in magnetically oriented chloroplasts. In both cases the dichroic ratio for the ..cap alpha.. band shows that the heme plane makes an angle greater than 35/sup 0/ with the membrane plane. Moreover, the dichroic ratio is not constant throughout the ..cap alpha.. and ..beta.. bands, for both cytochrome c/sub 558/ and b/sub 559/. Linear dichroism spectra of oriented pure horse heart cytochrome c and cytochrome c/sub 2/ of Rhodopseudomonas sphaeroides in stretched polyvinyl alcohol films show that the variations of the dichroic ratio in the ..cap alpha.. and ..beta.. bands can be explained by the occurrence of x and y polarized transitions absorbing at slightly different wavelengths