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In bacterial photosynthesis, light energy is absorbed by a network of antenna pigment proteins and efficiently transferred to the photochemical reaction center where a charge separation takes place providing the free energy for subsequent chemical reactions. Most photosynthetic purple bacteria contain two types of antenna complexes: light-harvesting complex 1 (LH1) and light-harvesting complex 2 (LH2). It is known that the photochemical reaction center is closely associated with the LH1 complex while the LH2 complexes are arranged around the perimeter of LH1 in a two-dimensional structure. The transfer of energy from LH2 to LH1 and subsequently to the reaction center occurs in vivo on a time scale of 30–40 ps, i.e., very fast compared to the decay of an isolated LH2 which has a fluorescence lifetime of about 1 ns. As yet, there is no consensus about the details of the mechanisms of the energy-transfer process, and the full three-dimensional structure of the whole photosynthetic unit is still unknown. The great difficulty encountered when determining the various parameters that play a role in the description of the electronic structure of light-harvesting complexes and the process of energy transfer, is the fact that the optical absorption lines are inhomogeneously broadened as a result of heterogeneity in the ensemble of absorbing pigments. To circumvent this problem we have applied single-molecule detection schemes to study the pigment-protein complexes individually thereby avoiding ensemble averaging. Here we present an overview of our work on LH1 and LH2 from .

Solutions of bacteriochlorophyll (BChl) in both monomeric and aggregated forms, and also in solutions of pigment-protein complexes, were examined by resonance-Raman spectroscopy. The results permit us to report and discuss:

Visualization and mapping of induced current density using the ipsocentric distributed-origin method reveals the global ring currents in anionic, free-base and metallo- forms of porphin, chlorin and bacterio chlorin. All exhibit an essentially monocyclic circulation, but with characteristic additional bifurcations through unsaturated pyrrole units. The orbital model specific to the ipsocentric approach rationalizes the diatropic ring currents in all these systems in terms of magnetic activity of just a few electrons. The greater part of the magnetic response is ascribed to virtual transitions involving the two highest occupied pπ and two lowest unoccupied p orbitals, the same set invoked in the longstanding four-orbital model of the electronic spectra of these systems. A model, based on charge screening, accounts for the different degrees of bifurcation observed in the delocalisation pathways.

The photosystems of plants and photosynthetic bacteria are robust, adaptable and highly efficient light-harvesting/charge-separating systems that use chlorophylls (Chls) or bacteriochlorophylls (BChls) as photoreceptors. These solar energy collectors are built for increased light absorbance by providing environmental control over large pigment arrays with as little protein mass as possible. We wish to resolve the features of protein architecture necessary for effective light harvesting and electron transfer per se from other features of the natural protein some of which may be simply historical accidents.

Molecular assembly of bacteriochlorophyll (BChl ) or zinc-substituted bacteriochlorophyll ([Zn]-BChl ) into an artificial antenna complex using synthetic light-harvesting (LH) model polypeptides has been achieved.

Reconstitution and pigment exchange are two experimental techniques that have proven extremely useful to elucidate structure-function relationships in chlorophyll (Chl)-protein complexes. In reconstitution experiments the Chl-binding apoproteins, usually in their recombinant form, are folded in the presence of pigments to form pigment-protein complexes that are often virtually indistinguishable from their native counterparts. Since both the protein and the pigment building blocks in such an assembly kit can easily be modified, this approach serves to elucidate the functional significance of the structural elements modified. Pigment exchange can be viewed as a partial reconstitution: rather than completely taking a Chl-protein complex apart and then reconstituting it, only a limited number of pigments is dissociated and then restored. This, too, allows alteration of the pigments bound to specific positions and, thus, to learn more about the functional contribution of these particular pigment binding sites. Reconstitution and pigment exchange are complementary techniques in that some complexes are accessible to pigment exchange that cannot (yet) be reconstituted in vitro.

Protein design is used as an approach to further the understanding of membrane protein assembly, in particular, the assembly of transmembrane (bacterio)chlorophyll-binding pockets. Pigment-protein interaction motifs have been explored by (i) use of model proteins in which the native amino acid sequence in the pigment binding pockets are substantially altered and (ii) theoretical analyses of binding pockets of natural photosynthetic proteins. The bacteriochlorophyll binding sites of light harvesting complex 2, LH2, are replaced by model sites and expressed in vivo by the use of a modified strain. The artificial helices are shown to bind bacteriochlorophyll and support the assembly of light-harvesting active complexes in the native membrane. A H-bond, which has been introduced at the membrane embedded bacteriochlorophyll/helix model site, is shown to drive the assembly of the model LH2 complex. Statistical analyses of natural (bacterio)chlorophyll binding pockets reveal the presence of distinct interaction motifs at the pigment/helix interface. One example is intra-membrane H-bonding between the pigments and the surrounding polypeptides, particularly between the chlorophylls’ C13 keto carbonyl groups and the residues of the binding helices. With this system at hand, specific interaction motifs, such as the H-bonding motif, and their contribution to the folding and assembly can now be directly addressed within a highly simplified sequence context and in the polypeptides’ native membrane environement.

Chlorophylls (Chls) are the signature pigments of photosynthetic organisms and have several distinct functions, including photochemical activity and antenna function. Chls carry out reversible photochemical oxidations and reductions, which determine the basic mechanism of functioning of the photosynthetic reaction center (RC). The light-harvesting function of chlorophylls is based on their ability to absorb light over a wide spectral region. The variety of Chls (and bacteriochlorophylls) that are found in photosynthetic systems is formed by peripheral substitutions and reductions of the molecule’s macrocycle. Chls undergo specific adjustments of their absorption properties due to pigment-pigment and pigment-protein interactions. Complexes of RC supplemented with light-harvesting antennas and additional electron transfer proteins are known as photosynthetic units (PSU). Despite the structural variety of the chlorophyll-based photosynthetic antennas known to date, the principles of the antenna design conform to fulfilling its biological function, capturing the energy of sun and conveying it via excitation energy transfer to the reaction center. A number of very different strategies for energy collection, delivery to the RC and regulation can be found in different photosynthetic systems. Chls are also involved in photoprotective processes of excess excitation deactivation in carotenoid-Chl complexes and Chl clusters, accumulation of stress-related Chl binding polypeptides and specific photoprotective electron transfer pathways.

The function of photosynthetic light harvesting complexes (LHCs) comprises absorption and regulated excitation energy transfer (EET) to the photochemical reaction centers (RCs). Photosynthesizing organisms have developed a variety of LHCs but, apart from phycobilins in cyanobacteria and certain algae, use only two types of pigments, (bacterio)chlorophylls ((B)Chl) and carotenoids.

This chapter summarizes the most recent results of investigations on carotenoid-to-bacteriochlorophyll (Carto-BChl) singlet-energy transfer reactions. These reactions strongly depend on the number, , of conjugated double bonds in the Car molecule. Subpicosecond time-resolved electronic-absorption data were analyzed by singular-value decomposition, followed by global-fitting based on a new energy diagram of the singlet and triplet excited states and also on a new scheme of singlet-to-singlet and singlet-to-triplet electronic conversions of Car molecules. The mechanisms and efficiencies of Car-to-BChl singlet-energy transfer varies from one Car to another depending on the conjugation length (); in particular, a sudden drop in energy transfer when is increased from 10 to 11 has been explained by the energetics and dynamics of Car excited states.