The light-harvesting core antenna (LH1) and the reaction centre (RC) of purple photosynthetic bacteria form a supramolecular complex (LH1-RC) to use sunlight energy in a highly efficient manner. Here we report the first near-atomic structure, to our knowledge, of a LH1-RC complex, namely that of a Ca(2+)-bound complex from Thermochromatium tepidum, which reveals detailed information on the arrangement and interactions of the protein subunits and the cofactors. The RC is surrounded by 16 heterodimers of the LH1 αβ-subunit that form a completely closed structure. The Ca(2+) ions are located at the periplasmic side of LH1. Thirty-two bacteriochlorophyll and 16 spirilloxanthin molecules in the LH1 ring form an elliptical assembly. The geometries of the pigment assembly involved in the absorption characteristics of the bacteriochlorophyll in LH1 and excitation energy transfer among the pigments are reported. In addition, possible ubiquinone channels in the closed LH1 complex are proposed based on the atomic structure.

Light-harvesting complex 1 (LH1) and the reaction centre (RC) form a membrane-protein supercomplex that performs the primary reactions of photosynthesis in purple photosynthetic bacteria. The structure of the LH1-RC complex can provide information on the arrangement of protein subunits and cofactors; however, so far it has been resolved only at a relatively low resolution. Here we report the crystal structure of the calcium-ion-bound LH1-RC supercomplex of Thermochromatium tepidum at a resolution of 1.9 Å. This atomic-resolution structure revealed several new features about the organization of protein subunits and cofactors. We describe the loop regions of RC in their intact states, the interaction of these loop regions with the LH1 subunits, the exchange route for the bound quinone Q with free quinone molecules, the transport of free quinones between the inside and outside of the LH1 ring structure, and the detailed calcium-ion-binding environment. This structure provides a solid basis for the detailed examination of the light reactions that occur during bacterial photosynthesis.

While the majority of the photochemical states and pathways related to the biological capture of solar energy are now well understood and provide paradigms for artificial device design, additional low-energy states have been discovered in many systems with obscure origins and significance. However, as low-energy states are naively expected to be critical to function, these observations pose important challenges. A review of known properties of low energy states covering eight photochemical systems, and options for their interpretation, are presented. A concerted experimental and theoretical research strategy is suggested and outlined, this being aimed at providing a fully comprehensive understanding.

The light-harvesting-reaction center complex (LH1-RC) from the purple phototrophic bacterium Thiorhodovibrio strain 970 exhibits an LH1 absorption maximum at 960 nm, the most red-shifted absorption for any bacteriochlorophyll (BChl) a-containing species. Here we present a cryo-EM structure of the strain 970 LH1-RC complex at 2.82 Å resolution. The LH1 forms a closed ring structure composed of sixteen pairs of the αβ-polypeptides. Sixteen Ca ions are present in the LH1 C-terminal domain and are coordinated by residues from the αβ-polypeptides that are hydrogen-bonded to BChl a. The Ca2+-facilitated hydrogen-bonding network forms the structural basis of the unusual LH1 redshift. The structure also revealed the arrangement of multiple forms of α- and β-polypeptides in an individual LH1 ring. Such organization indicates a mechanism of interplay between the expression and assembly of the LH1 complex that is regulated through interactions with the RC subunits inside.

Carotenoids (Cars) in bacterial photosynthesis are known as accessory light harvesters and photoprotectors. Recently, the singlet fission (SF) reaction initiated by Car photoabsorption has been recognized to be an effective excitation deactivation channel disfavoring the light harvesting function. Since the SF reaction and the triplet sensitization reaction underlying photoprotection both yield triplet excited state Cars (Car*), their contribution to the overall Car* photoproduction are difficult to disentangle. To tackle this problem, we resorted to the triplet excitation profiles (TEPs), i.e., the actinic spectra of the overallCar* photoproduction. The TEPs combined with the conventional fluorescence excitation spectra allowed us to extract the neat SF contribution, which can serve as a spectroscopic measure for the SF reactivity. This novel spectroscopic strategy was applied to analyze the light harvesting complexes (LHs) from Tch. tepidum and Rba. sphaeroides 2.4.1. The results unambiguously showed that the SF reaction of Cars proceeds with an intramolecular scheme, even in the case of LH1-RC from Rba. sphaeroides 2.4.1 likely binding a secondary pool of Cars. Regarding the SF-reactivity, the geometric distortion in the conjugated backbone of Cars was shown to be the structural determinant, while the length of the Car conjugation was suggested to be relevant to the effective localization of the geminate triplets to avoid being annihilated. The SF reaction scheme and structure-activity relationship revealed herein will be useful not only in deepening our understanding of the roles of Cars in photosynthesis, but also in enlightening the applications of Cars in artificial light conversion systems.