Spectroscopy and Dynamics of Excitation Transfer and Trapping in Purple Bacteria. In: Hunter CN, Daldal F, Thurnauer MC, Beatty JT, editors. The Purple Phototrophic Bacteria. Dordrecht: Springer Netherlands; 2009. pp. 231-52. [DOI: 10.1007/978-1-4020-8815-5_13]

Structural Basis for the Unusual Qy Red-Shift and Enhanced Thermostability of the LH1 Complex from Thermochromatium tepidum

While the majority of the core light-harvesting complexes (LH1) in purple photosynthetic bacteria exhibit a Q_y absorption band in the range of 870-890 nm, LH1 from the thermophilic bacterium Thermochromatium tepidum displays the Q_y band at 915 nm with an enhanced thermostability. These properties are regulated by Ca²⁺ ions. Substitution of the Ca²⁺ with other divalent metal ions results in a complex with the Q_y band blue-shifted to 880-890 nm and a reduced thermostability. Following the recent publication of the structure of the Ca-bound LH1-reaction center (RC) complex [Niwa, S., et al. (2014) Nature 508, 228], we have determined the crystal structures of the Sr- and Ba-substituted LH1-RC complexes with the LH1 Q_y band at 888 nm. Sixteen Sr²⁺ and Ba²⁺ ions are identified in the LH1 complexes. Both Sr²⁺ and Ba²⁺ are located at the same positions, and these are clearly different from, though close to, the Ca²⁺-binding sites. Conformational rearrangement induced by the substitution is limited to the metal-binding sites. Unlike the Ca-LH1-RC complex, only the α-polypeptides are involved in the Sr and Ba coordinations in LH1. The difference in the thermostability between these complexes can be attributed to the different patterns of the network formed by metal binding. The Sr- and Ba-LH1-RC complexes form a single-ring network by the LH1 α-polypeptides only, in contrast to the double-ring network composed of both α- and β-polypeptides in the Ca-LH1-RC complex. On the basis of the structural information, a combined effect of hydrogen bonding, structural integrity, and charge distribution is considered to influence the spectral properties of the core antenna complex.

Förster energy transfer theory as reflected in the structures of photosynthetic light-harvesting systems

Förster’s theory of resonant energy transfer underlies a fundamental process in nature, namely the harvesting of sunlight by photosynthetic life forms. The theoretical framework developed by Förster and others describes how electronic excitation migrates in the photosynthetic apparatus of plants, algae, and bacteria from light absorbing pigments to reaction centers where light energy is utilized for the eventual conversion into chemical energy. The demand for highest possible efficiency of light harvesting appears to have shaped the evolution of photosynthetic species from bacteria to plants which, despite a great variation in architecture, display common structural themes founded on the quantum physics of energy transfer as described first by Förster. Herein, Förster’s theory of excitation transfer is summarized, including recent extensions, and the relevance of the theory to photosynthetic systems as evolved in purple bacteria, cyanobacteria, and plants is demonstrated. Förster’s energy transfer formula, as used widely today in many fields of science, is also derived.