Temperature dependent LH1 → RC energy transfer in purple bacteria Tch. tepidum with shiftable LH1-Q_y band: A natural system to investigate thermally activated energy transfer in photosynthesis

The native LH1-RC complex of the purple bacterium Thermochromatium (Tch.) tepidum has an ultra-red LH1-Q_y absorption at 915 nm, which can shift to 893 and 882 nm by means of chemical modifications. These unique complexes are a good natural system to investigate the thermally activated energy transfer process, with the donor energies different while the other factors (such as the acceptor energy, special pair at 890 nm, and the distance/relative orientation between the donor and acceptor) remain the same. The native B915-RC, B893-RC and B882-RC complexes, as well as the LH1-RC complex of Rhodobacter (Rba.) sphaeroides were studied by temperature-dependent time-resolved absorption spectroscopy. The energy transfer time constants, k_ET^{- 1}, are 65, 45, 46 and 45 ps at room temperature while 225, 58, 85, 33 ps at 77 K for the B915-RC, B893-RC, B882-RC and Rba. sphaeroides LH1-RC, respectively. The dependences of k_ET on temperature have different trends. The reorganization energies are determined to be 70, 290, 200 and 45 cm^{- 1}, respectively, by fitting k_ET vs temperature using Marcus equation. The activation energies are 200, 60, 115 and 20 cm^{- 1}, respectively. The influences of the structure (the arrangement of the 32 BChl a molecules) on k_ET are discussed based on these results, to reveal how the B915-RC complex accomplishes its energy transfer function with a large uphill energy of 290 cm^{- 1}.