Energy transfer within the isolated B875 light‐harvesting pigment‐protein complex of Rhodobacter sphaeroides at 77 K studied by picosecond absorption spectroscopy

Abstract

The energy transfer dynamics at 77 K within detergent solubilized and purified preparations of the B875 pigment-protein complex of Rhodobacter sphaeroides have been investigated by picosecond absorption spectroscopy. Isotropic absorption recovery and decay of induced absorption anisotropy provide clear evidence that B875 is inhomogeneous in these preparations. We interpret the results as fast (r = 15 + 5 ps) energy transfer from the major BChl 875 pigments to a minor pigment pool, B896. The excited state of B896 decays with a time constant of 650 + 50 ps. We suggest that B896 is intrinsic to B875 complexes and exists in a highly organized state, close to the reaction center. In the intact membrane, B896 may concentrate excitation energy in the vicinity of the reaction center special pair, thereby increasing the efficiency of the final energy transfer step.Antenna bacteriochlorophyll-protein complex; Bacterial photosynthesis; Energy transfer; (Rhodobacter sphaeriodes)

Characterization of excitation energy trapping in photosynthetic purple bacteria at 77 K

“…Since the solvent is mostly glycerol we have used n = 1.5 as the refractive in-dex [21]. The rate of energy transfer is kET = (37 × 10 -12 S) -1 --(650 x 10 -12 s) -1 where 650 x 10 -12 is the excited-state lifetime of BChl896 in the absence of trapping as measured in the isolated B875 complex at 77 K [22]. When the steady-state fluorescence spectrum of the B875/896 complex is used to obtain f(~) of the energy donor (B896) for the calculation of R0, there is also a small contribution from B875 fluorescence.…”

“…BChl896 is assumed to have the Qy transition parallel to the membrane plane [5], and judging from measurements of the anisotropy decay at 77 K [2,22], which suggest transfer among differently oriented BCh1896 molecules, we assume, for simplicity, the BChl896 transition dipole moments to have a circularly degenerate orientation in the plane. This allows us to calculate a value of 1,2 for x 2.…”

Exciton states of the antenna and energy trapping by the reaction center

“…Several experimental facts obtained with the antenna of purple photosynthetic bacteria such as the shape of picosecond absorbance difference spectra Razjivin et al 1982;Nuijs et al 1985;Danielius et al 1989), the absorption anisotropy kinetics (Sundstrom et al 1986;Bergstrom et al 1988), the anomalously high value of absorption changes per absorbed quantum (Novoderezhkin and Razjivin, 1993b) are difficult to interpret using traditional models based on the assumption of localized exciton migration Razjivin et al 1982;Van Grondelle et al 1988;Hunter et al 1990;Sundstrom et al 1986;Pullerits and Freiberg 1991;Timpmann et al 1991;Visschers et al 1993;Van Mourik et al 1992;Pullerits et al 1994). …”

The cooperativity phenomena in a pigment‐protein complex of light‐harvesting antenna revealed by picosecond absorbance difference spectroscopy

“…Spectral heterogeneity among the light-harvesting antennae of the abovementioned objects was also revealed by derivative spectroscopy, spectrofluorimetry with laser excitation, fluorescence polarization at 4 K (see [7]), and picosecond fluorescence spectrochronography at 77 K [8]. According to the first [3] and now widespread [6,9] interpretation, the second signal results from the existence of a minor fraction of light-harvesting BChl molecules having an absorption band at longer wavelengths (-20 nm) as compared to the major BChl antenna molecules. Other interpretations of the minor and major signals have also been proposed [5,10,11].…”