Energy trapping and detrapping by wild type and mutant reaction centers of purple non-sulfur bacteria

Arvi Freiberg, James Paul Allen, JoAnn C. Williams, Neal W. Woodbury

Research output: Contribution to journalArticle

32 Citations (Scopus)

Abstract

Time-correlated single photon counting was used to study energy trapping and detrapping kinetics at 295 K in Rhodobacter sphaeroides chromatophore membranes containing mutant reaction centers. The mutant reaction centers were expressed in a background strain of Rb. sphaeroides which contained only B880 antenna complexes and no B800-850 antenna complexes. The excited state decay times in the isolated reaction centers from these strains were previously shown to vary by roughly 15-fold, from 3.4 to 52 ps, due to differences in the charge separation rates in the different mutants (Allen and Williams (1995) J Bioenerg Biomembr 27: 275- 283). In this study, measurements were also performed on wild type Rhodospirillum rubrum and Rb. sphaeroides B880 antenna-only mutant chromatophores for comparison. The emission kinetics in membranes containing mutant reaction centers was complex. The experimental data were analyzed in terms of a kinetic model that involved fast excitation migration between antenna complexes followed by reversible energy transfer to the reaction center and charge separation. Three emission time constants were identified by fitting the data to a sum of exponential decay components. They were assigned to trapping/quenching of antenna excitations by the reaction center, recombination of the P+H- charge-separated state of the reaction center reforming an emitting state, and emission from uncoupled antenna pigment-protein complexes. The first varied from 60 to 160 ps, depending on the reaction center mutation; the second was 200-300 ps, and the third was about 700 ps. The observed weak linear dependence of the trapping time on the primary charge separation time, together with the known sub-picosecond exciton migration time within the antenna, supports the concept that it is energy transfer from the antenna to the reaction center, rather than charge separation, that limits the overall energy trapping time in wild type chromatophores. The component due to charge recombination reforming the excited state is minor in wild type membranes, but increases substantially in mutants due to the decreasing free energy gap between the states P* and P+H-.

Original languageEnglish
Pages (from-to)309-319
Number of pages11
JournalPhotosynthesis Research
Volume48
Issue number1-2
Publication statusPublished - 1996

Fingerprint

Chromatiaceae
Rhodospirillaceae
antennae
trapping
Bacteria
Antennas
mutants
Chromatophores
energy
chromatophores
Energy Transfer
Genetic Recombination
Reforming reactions
energy transfer
Membranes
kinetics
Excited states
Energy transfer
Kinetics
Rhodospirillum rubrum

Keywords

  • bacteriochlorophyll
  • energy transfer
  • exciton
  • picosecond fluorescence
  • Rhodobacter sphaeroides
  • Rhodospirillum rubrum

ASJC Scopus subject areas

  • Plant Science

Cite this

Energy trapping and detrapping by wild type and mutant reaction centers of purple non-sulfur bacteria. / Freiberg, Arvi; Allen, James Paul; Williams, JoAnn C.; Woodbury, Neal W.

In: Photosynthesis Research, Vol. 48, No. 1-2, 1996, p. 309-319.

Research output: Contribution to journalArticle

Freiberg, Arvi ; Allen, James Paul ; Williams, JoAnn C. ; Woodbury, Neal W. / Energy trapping and detrapping by wild type and mutant reaction centers of purple non-sulfur bacteria. In: Photosynthesis Research. 1996 ; Vol. 48, No. 1-2. pp. 309-319.
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AB - Time-correlated single photon counting was used to study energy trapping and detrapping kinetics at 295 K in Rhodobacter sphaeroides chromatophore membranes containing mutant reaction centers. The mutant reaction centers were expressed in a background strain of Rb. sphaeroides which contained only B880 antenna complexes and no B800-850 antenna complexes. The excited state decay times in the isolated reaction centers from these strains were previously shown to vary by roughly 15-fold, from 3.4 to 52 ps, due to differences in the charge separation rates in the different mutants (Allen and Williams (1995) J Bioenerg Biomembr 27: 275- 283). In this study, measurements were also performed on wild type Rhodospirillum rubrum and Rb. sphaeroides B880 antenna-only mutant chromatophores for comparison. The emission kinetics in membranes containing mutant reaction centers was complex. The experimental data were analyzed in terms of a kinetic model that involved fast excitation migration between antenna complexes followed by reversible energy transfer to the reaction center and charge separation. Three emission time constants were identified by fitting the data to a sum of exponential decay components. They were assigned to trapping/quenching of antenna excitations by the reaction center, recombination of the P+H- charge-separated state of the reaction center reforming an emitting state, and emission from uncoupled antenna pigment-protein complexes. The first varied from 60 to 160 ps, depending on the reaction center mutation; the second was 200-300 ps, and the third was about 700 ps. The observed weak linear dependence of the trapping time on the primary charge separation time, together with the known sub-picosecond exciton migration time within the antenna, supports the concept that it is energy transfer from the antenna to the reaction center, rather than charge separation, that limits the overall energy trapping time in wild type chromatophores. The component due to charge recombination reforming the excited state is minor in wild type membranes, but increases substantially in mutants due to the decreasing free energy gap between the states P* and P+H-.

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KW - Rhodospirillum rubrum

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