Low-temperature optical properties and pigment organization of the B875 light-harvesting bacteriochlorophyll-protein complex of purple photosynthetic bacteria

Herman J.M. Kramer, Jeffrey D. Pennoyer, Rienk Van Grondelle, Willem H.J. Westerhuis, Robert A. Niederman, Jan Amesz

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103 Scopus citations

Abstract

Optical and structural properties of the B875 light-harvesting complex of purple bacteria were examined by measurements of low-temperature circular dichroism (CD) and excitation spectra of fluorescence polarization. In the B875 complex isolated from wild-type Rhodopseudomonas sphaeroides, fluorescence polarization increased steeply across the long-wavelength Qy bacteriochlorophyll a (BChl) absorption band at both 4 and approx. 300 K. With the native complex in the photosynthetic membranes of Rhodospirillum rubrum and Rps. sphaeroides wild-type and R26-carotenoidless strains, this significant increase in polarization from 0.12 to 0.40 was only observed at low temperature. A polarization of -0.2 was observed upon excitation in the Qx BChl band. The results indicate that about 15% of the BChl molecules in the complex absorb at wavelengths about 12 nm longer than the other BChls. All BChls have approximately the same orientation with their Qy transition dipoles essentially parallel and their Qx transitions perpendicular to the plane of the membrane. At low temperature, energy transfer to the long-wavelength BChls is irreversible, yielding a high degree of polarization upon direct excitation, whereas at room temperature a partial depolarization of fluorescence by energy transfer between different subunits occurs in the membrane, but not in the isolated complex. CD spectra appear to reflect the two spectral forms of B875 BChl in Rps. sphaeroides membranes. They also reveal structural differences between the complexes of Rps. sphaeroides and Rhs. rubrum, in both BChl and carotenoid regions. The CD spectrum of isolated B875 indicates that the interactions between the BChls but not the carotenoids are altered upon isolation.

Original languageEnglish (US)
Pages (from-to)335-344
Number of pages10
JournalBBA - Bioenergetics
Volume767
Issue number2
DOIs
StatePublished - Nov 26 1984

All Science Journal Classification (ASJC) codes

  • Biophysics
  • Biochemistry
  • Cell Biology

Keywords

  • (Rps. sphaeroides)
  • Bacterial photosynthesis
  • Chlorophyll-protein complex
  • Circular dichroism
  • Fluorescence polarization
  • Light-harvesting complex

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