Binding sites of quinones in photosynthetic bacterial reaction centers investigated by light-induced FTIR difference spectroscopy: Symmetry of the carbonyl interactions and close equivalence of the Q{sub B} vibrations in Rhodobacter sphaeroides and Rhodopseudomonas viridis probed by isotope labeling
- SBE/DBCM and SMM/DBCM, CEA-Saclay (France); and others
The photoreduction of the secondary quinone acceptor Q{sub B} in reaction centers (RCs) of the photosynthetic bacteria Rhodobacter sphaeroides and Rhodopseudomonas viridis has been investigated by light-induced FTIR difference spectroscopy of RCs reconstituted with several isotopically labeled ubiquinones. The labels used were {sup 18}O on both carbonyls and {sup 13}C either uniformly or selectively at the 1- or the 4-position, i.e., on either one of the two carbonyls. The Q{sub B}{sup {minus}}/Q{sub B} spectra of RCs reconstituted with the isotopically labeled and unlabeled quinones as well as the double differences calculated form these spectra exhibit distinct isotopic shifts for a numer of bands attributed to vibrations of Q{sub B} and Q{sub B}{sup {minus}}. The vibrational modes of the quinone in the Q{sub B} site are compared to those of ubiquinone in vitro, leading to band assignments for the C{double_bond}O and C{double_bond}C vibrations of the neutral Q{sub B} and for the C---O and C---C of the semiquinone. The C{double_bond}O frequency of each of the carbonyls of the unlabeled quinone is revealed at 1641 cm{sup {minus}1} for both species. This demonstrates symmetrical and weak hydrogen bonding of the two C{double_bond}O groups to the protein at the Q{sub B} site. In contrast, the C{double_bond}C vibrations are not equivalent for selective labeling at C{sub 1} or at C{sub 4}, although they both contribute to the {approximately}1611-cm{sup {minus}1} band in the Q{sub B}{sup {minus}}/Q{sub B} spectra of the two species. Compared to the vibrations of isolated ubiquinone, the C{double_bond}C mode of Q{sub B} does not involve displacement of the C{sub 4} carbon atom, while the motion of C{sub 1} is not hindered. Further analysis of the spectra suggests that the protein at the binding site imposes a specific constraint on the methoxy and/or the methyl group proximal to the C{sub 4} carbonyl. 49 refs., 5 figs.
- Sponsoring Organization:
- USDOE
- OSTI ID:
- 245246
- Journal Information:
- Biochemistry (Eaton), Vol. 34, Issue 36; Other Information: PBD: 12 Sep 1995
- Country of Publication:
- United States
- Language:
- English
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