Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i>
The pH dependence of the free energy level of the flash-induced primary charge pair P<sup>+</sup>I<sub>A</sub><sup>−</sup> was determined by a combination of the results from the indirect charge recombination of P<sup>+</sup>Q<sub>A</sub>&l...
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2024-10-01
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| description | The pH dependence of the free energy level of the flash-induced primary charge pair P<sup>+</sup>I<sub>A</sub><sup>−</sup> was determined by a combination of the results from the indirect charge recombination of P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and from the delayed fluorescence of the excited dimer (P*) in the reaction center of the photosynthetic bacterium <i>Rhodobacter sphaeroides</i>, where the native ubiquinone at the primary quinone binding site Q<sub>A</sub> was replaced by low-potential anthraquinone (AQ) derivatives. The following observations were made: (1) The free energy state of P<sup>+</sup>I<sub>A</sub><sup>−</sup> was pH independent below pH 10 (–370 ± 10 meV relative to that of the excited dimer P*) and showed a remarkable decrease (about 20 meV/pH unit) above pH 10. A part of the dielectric relaxation of the P<sup>+</sup>I<sub>A</sub><sup>−</sup> charge pair that is not insignificant (about 120 meV) should come from protonation-related changes. (2) The single exponential decay character of the kinetics proves that the protonated/unprotonated P<sup>+</sup>I<sub>A</sub><sup>−</sup> and P<sup>+</sup>Q<sub>A</sub><sup>−</sup> states are in equilibria and the rate constants of protonation <i>k</i><sub>on</sub><sup>H</sup> +<i>k</i><sub>off</sub><sup>H</sup> are much larger than those of the charge back reaction <i>k</i><sub>back</sub> ~10<sup>3</sup> s<sup>−1</sup>. (3) Highly similar pH profiles were measured to determine the free energy states of P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and P<sup>+</sup>I<sub>A</sub><sup>−</sup>, indicating that the same acidic cluster at around Q<sub>B</sub> should respond to both anionic species. This was supported by model calculations based on anticooperative proton distribution in the cluster with key residues of GluL212, AspL213, AspM17, and GluH173, and the effect of the polarization of the aqueous phase on electrostatic interactions. The larger distance of I<sub>A</sub><sup>−</sup> from the cluster (25.2 Å) compared to that of Q<sub>A</sub><sup>−</sup> (14.5 Å) is compensated by a smaller effective dielectric constant (6.5 ± 0.5 and 10.0 ± 0.5, respectively). (4) The P* → P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and I<sub>A</sub><sup>−</sup>Q<sub>A</sub> → I<sub>A</sub>Q<sub>A</sub><sup>−</sup> electron transfers are enthalpy-driven reactions with the exemption of very large (>60%) or negligible entropic contributions in cases of substitution by 2,3-dimethyl-AQ or 1-chloro-AQ, respectively. The possible structural consequences are discussed. |
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| spelling | doaj-art-0760dd8cb32d42f7938e92e5a54224102024-11-26T17:53:59ZengMDPI AGBiomolecules2218-273X2024-10-011411136710.3390/biom14111367Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i>Gábor Sipka0Péter Maróti1Institute of Medical Physics, University of Szeged, 6720 Szeged, HungaryInstitute of Medical Physics, University of Szeged, 6720 Szeged, HungaryThe pH dependence of the free energy level of the flash-induced primary charge pair P<sup>+</sup>I<sub>A</sub><sup>−</sup> was determined by a combination of the results from the indirect charge recombination of P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and from the delayed fluorescence of the excited dimer (P*) in the reaction center of the photosynthetic bacterium <i>Rhodobacter sphaeroides</i>, where the native ubiquinone at the primary quinone binding site Q<sub>A</sub> was replaced by low-potential anthraquinone (AQ) derivatives. The following observations were made: (1) The free energy state of P<sup>+</sup>I<sub>A</sub><sup>−</sup> was pH independent below pH 10 (–370 ± 10 meV relative to that of the excited dimer P*) and showed a remarkable decrease (about 20 meV/pH unit) above pH 10. A part of the dielectric relaxation of the P<sup>+</sup>I<sub>A</sub><sup>−</sup> charge pair that is not insignificant (about 120 meV) should come from protonation-related changes. (2) The single exponential decay character of the kinetics proves that the protonated/unprotonated P<sup>+</sup>I<sub>A</sub><sup>−</sup> and P<sup>+</sup>Q<sub>A</sub><sup>−</sup> states are in equilibria and the rate constants of protonation <i>k</i><sub>on</sub><sup>H</sup> +<i>k</i><sub>off</sub><sup>H</sup> are much larger than those of the charge back reaction <i>k</i><sub>back</sub> ~10<sup>3</sup> s<sup>−1</sup>. (3) Highly similar pH profiles were measured to determine the free energy states of P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and P<sup>+</sup>I<sub>A</sub><sup>−</sup>, indicating that the same acidic cluster at around Q<sub>B</sub> should respond to both anionic species. This was supported by model calculations based on anticooperative proton distribution in the cluster with key residues of GluL212, AspL213, AspM17, and GluH173, and the effect of the polarization of the aqueous phase on electrostatic interactions. The larger distance of I<sub>A</sub><sup>−</sup> from the cluster (25.2 Å) compared to that of Q<sub>A</sub><sup>−</sup> (14.5 Å) is compensated by a smaller effective dielectric constant (6.5 ± 0.5 and 10.0 ± 0.5, respectively). (4) The P* → P<sup>+</sup>Q<sub>A</sub><sup>−</sup> and I<sub>A</sub><sup>−</sup>Q<sub>A</sub> → I<sub>A</sub>Q<sub>A</sub><sup>−</sup> electron transfers are enthalpy-driven reactions with the exemption of very large (>60%) or negligible entropic contributions in cases of substitution by 2,3-dimethyl-AQ or 1-chloro-AQ, respectively. The possible structural consequences are discussed.https://www.mdpi.com/2218-273X/14/11/1367<i>Rhodobacter sphaeroides</i>reaction centerbacteriophyeophytinquinone substitutionacidic clusterthermodynamics |
| spellingShingle | Gábor Sipka Péter Maróti Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> Biomolecules <i>Rhodobacter sphaeroides</i> reaction center bacteriophyeophytin quinone substitution acidic cluster thermodynamics |
| title | Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> |
| title_full | Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> |
| title_fullStr | Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> |
| title_full_unstemmed | Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> |
| title_short | Contribution of Protonation to the Dielectric Relaxation Arising from Bacteriopheophytin Reductions in the Photosynthetic Reaction Centers of <i>Rhodobacter sphaeroides</i> |
| title_sort | contribution of protonation to the dielectric relaxation arising from bacteriopheophytin reductions in the photosynthetic reaction centers of i rhodobacter sphaeroides i |
| topic | <i>Rhodobacter sphaeroides</i> reaction center bacteriophyeophytin quinone substitution acidic cluster thermodynamics |
| url | https://www.mdpi.com/2218-273X/14/11/1367 |
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