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The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers.

de Almeida WB, Taguchi AT, Dikanov SA, Wraight CA, O'Malley PJ - J Phys Chem Lett (2014)

Bottom Line: A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference.This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV.The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

View Article: PubMed Central - PubMed

Affiliation: LQC-MM, Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG) , Campus Pampulh, Belo Horizonte, MG 31.910-270, Brazil.

ABSTRACT
Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. (13)C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQA and SQB, were compared with DFT calculations of the (13)C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE m) between QA and QB of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

No MeSH data available.


Variation in the 2-methoxy 13Cm isotropic hfc as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). The estimates for the 2-methoxy dihedral angles,giving agreement with experimental determinations, are indicated bygreen (QA) and pink (QB) vertical arrows, QA (−155°, −50°, 50°, 155°)and QB (−130°,-82°, 82°,130°).The best agreement with X-ray CmOmC2C1 dihedral anglevalues are highlighted in bold vertical arrows. Experimental QA and QB 2-methoxy 13C hfc values forthe ubsemiquinone radical are indicated as solid horizontal lines.
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fig2: Variation in the 2-methoxy 13Cm isotropic hfc as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). The estimates for the 2-methoxy dihedral angles,giving agreement with experimental determinations, are indicated bygreen (QA) and pink (QB) vertical arrows, QA (−155°, −50°, 50°, 155°)and QB (−130°,-82°, 82°,130°).The best agreement with X-ray CmOmC2C1 dihedral anglevalues are highlighted in bold vertical arrows. Experimental QA and QB 2-methoxy 13C hfc values forthe ubsemiquinone radical are indicated as solid horizontal lines.

Mentions: The theoretical dependence of the 2-methoxy isotropic 13C hfc on the methoxy orientation for our ubisemiquinone model isshown in Figure 2. The 13C couplings(Aiso) for the 2-methoxy group in SQA (1.3 MHz) and SQB (5.7 MHz, adjusted to the sameunpaired spin density (0.11) on C2)12 are indicated by the solid horizontal lines. This definesfour possible dihedral angles in the two SQs (see Figure 2 legend). In a survey of X-ray structures at resolutionsof at least 2.8 Å, the average values for the 2-methoxy dihedralangles (CmOmC2C1) of QA and QB wereQA = +139 ± 25° and QB = −90± 9°, showing that the 2-methoxy group is located on oppositesides of the ring for the two quinone sites and that the QA site quinone has a 2-methoxy orientation relatively closer to thering plane.10 One can see that the bestagreement when comparing our estimated dihedral angles with the experimentalX-ray range is +155° for SQA and −82°for SQB. These are shown in Figure 2 by the solid vertical arrows.


The 2-Methoxy Group Orientation Regulates the Redox Potential Difference between the Primary (QA) and Secondary (QB) Quinones of Type II Bacterial Photosynthetic Reaction Centers.

de Almeida WB, Taguchi AT, Dikanov SA, Wraight CA, O'Malley PJ - J Phys Chem Lett (2014)

Variation in the 2-methoxy 13Cm isotropic hfc as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). The estimates for the 2-methoxy dihedral angles,giving agreement with experimental determinations, are indicated bygreen (QA) and pink (QB) vertical arrows, QA (−155°, −50°, 50°, 155°)and QB (−130°,-82°, 82°,130°).The best agreement with X-ray CmOmC2C1 dihedral anglevalues are highlighted in bold vertical arrows. Experimental QA and QB 2-methoxy 13C hfc values forthe ubsemiquinone radical are indicated as solid horizontal lines.
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Related In: Results  -  Collection

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Show All Figures
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fig2: Variation in the 2-methoxy 13Cm isotropic hfc as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). The estimates for the 2-methoxy dihedral angles,giving agreement with experimental determinations, are indicated bygreen (QA) and pink (QB) vertical arrows, QA (−155°, −50°, 50°, 155°)and QB (−130°,-82°, 82°,130°).The best agreement with X-ray CmOmC2C1 dihedral anglevalues are highlighted in bold vertical arrows. Experimental QA and QB 2-methoxy 13C hfc values forthe ubsemiquinone radical are indicated as solid horizontal lines.
Mentions: The theoretical dependence of the 2-methoxy isotropic 13C hfc on the methoxy orientation for our ubisemiquinone model isshown in Figure 2. The 13C couplings(Aiso) for the 2-methoxy group in SQA (1.3 MHz) and SQB (5.7 MHz, adjusted to the sameunpaired spin density (0.11) on C2)12 are indicated by the solid horizontal lines. This definesfour possible dihedral angles in the two SQs (see Figure 2 legend). In a survey of X-ray structures at resolutionsof at least 2.8 Å, the average values for the 2-methoxy dihedralangles (CmOmC2C1) of QA and QB wereQA = +139 ± 25° and QB = −90± 9°, showing that the 2-methoxy group is located on oppositesides of the ring for the two quinone sites and that the QA site quinone has a 2-methoxy orientation relatively closer to thering plane.10 One can see that the bestagreement when comparing our estimated dihedral angles with the experimentalX-ray range is +155° for SQA and −82°for SQB. These are shown in Figure 2 by the solid vertical arrows.

Bottom Line: A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference.This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV.The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

View Article: PubMed Central - PubMed

Affiliation: LQC-MM, Departamento de Química, ICEx, Universidade Federal de Minas Gerais (UFMG) , Campus Pampulh, Belo Horizonte, MG 31.910-270, Brazil.

ABSTRACT
Recent studies have shown that only quinones with a 2-methoxy group can act simultaneously as the primary (QA) and secondary (QB) electron acceptors in photosynthetic reaction centers from purple bacteria such as Rb. sphaeroides. (13)C HYSCORE measurements of the 2-methoxy group in the semiquinone states, SQA and SQB, were compared with DFT calculations of the (13)C hyperfine couplings as a function of the 2-methoxy dihedral angle. X-ray structure comparisons support 2-methoxy dihedral angle assignments corresponding to a redox potential gap (ΔE m) between QA and QB of 175-193 mV. A model having a methyl group substituted for the 2-methoxy group exhibits no electron affinity difference. This is consistent with the failure of a 2-methyl ubiquinone analogue to function as QB in mutant reaction centers with a ΔE m of ∼160-195 mV. The conclusion reached is that the 2-methoxy group is the principal determinant of electron transfer from QA to QB in type II photosynthetic reaction centers with ubiquinone serving as both acceptor quinones.

No MeSH data available.