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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 EA as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). Bold vertical arrows indicate dihedral anglevalues for SQA and SQB estimated from Figure 2. The 3-methoxy dihedral angles (CmOmC3C4) forthe QA and QB points are, respectively, −63.5and −66.1°. The horizontal lines are the EA values (QA and QB) for ubiquinone.
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fig3: Variation in EA as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). Bold vertical arrows indicate dihedral anglevalues for SQA and SQB estimated from Figure 2. The 3-methoxy dihedral angles (CmOmC3C4) forthe QA and QB points are, respectively, −63.5and −66.1°. The horizontal lines are the EA values (QA and QB) for ubiquinone.

Mentions: Figure 3 gives the variation in electronaffinity value as a function of the 2-methoxy dihedral angle. Again,the orientations corresponding most closely to the crystal structureanalysis are indicated by the vertical arrows. For the dihedral anglesgiven above, the QB site quinone is estimated to have anelectron affinity 175 meV higher than that of QA. Thisis similar to our previous calculated value using a smaller modeland restricted scan.11,12 Also included in Table 1 is the ΔEA value calculated when the 3-methoxygroup (CmOmC3C4) is fixed at its midrange value from thecrystal structure analysis, −77° for QA and+88° for QB.10 This leadsto an elevation of the ΔEA value to 193 meV compared with 175meV using the optimized 3-methoxy dihedral angle values. This illustrates,as expected, that the 3-methoxy orientation influences the electronaffinity as well but that the orientation of this group is similarfor both QA and QB, in contrast to the 2-methoxygroup where each has a significantly different orientation.


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 EA as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). Bold vertical arrows indicate dihedral anglevalues for SQA and SQB estimated from Figure 2. The 3-methoxy dihedral angles (CmOmC3C4) forthe QA and QB points are, respectively, −63.5and −66.1°. The horizontal lines are the EA values (QA and QB) for ubiquinone.
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Related In: Results  -  Collection

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Show All Figures
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fig3: Variation in EA as a function of the CmOmC2C1 dihedralangle for the model shown in Figure S1a (SupportingInformation). Bold vertical arrows indicate dihedral anglevalues for SQA and SQB estimated from Figure 2. The 3-methoxy dihedral angles (CmOmC3C4) forthe QA and QB points are, respectively, −63.5and −66.1°. The horizontal lines are the EA values (QA and QB) for ubiquinone.
Mentions: Figure 3 gives the variation in electronaffinity value as a function of the 2-methoxy dihedral angle. Again,the orientations corresponding most closely to the crystal structureanalysis are indicated by the vertical arrows. For the dihedral anglesgiven above, the QB site quinone is estimated to have anelectron affinity 175 meV higher than that of QA. Thisis similar to our previous calculated value using a smaller modeland restricted scan.11,12 Also included in Table 1 is the ΔEA value calculated when the 3-methoxygroup (CmOmC3C4) is fixed at its midrange value from thecrystal structure analysis, −77° for QA and+88° for QB.10 This leadsto an elevation of the ΔEA value to 193 meV compared with 175meV using the optimized 3-methoxy dihedral angle values. This illustrates,as expected, that the 3-methoxy orientation influences the electronaffinity as well but that the orientation of this group is similarfor both QA and QB, in contrast to the 2-methoxygroup where each has a significantly different orientation.

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.