The Effect of Axial Mg Ligation on the Geometry and Spin Density Distribution of Chlorophyll and Bacteriochlorophyll Cation Free Radical Models:  A Density Functional Study

O’Malley, Patrick J.; Collins, Simon J.

doi:10.1021/ja010522u

Cited by 35 publications

(26 citation statements)

References 16 publications

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“…Preliminary calculation of isotropic hfc's of model complexes of individual chlorophyll a radical cation with different amino acid residues has shown a reasonable agreement between parameters calculated in 3-21G and the 6-31G(d) basis sets (Petrenko, unpublished results). The B3LYP 6-31G(d) basis set was successfully used in previous calculations of the spin density distribution and hyperfine coupling constants of individual chlorophyll cations (33). (Unlike the EPR-II basis set, magnesium is supported by the 6-31G(d) and 3-21G basis sets.…”

Section: Methodsmentioning

confidence: 99%

Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I

Lucas

Konovalova

et al. 2004

Biochemistry

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The primary electron donor of photosystem I (PS1), called P 700 , is a heterodimer of chlorophyll (Chl) a and a′. The crystal structure of photosystem I reveals that the chlorophyll a′ (P A ) could be hydrogenbonded to the protein via a threonine residue, while the chlorophyll a (P B ) does not have such a hydrogen bond. To investigate the influence of this hydrogen bond on P 700 , PsaA-Thr739 was converted to alanine to remove the H-bond to the 13 1 -keto group of the chlorophyll a′ in Chlamydomonas reinhardtii. The PsaA-T739A mutant was capable of assembling active PS1. Furthermore the mutant PS1 contained approximately one chlorophyll a′ molecule per reaction center, indicating that P 700 was still a Chl a/a′ heterodimer in the mutant. However, the mutation induced several band shifts in the visible P 700 + -P 700 absorbance difference spectrum. Redox titration of P 700 revealed a 60 mV decrease in the P 700 /P 700 + midpoint potential of the mutant, consistent with loss of a H-bond. Fourier transform infrared (FTIR) spectroscopy indicates that the ground state of P 700 is somewhat modified by mutation of ThrA739 to alanine. Comparison of FTIR difference band shifts upon P 700 + formation in WT and mutant PS1 suggests that the mutation modifies the charge distribution over the pigments in the P 700 + state, with ∼14-18% of the positive charge on P B in WT being relocated onto P A in the mutant. 1 H-electron-nuclear double resonance (ENDOR) analysis of the P 700 + cation radical was also consistent with a slight redistribution of spin from the P B chlorophyll to P A , as well as some redistribution of spin within the P B chlorophyll. High-field electron paramagnetic resonance (EPR) spectroscopy at 330-GHz was used to resolve the g-tensor of P 700 + , but no significant differences from wild-type were observed, except for a slight decrease of anisotropy. The mutation did, however, provoke changes in the zero-field splitting parameters of the triplet state of P 700 ( 3 P 700 ), as determined by EPR. Interestingly, the mutation-induced change in asymmetry of P 700 did not cause an observable change in the directionality of electron transfer within PS1.In higher plants and green algae, the photosynthetic reaction occurs within two large pigment-protein complexes located in the thylakoid membranes of the chloroplast: photosystems I and II (PS1 and PS2). 1 These two systems exemplify the type 1 ("iron-sulfur") and type 2 ("quinone") reaction centers, respectively, based on their terminal electron acceptors. In oxygenic phototrophs, PS2 and PS1 act in tandem to oxidize water and reduce NADP + , but there are several anoxygenic photosynthetic bacteria that use only one type of reaction center protein.All known reaction centers share a similar structural motif with a core of reaction centers composed of two similar or identical integral membrane subunits, to which the various redox cofactors are bound. Both the protein structural motifs and the cofactor arrangements are characterized by a pseudo-C 2 symmetry ...

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Section: Methodsmentioning

confidence: 99%

Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I

Lucas

Konovalova

et al. 2004

Biochemistry

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“…It is now desirable to extend the systems under consideration to (macro)molecules involved in important biological processes. This approach has so far been successfully applied to study, e.g., [NiFe] hydrogenase [20,21], Blz-dependent glutamate mutase [22], (bacterio-)chlorophylls [23,24], blue copper proteins [25,26], coenzyme B~2 [27], small cz-helix and [3-sheet protein fragments [28], the hammerhead ribozyme [29], ribonucleotide reductase [30], cytochrome P450 [31] and quinol oxidase [32].…”

Section: Lntroductionmentioning

confidence: 99%

Theoretical investigation of Q A −· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

2006

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Density functional theory was used to caleulate magnetic resonance parameters for the primary stable electron acceptor anion radical (Q2") in its binding site in the bacterial reaction center (bRC) of Rhodobacter sphaeroides. The models used for the calculations of the QA" binding pocket included all short-range interactions of the ubiquinone with the protein surroundings in a gradual manner and thus allowed a decomposition and detailed analysis of the different specific interactions. Comparison of the obtained hyperfine and quadrupole couplings with experimental data demonstrates the feasibility and reliability of calculations on such complex biologically relevant systems. With these results, the interpretation of previously published 3-pulse electron spin echo envelope modulation data could be extended and an assignment of the observed double quantum peak to a specific amino acid is proposed. The computations provide evidence for a slightly altered binding site geometry for the QA ground state as investigated by X-ray crystallography with respect to the QA" anion radical state as accessible vŸ EPR spectroscopy. This new geometry leads to improved fits of the W-band correlated-coupled radical pair spectra of Q2"-P~-91 compared to orientation data from the crystal structure. Finally, a correlation of the 14N quadrupole parameters of His219 with the hydrogen bond geometry anda comparison with previous systematic studies on the influence of hydrogen bond geometry on quadrupole coupling parameters (J. Fritscher: Phys. Chem. Chem. Phys. 6, 4950~-956, 2004) is presented.

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“…Indeed, the suitability of the use of current DFT methods for quantitatively interpreting the nature of the HOMO and SHOMO orbitals in Chl-a has already been questioned by others. 46,47 In the present context, we conclude that the calculations are not sufficiently accurate to permit authoritative assignment of the SH transition in photosynthetic monomer or special-pair cation radicals, but the established accuracy is sufficient to indicate likely scenarios.…”

Section: Location Of the Shomo\homo Transition Energymentioning

confidence: 70%

“…Also, environmental effects may change the relative energies by amounts considerably greater than 500 cm Ϫ1 . 46 In going from BChl-a to BChl-g an increase in the SHOMO→HOMO transition energy of 1100 cm Ϫ1 is predicted. In addition, a redshift of order 500 cm Ϫ1 is observed for the HT bands in these systems, and the combined effect would take the GS→SH transition into the tail region of the observed GS→HT band.…”

Section: Variations Of the Shomo\homo Transition Energy With Changmentioning

confidence: 95%

Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition

Reimers

Shapley

Hush

2003

The Journal of Chemical Physics

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Modeling the bacterial photosynthetic reaction center. VII. Full simulation of the intervalence hole-transfer absorption spectrum of the special-pair radical cation Modelling the bacterial photosynthetic reaction center. VI. Use of density-functional theory to determine the nature of the vibronic coupling between the four lowest-energy electronic states of the special-pair radical cation Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm À1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition Primary charge separation in photoexcited photosynthetic reaction centers produces the radical cation P ϩ of a bacteriochlorophyll dimer known as the special-pair P. P ϩ has an intense electronic transition in the vicinity of 1800-5000 cm Ϫ1 which is usually assigned to the interchromophore hole-transfer excitation of the dimer radical cation; in principle, this spectrum can give much insight into key steps of the solar-to-electrical energy-conversion process. The extent to which this transition is localized on one-half of the dimer or delocalized over both is of utmost importance; an authoritative deduction of this quantity from purely spectroscopic arguments requires the detailed assignment of the observed high to medium resolution spectra. For reaction centers containing bacteriochlorophylls a or b, a shoulder is observed at 2200 cm Ϫ1 on the low-energy side of the main hole-transfer absorption band, a band whose maximum is near 2700 cm Ϫ1 . Before quantitative analysis of the hole-transfer absorption in these well-studied systems can be attempted, the nature of the processes leading to this shoulder must be determined. We interpret it as arising from an intrachromophore SHOMO to HOMO transition whose intensity arises wholly through vibronic coupling with the hole-transfer band. A range of ab initio and density-functional calculations are performed to estimate the energy of this transition both for monomeric cations and for P ϩ of Blastochloris viridis, Rhodobacter sphaeroides, Chlorobium limicola, Chlorobium tepidum, Chlamydomonas reinhardtii, Synochocystis S.6803, spinach photosystems I and II, Heliobacillus mobilis, and finally Heliobacterium modesticaldum, with the results found to qualitatively describe the available experimental data. Subsequent papers in this series provide quantitative analyses of the vibronic coupling and complete spectral simulations based on the model developed herein.

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The Effect of Axial Mg Ligation on the Geometry and Spin Density Distribution of Chlorophyll and Bacteriochlorophyll Cation Free Radical Models: A Density Functional Study

Cited by 35 publications

References 16 publications

Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I

Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I

Theoretical investigation of Q A −· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition

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The Effect of Axial Mg Ligation on the Geometry and Spin Density Distribution of Chlorophyll and Bacteriochlorophyll Cation Free Radical Models: A Density Functional Study

Cited by 35 publications

References 16 publications

Mutation of the Putative Hydrogen-Bond Donor to P700 of Photosystem I

Mutation of the Putative Hydrogen-Bond Donor to P700 of Photosystem I

Theoretical investigation of Q A −· -ligand interactions in bacterial reaction centers ofRhodobacter sphaeroides

Modelling the bacterial photosynthetic reaction center. V. Assignment of the electronic transition observed at 2200 cm−1 in the special-pair radical-cation as a second-highest occupied molecular orbital to highest occupied molecular orbital transition

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Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I

Mutation of the Putative Hydrogen-Bond Donor to P₇₀₀ of Photosystem I