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Chiesa M, Giamello E, Livraghi S, Paganini MC, Polliotto V, Salvadori E. Electron magnetic resonance in heterogeneous photocatalysis research. JOURNAL OF PHYSICS. CONDENSED MATTER : AN INSTITUTE OF PHYSICS JOURNAL 2019; 31:444001. [PMID: 31311893 DOI: 10.1088/1361-648x/ab32c6] [Citation(s) in RCA: 9] [Impact Index Per Article: 1.8] [Reference Citation Analysis] [Abstract] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 06/10/2023]
Abstract
The contribution of electron magnetic resonance techniques, and in particular of CW-EPR, to the experimental research on photocatalytic phenomena is illustrated in this paper with selected examples. In the first part of the paper the role of EPR in unravelling the nature and the features of extrinsic point defects in semiconducting oxides is epitomized using the important example of the photoactive nitrogen center in various semiconducting oxides. In the second part we describe how EPR can monitor the processes that follow the initial photoinduced charge separation in photocatalysis, namely the stabilisation, migration and surface reactivity of electrons and holes. Finally, we will discuss how the role of EPR in photocatalysis is not limited to monitor phenomena occurring in the solid or at its surface but it can be extended to the investigation of the liquid phase by employing the spin trapping techniques to monitor the nature and the concentration of the reactive free radicals formed along the photocatalytic process.
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Affiliation(s)
- Mario Chiesa
- Dipartimento di Chimica, Università di Torino, Via P. Giuria 7, 10125., Torino, Italy
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Möbius K, Lubitz W, Savitsky A. Jim Hyde and the ENDOR Connection: A Personal Account. APPLIED MAGNETIC RESONANCE 2017; 48:1149-1183. [PMID: 29151676 PMCID: PMC5668355 DOI: 10.1007/s00723-017-0959-9] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Figures] [Subscribe] [Scholar Register] [Received: 09/29/2017] [Indexed: 06/07/2023]
Abstract
In this minireview, we report on our year-long EPR work, such as electron-nuclear double resonance (ENDOR), pulse electron double resonance (PELDOR) and ELDOR-detected NMR (EDNMR) at X-band and W-band microwave frequencies and magnetic fields. This report is dedicated to James S. Hyde and honors his pioneering contributions to the measurement of spin interactions in large (bio)molecules. From these interactions, detailed information is revealed on structure and dynamics of macromolecules embedded in liquid-solution or solid-state environments. New developments in pulsed microwave and sweepable cryomagnet technology as well as ultra-fast electronics for signal data handling and processing have pushed the limits of EPR spectroscopy and its multi-frequency extensions to new horizons concerning sensitivity of detection, selectivity of molecular interactions and time resolution. Among the most important advances is the upgrading of EPR to high magnetic fields, very much in analogy to what happened in NMR. The ongoing progress in EPR spectroscopy is exemplified by reviewing various multi-frequency electron-nuclear double-resonance experiments on organic radicals, light-generated donor-acceptor radical pairs in photosynthesis, and site-specifically nitroxide spin-labeled bacteriorhodopsin, the light-driven proton pump, as well as EDNMR and ENDOR on nitroxides. Signal and resolution enhancements are particularly spectacular for ENDOR, EDNMR and PELDOR on frozen-solution samples at high Zeeman fields. They provide orientation selection for disordered samples approaching single-crystal resolution at canonical g-tensor orientations-even for molecules with small g-anisotropies. Dramatic improvements of EPR detection sensitivity could be achieved, even for short-lived paramagnetic reaction intermediates. Thus, unique structural and dynamic information is revealed that can hardly be obtained by other analytical techniques. Micromolar concentrations of sample molecules have become sufficient to characterize stable and transient reaction intermediates of complex molecular systems-offering exciting applications for physicists, chemists, biochemists and molecular biologists.
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Affiliation(s)
- Klaus Möbius
- Department of Physics, Free University Berlin, Arnimallee 14, 14195 Berlin, Germany
- Max-Planck-Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany
| | - Wolfgang Lubitz
- Max-Planck-Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany
| | - Anton Savitsky
- Max-Planck-Institute for Chemical Energy Conversion, 45470 Mülheim an der Ruhr, Germany
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Sinnecker S, Lubitz W. Probing the Electronic Structure of Bacteriochlorophyll Radical Ions-A Theoretical Study of the Effect of Substituents on Hyperfine Parameters. Photochem Photobiol 2017; 93:755-761. [PMID: 28120345 DOI: 10.1111/php.12724] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.1] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/19/2016] [Accepted: 12/01/2016] [Indexed: 11/29/2022]
Abstract
In reaction centers (RCs) of photosynthesis, a light-induced charge separation takes place creating radical cations and anions of the participating cofactors. In photosynthetic bacteria, different bacteriochlorophylls (BChl) are involved in this process. Information about the electronic structure of the BChl radical cations and anions can be obtained by measuring the electron spin density distribution via the electron-nuclear hyperfine interaction using EPR and ENDOR techniques. In this communication, we report isotropic hyperfine coupling constants (hfcs) of the BChl b and g radical cations and anions, calculated by density functional theory, and compare them with the more common radical ions of BChl a and with available experimental data. The observed differences in the computed hyperfine data are discussed in view of a possible distinction between these species by EPR/ENDOR methods. In addition, 14 N nuclear quadrupole coupling constants (nqcs) computed for BChl a, b, g, and also for Chl a in their charge neutral, radical cation and radical anion states are presented. These nqcs are compared with experimental values obtained by ESEEM spectroscopy on several different radical ions.
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Affiliation(s)
- Sebastian Sinnecker
- Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr, Germany
| | - Wolfgang Lubitz
- Max-Planck-Institut für Chemische Energiekonversion, Mülheim an der Ruhr, Germany
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Sun C, Taguchi AT, Vermaas JV, Beal NJ, O'Malley PJ, Tajkhorshid E, Gennis RB, Dikanov SA. Q-Band Electron-Nuclear Double Resonance Reveals Out-of-Plane Hydrogen Bonds Stabilize an Anionic Ubisemiquinone in Cytochrome bo 3 from Escherichia coli. Biochemistry 2016; 55:5714-5725. [PMID: 27622672 DOI: 10.1021/acs.biochem.6b00669] [Citation(s) in RCA: 8] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
The respiratory cytochrome bo3 ubiquinol oxidase from Escherichia coli has a high-affinity ubiquinone binding site that stabilizes the one-electron reduced ubisemiquinone (SQH), which is a transient intermediate during the electron-mediated reduction of O2 to water. It is known that SQH is stabilized by two strong hydrogen bonds from R71 and D75 to ubiquinone carbonyl oxygen O1 and weak hydrogen bonds from H98 and Q101 to O4. In this work, SQH was investigated with orientation-selective Q-band (∼34 GHz) pulsed 1H electron-nuclear double resonance (ENDOR) spectroscopy on fully deuterated cytochrome (cyt) bo3 in a H2O solvent so that only exchangeable protons contribute to the observed ENDOR spectra. Simulations of the experimental ENDOR spectra provided the principal values and directions of the hyperfine (hfi) tensors for the two strongly coupled H-bond protons (H1 and H2). For H1, the largest principal component of the proton anisotropic hfi tensor Tz' = 11.8 MHz, whereas for H2, Tz' = 8.6 MHz. Remarkably, the data show that the direction of the H1 H-bond is nearly perpendicular to the quinone plane (∼70° out of plane). The orientation of the second strong hydrogen bond, H2, is out of plane by ∼25°. Equilibrium molecular dynamics simulations on a membrane-embedded model of the cyt bo3 QH site show that these H-bond orientations are plausible but do not distinguish which H-bond, from R71 or D75, is nearly perpendicular to the quinone ring. Density functional theory calculations support the idea that the distances and geometries of the H-bonds to the ubiquinone carbonyl oxygens, along with the measured proton anisotropic hfi couplings, are most compatible with an anionic (deprotonated) ubisemiquinone.
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Affiliation(s)
- Chang Sun
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Alexander T Taguchi
- Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Josh V Vermaas
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Nathan J Beal
- School of Chemistry, The University of Manchester , Manchester M13 9PL, U.K
| | - Patrick J O'Malley
- School of Chemistry, The University of Manchester , Manchester M13 9PL, U.K
| | - Emad Tajkhorshid
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Beckman Institute, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Robert B Gennis
- Department of Biochemistry, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States.,Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
| | - Sergei A Dikanov
- Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
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Taguchi AT, Baldansuren A, Dikanov SA. Basic and Combination Cross-Features in X- and Q-band HYSCORE of the 15N Labeled Bacteriochlorophyll a Cation Radical. ACTA ACUST UNITED AC 2016. [DOI: 10.1515/zpch-2016-0815] [Citation(s) in RCA: 5] [Impact Index Per Article: 0.6] [Reference Citation Analysis] [Abstract] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/15/2022]
Abstract
Abstract
Chlorophylls are an essential class of cofactors found in all photosynthetic organisms. Upon absorbing a photon, the excited state energy of the chlorophyll can either be transferred to another acceptor molecule, or be used to drive electron transfer. When acting as the primary donor in the bacterial photosynthetic reaction center, light-induced charge separation results in the formation of a cationic bacteriochlorophyll dimer. The hyperfine interactions between the unpaired electron of the 15N labeled bacteriochlorophyll cation radical and its four pyrrole nitrogens are probed with X- and Q-band 15N HYSCORE spectroscopy in frozen solution. The powder-type HYSCORE shows the basic (ν
α(β), ν
β(α)) cross-features as well as several types of combination cross-features. The nitrogen tensors were resolved in the squared-frequency representation of the HYSCORE spectra, and simulations of the combination peaks allowed for further refinement of the hyperfine coupling constants. The nitrogen tensors were found to have coupling constants a=3.28 MHz, T=1.23 MHz (N1 and N2), a=4.10 MHz, T=1.25 MHz (N3), and a=4.35 MHz, T=1.70 MHz (N4). The combination features were assigned based on a linear regression analysis of the cross-ridges in the squared-frequency representation as well as spectral simulations. The methodology discussed here will provide an important foundation for analyzing and understanding complex two-dimensional spectra from several I=1/2 nuclei.
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Affiliation(s)
- Alexander T. Taguchi
- Center for Biophysics and Computational Biology, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Amgalanbaatar Baldansuren
- Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
| | - Sergei A. Dikanov
- Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA , Phone: +(217) 300-2209
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Taguchi AT, O'Malley PJ, Wraight CA, Dikanov SA. Hyperfine and nuclear quadrupole tensors of nitrogen donors in the Q(A) site of bacterial reaction centers: correlation of the histidine N(δ) tensors with hydrogen bond strength. J Phys Chem B 2014; 118:9225-37. [PMID: 25026433 PMCID: PMC4126732 DOI: 10.1021/jp5051029] [Citation(s) in RCA: 17] [Impact Index Per Article: 1.7] [Reference Citation Analysis] [Abstract] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/29/2022]
Abstract
![]()
X-
and Q-band pulsed EPR spectroscopy was applied to study the
interaction of the QA site semiquinone (SQA)
with nitrogens from the local protein environment in natural abundance 14N and in 15N uniformly labeled photosynthetic
reaction centers of Rhodobacter sphaeroides. The hyperfine and nuclear quadrupole tensors for His-M219 Nδ and Ala-M260 peptide nitrogen (Np) were
estimated through simultaneous simulation of the Q-band 15N Davies ENDOR, X- and Q-band 14,15N HYSCORE, and X-band 14N three-pulse ESEEM spectra, with support from DFT calculations.
The hyperfine coupling constants were found to be a(14N) = 2.3 MHz, T = 0.3 MHz for His-M219
Nδ and a(14N) = 2.6 MHz, T = 0.3 MHz for Ala-M260 Np. Despite that His-M219
Nδ is established as the stronger of the two H-bond
donors, Ala-M260 Np is found to have the larger value of a(14N). The nuclear quadrupole coupling constants
were estimated as e2Qq/4h = 0.38 MHz, η = 0.97 and e2Qq/4h = 0.74 MHz, η = 0.59 for His-M219 Nδ and Ala-M260 Np, respectively. An analysis of the available
data on nuclear quadrupole tensors for imidazole nitrogens found in
semiquinone-binding proteins and copper complexes reveals these systems
share similar electron occupancies of the protonated nitrogen orbitals.
By applying the Townes–Dailey model, developed previously for
copper complexes, to the semiquinones, we find the asymmetry parameter
η to be a sensitive probe of the histidine Nδ–semiquinone hydrogen bond strength. This is supported by
a strong correlation observed between η and the isotropic coupling
constant a(14N) and is consistent with
previous computational works and our own semiquinone-histidine model
calculations. The empirical relationship presented here for a(14N) and η will provide an important
structural characterization tool in future studies of semiquinone-binding
proteins.
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Affiliation(s)
- Alexander T Taguchi
- Center for Biophysics and Computational Biology, §Department of Biochemistry, and ∥Department of Veterinary Clinical Medicine, University of Illinois at Urbana-Champaign , Urbana, Illinois 61801, United States
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Savitsky A, Niklas J, Golbeck JH, Möbius K, Lubitz W. Orientation Resolving Dipolar High-Field EPR Spectroscopy on Disordered Solids: II. Structure of Spin-Correlated Radical Pairs in Photosystem I. J Phys Chem B 2013; 117:11184-99. [DOI: 10.1021/jp401573z] [Citation(s) in RCA: 22] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
Affiliation(s)
- A. Savitsky
- Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470
Mülheim an der Ruhr, Germany
| | - J. Niklas
- Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470
Mülheim an der Ruhr, Germany
| | - J. H. Golbeck
- Department of Biochemistry
and
Molecular Biology, Department of Chemistry, The Pennsylvania State University, University Park, Pennsylvania 16802,
United States
| | - K. Möbius
- Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470
Mülheim an der Ruhr, Germany
- Department of Physics, Free University Berlin, Arnimallee 14, D-14195 Berlin,
Germany
| | - W. Lubitz
- Max Planck Institute for Chemical Energy Conversion, Stiftstr. 34-36, D-45470
Mülheim an der Ruhr, Germany
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Di Valentin M, Tait C, Salvadori E, Ceola S, Scheer H, Hiller RG, Carbonera D. Conservation of Spin Polarization during Triplet–Triplet Energy Transfer in Reconstituted Peridinin–Chlorophyll–Protein Complexes. J Phys Chem B 2011; 115:13371-80. [DOI: 10.1021/jp206978y] [Citation(s) in RCA: 16] [Impact Index Per Article: 1.2] [Reference Citation Analysis] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/19/2023]
Affiliation(s)
- Marilena Di Valentin
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
| | - Claudia Tait
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
| | - Enrico Salvadori
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
| | - Stefano Ceola
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
| | - Hugo Scheer
- Department Biologie I-Botanik, Ludwig-Maximilians-Universität München, Menziger Strasse 67, D-80638 München, Germany
| | - Roger G. Hiller
- Department of Biological Sciences, Macquarie University, North Ryde, NSW 2109, Australia
| | - Donatella Carbonera
- Dipartimento di Scienze Chimiche, Università degli Studi di Padova, via Marzolo 1, 35131 Padova, Italy
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Savitsky A, Möbius K. High-field EPR. PHOTOSYNTHESIS RESEARCH 2009; 102:311-333. [PMID: 19468856 DOI: 10.1007/s11120-009-9432-4] [Citation(s) in RCA: 19] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Subscribe] [Scholar Register] [Received: 11/24/2008] [Accepted: 04/29/2009] [Indexed: 05/27/2023]
Abstract
Among the numerous spectroscopic techniques utilized in photosynthesis research, high-field/high-frequency EPR and its pulse extensions ESE, ENDOR, ESEEM, and PELDOR play an important role in the endeavor to understand, on the basis of structure and dynamics data, dominant factors that control specificity and efficiency of light-induced electron- and proton-transfer processes in primary photosynthesis. Short-lived transient intermediates of the photocycle can be characterized by high-field EPR techniques, and detailed structural information can be obtained even from disordered sample preparations. The chapter describes how multifrequency high-field EPR methodology, in conjunction with mutation strategies for site-specific isotope or spin labeling and with the support of modern quantum-chemical computation methods for data interpretation, is capable of providing new insights into the photosynthetic transfer processes. The information obtained is complementary to that of protein crystallography, solid-state NMR and laser spectroscopy.
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Affiliation(s)
- Anton Savitsky
- Department of Physics, Free University Berlin, Arnimallee 14, 14195 Berlin, Germany
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High-Field/High-Frequency Electron Paramagnetic Resonance Involving Single- and Multiple-Transition Schemes. BIOPHYSICAL TECHNIQUES IN PHOTOSYNTHESIS 2008. [DOI: 10.1007/978-1-4020-8250-4_14] [Citation(s) in RCA: 7] [Impact Index Per Article: 0.4] [Reference Citation Analysis] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 12/15/2022]
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Schiemann O, Prisner TF. Long-range distance determinations in biomacromolecules by EPR spectroscopy. Q Rev Biophys 2007; 40:1-53. [PMID: 17565764 DOI: 10.1017/s003358350700460x] [Citation(s) in RCA: 423] [Impact Index Per Article: 24.9] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022]
Abstract
Electron paramagnetic resonance (EPR) spectroscopy provides a variety of tools to study structures and structural changes of large biomolecules or complexes thereof. In order to unravel secondary structure elements, domain arrangements or complex formation, continuous wave and pulsed EPR methods capable of measuring the magnetic dipole coupling between two unpaired electrons can be used to obtain long-range distance constraints on the nanometer scale. Such methods yield reliably and precisely distances of up to 80 A, can be applied to biomolecules in aqueous buffer solutions or membranes, and are not size limited. They can be applied either at cryogenic or physiological temperatures and down to amounts of a few nanomoles. Spin centers may be metal ions, metal clusters, cofactor radicals, amino acid radicals, or spin labels. In this review, we discuss the advantages and limitations of the different EPR spectroscopic methods, briefly describe their theoretical background, and summarize important biological applications. The main focus of this article will be on pulsed EPR methods like pulsed electron-electron double resonance (PELDOR) and their applications to spin-labeled biosystems.
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Affiliation(s)
- Olav Schiemann
- Institute of Physical and Theoretical Chemistry, Center for Biomolecular Magnetic Resonance, J. W. Goethe-University Frankfurt, 60438 Frankfurt am Main, Germany.
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Piton N, Mu Y, Stock G, Prisner TF, Schiemann O, Engels JW. Base-specific spin-labeling of RNA for structure determination. Nucleic Acids Res 2007; 35:3128-43. [PMID: 17452362 PMCID: PMC1891445 DOI: 10.1093/nar/gkm169] [Citation(s) in RCA: 123] [Impact Index Per Article: 7.2] [Reference Citation Analysis] [Abstract] [MESH Headings] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 01/16/2023] Open
Abstract
To facilitate the measurement of intramolecular distances in solvated RNA systems, a combination of spin-labeling, electron paramagnetic resonance (EPR), and molecular dynamics (MD) simulation is presented. The fairly rigid spin label 2,2,5,5-tetramethyl-pyrrolin-1-yloxyl-3-acetylene (TPA) was base and site specifically introduced into RNA through a Sonogashira palladium catalyzed cross-coupling on column. For this purpose 5-iodo-uridine, 5-iodo-cytidine and 2-iodo-adenosine phosphoramidites were synthesized and incorporated into RNA-sequences. Application of the recently developed ACE chemistry presented the main advantage to limit the reduction of the nitroxide to an amine during the oligonucleotide automated synthesis and thus to increase substantially the reliability of the synthesis and the yield of labeled oligonucleotides. 4-Pulse Electron Double Resonance (PELDOR) was then successfully used to measure the intramolecular spin-spin distances in six doubly labeled RNA-duplexes. Comparison of these results with our previous work on DNA showed that A- and B-Form can be differentiated. Using an all-atom force field with explicit solvent, MD simulations gave results in good agreement with the measured distances and indicated that the RNA A-Form was conserved despite a local destabilization effect of the nitroxide label. The applicability of the method to more complex biological systems is discussed.
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Affiliation(s)
- Nelly Piton
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Yuguang Mu
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Gerhard Stock
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Thomas F. Prisner
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Olav Schiemann
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
| | - Joachim W. Engels
- Institute of Organic Chemistry and Chemical Biology, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany, Institute of Physical and Theoretical Chemistry, J. W. Goethe-University, Max-von-Laue Strasse 7, 60438 Frankfurt am Main, Germany and Center of Biological Magnetic Resonance, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551, Singapore
- *To whom correspondence should be addressed.+49-69-798-29150+49-69-798-29148 Correspondence may also be addressed to Olav Schiemann. +49-69-798-29786+49-69-798-29404
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