Solution Tetrahydrobiopterin Radical vs. the Enzyme-Bound Radical: A Paramagnetic Reconciliation

2021 ◽  
Vol 16 ◽  
Author(s):  
Yaser Nejaty Jahromy
Keyword(s):  
Nitric Oxide ◽  
Density Functional ◽  
Double Resonance ◽  
Nicotinamide Adenine Dinucleotide Phosphate ◽  
Cation Radical ◽  
Spectral Simulation ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Isotropic Hyperfine Coupling ◽  
The 1960S

Background: Nitric oxide synthase (NOS) catalyzes the formation of nitric oxide (NO) and citrulline from L-arginine, dioxygen (O2), and nicotinamide adenine dinucleotide phosphate (NADPH) in a two-step reaction, with the enzyme-bound intermediate Nω-hydroxy-L-arginine (NHA). Previous electron paramagnetic resonance (EPR) studies of NOS reaction have shown that (6R, 1'R, 2'S)-6-(l',2'-dihydroxypropyl)-5,6,7,8-tetrahydropterin (H4B) acts as a single electron donor in both steps of the reaction, resulting in the transient generation of a tetrahydropterin cation radical (H4B•+). Results: H4B•+ can also be chemically generated in strongly acidic solutions. EPR studies of chemically generated H4B•+ and similar pterin radicals date back to the 1960s. However, the reported paramagnetic parameters of H4B•+ in NOS do not seem to match the corresponding reported parameters for either H4B•+ or other pterin centered radicals chemically generated in solution. In particular, the rather isotropic hyperfine coupling of ca. 45 MHz for 1H6 of H4B•+ in NOS is at least 15 MHz larger than that of H4B•+ or any other previously studies pterin solution radical. In the work reported here, a combination of 9.5 - 9.8 GHz contentious wave (cw-) EPR, 34GHz 1H electron nuclear double resonance (ENDOR), spectral simulation and Density Functional Theory (DFT) calculations were used to investigate this seeming discrepancy. Conclusion: We demonstrated that the differences in the paramagnetic parameters of the chemically generated H4B radicals in solutions and those of the H4B radicals in NOS are consistent with the presence of two different conformers of the same cation radical in the two media.

scholarly journals Probing the C 60 triplet state coupling to nuclear spins inside and out

2013 ◽  
Vol 371 (1998) ◽  
pp. 20120475 ◽  
Author(s):  
Vasileia Filidou ◽  
Salvatore Mamone ◽  
Stephanie Simmons ◽  
Steven D. Karlen ◽  
Harry L. Anderson ◽  
...  
Keyword(s):  
Small Molecules ◽  
Density Functional ◽  
Double Resonance ◽  
Nuclear Spins ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Endohedral Fullerenes ◽  
New Class ◽  
Pulsed Electron Paramagnetic Resonance ◽  
Electron Paramagnetic

The photoexcitation of functionalized fullerenes to their paramagnetic triplet electronic state can be studied by pulsed electron paramagnetic resonance (EPR) spectroscopy, whereas the interactions of this state with the surrounding nuclear spins can be observed by a related technique: electron nuclear double resonance (ENDOR). In this study, we present EPR and ENDOR studies on a functionalized exohedral fullerene system, dimethyl[9-hydro (C60-Ih)[5,6]fulleren-1(9 H )-yl]phosphonate (DMHFP), where the triplet electron spin has been used to hyperpolarize, couple and measure two nuclear spins. We go on to discuss the extension of these methods to study a new class of endohedral fullerenes filled with small molecules, such as H 2 @C 60 , and we relate the results to density functional calculations.

scholarly journals Statistical analysis of ENDOR spectra

2021 ◽  
Vol 118 (27) ◽  
pp. e2023615118
Author(s):  
Yvo Pokern ◽  
Benjamin Eltzner ◽  
Stephan F. Huckemann ◽  
Clemens Beeken ◽  
JoAnne Stubbe ◽  
...  
Keyword(s):  
Statistical Analysis ◽  
Density Functional ◽  
Double Resonance ◽  
Larmor Frequency ◽  
Isotopic Labeling ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Molecular Conformations ◽  
Biologically Relevant ◽  
Quantitative Spectral Analysis

Electron–nuclear double resonance (ENDOR) measures the hyperfine interaction of magnetic nuclei with paramagnetic centers and is hence a powerful tool for spectroscopic investigations extending from biophysics to material science. Progress in microwave technology and the recent availability of commercial electron paramagnetic resonance (EPR) spectrometers up to an electron Larmor frequency of 263 GHz now open the opportunity for a more quantitative spectral analysis. Using representative spectra of a prototype amino acid radical in a biologically relevant enzyme, the Y122• in Escherichia coli ribonucleotide reductase, we developed a statistical model for ENDOR data and conducted statistical inference on the spectra including uncertainty estimation and hypothesis testing. Our approach in conjunction with 1H/2H isotopic labeling of Y122• in the protein unambiguously established new unexpected spectral contributions. Density functional theory (DFT) calculations and ENDOR spectral simulations indicated that these features result from the beta-methylene hyperfine coupling and are caused by a distribution of molecular conformations, likely important for the biological function of this essential radical. The results demonstrate that model-based statistical analysis in combination with state-of-the-art spectroscopy accesses information hitherto beyond standard approaches.

The Microscopic Structure Of Shallow Donors In Silicon Carbide

1996 ◽  
Vol 442 ◽  
Author(s):  
J.-M. Spaeth ◽  
S. Greulich-Weber ◽  
M. März ◽  
E. N. Kalabukhova ◽  
S. N. Lukin
Keyword(s):  
High Temperature ◽  
Low Temperature ◽  
Temperature Dependence ◽  
Double Resonance ◽  
Epr Spectra ◽  
Electron Nuclear Double Resonance ◽  
Paramagnetic Resonance ◽  
Vacancy Pair ◽  
Temperature Spectra ◽  
Electron Paramagnetic

AbstractThe electronic structure of nitrogen donors in 6H-, 4H- and 3C-SiC is investigated by measuring the nitrogen hyperfine (hf) interactions with electron nuclear double resonance (ENDOR) and the temperature dependence of the hf split electron paramagnetic resonance (EPR) spectra. Superhyperfine (shf) interactions with many shells of 13C and 29Si were measured in 6H-SiC. The hf and shf interactions are discussed in the framework of effective mass theory. The temperature dependence is explained with the thermal occupation of the lowest valley-orbit split A1 and E states. It is proposed that the EPR spectra of P donors observed previously in neutron transmuted 6H-SiC at low temperature (<10K) and high temperature (>60K) are all due to substitutional P donors on the two quasi-cubic and hexagonal Si sites, whereby at low temperature the E state is occupied and at high temperature the A1 state. The low temperature spectra are thus thought not to be due to P-vacancy pair defects as proposed previously.

scholarly journals Selective 13C labelling reveals the electronic structure of flavocoenzyme radicals

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Erik Schleicher ◽  
Stephan Rein ◽  
Boris Illarionov ◽  
Ariane Lehmann ◽  
Tarek Al Said ◽  
...  
Keyword(s):  
Electronic Structure ◽  
Blue Light ◽  
Density Functional ◽  
Double Resonance ◽  
Density Functional Theory Calculations ◽  
Microwave Frequency ◽  
Fine Tuning ◽  
Electron Nuclear Double Resonance ◽  
Intermediate Radical ◽  
Wide Range

AbstractFlavocoenzymes are nearly ubiquitous cofactors that are involved in the catalysis and regulation of a wide range of biological processes including some light-induced ones, such as the photolyase-mediated DNA repair, magnetoreception of migratory birds, and the blue-light driven phototropism in plants. One of the factors that enable versatile flavin-coenzyme biochemistry and biophysics is the fine-tuning of the cofactor’s frontier orbital by interactions with the protein environment. Probing the singly-occupied molecular orbital (SOMO) of the intermediate radical state of flavins is therefore a prerequisite for a thorough understanding of the diverse functions of the flavoprotein family. This may be ultimately achieved by unravelling the hyperfine structure of a flavin by electron paramagnetic resonance. In this contribution we present a rigorous approach to obtaining a hyperfine map of the flavin’s chromophoric 7,8-dimethyl isoalloxazine unit at an as yet unprecedented level of resolution and accuracy. We combine powerful high-microwave-frequency/high-magnetic-field electron–nuclear double resonance (ENDOR) with 13C isotopologue editing as well as spectral simulations and density functional theory calculations to measure and analyse 13C hyperfine couplings of the flavin cofactor in DNA photolyase. Our data will provide the basis for electronic structure considerations for a number of flavin radical intermediates occurring in blue-light photoreceptor proteins.

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