19F Electron-Nuclear Double Resonance Reveals Interaction between Redox-Active Tyrosines across the α/ß Interface of E. coli Ribonucleotide Reductase.

Ribonucleotide reductases (RNRs) catalyze the reduction of ribonucleotides to deoxyribonucleotides, thereby playing a key role in DNA replication and repair. Escherichia coli class Ia RNR is an α2ß2 enzyme complex that uses a reversible multistep radical transfer (RT) over 32 Å across its two subunits, α and ß, to initiate, using its metallo-cofactor in ß2, nucleotide reduction in α2. Each step is proposed to involve a distinct proton-coupled electron-transfer (PCET) process. An unresolved step is the RT involving Y356(ß) and Y731(α) across the α/ß interface. Using 2,3,5-F3Y122-ß2 with 3,5-F2Y731-α2, GDP (substrate) and TTP (allosteric effector), a Y356• intermediate was trapped and its identity was verified by 263 GHz electron paramagnetic resonance (EPR) and 34 GHz pulse electron-electron double resonance spectroscopies. 94 GHz 19F electron-nuclear double resonance spectroscopy allowed measuring the interspin distances between Y356• and the 19F nuclei of 3,5-F2Y731 in this RNR mutant. Similar experiments with the double mutant E52Q/F3Y122-ß2 were carried out for comparison to the recently published cryo-EM structure of a holo RNR complex. For both mutant combinations, the distance measurements reveal two conformations of 3,5-F2Y731. Remarkably, one conformation is consistent with 3,5-F2Y731 within the H-bond distance to Y356•, whereas the second one is consistent with the conformation observed in the cryo-EM structure. The observations unexpectedly suggest the possibility of a colinear PCET, in which electron and proton are transferred from the same donor to the same acceptor between Y356 and Y731. The results highlight the important role of state-of-the-art EPR spectroscopy to decipher this mechanism.

Resolution of chemical shift anisotropy in 19F ENDOR spectroscopy at 263 GHz/9.4 T.

Pulsed 19F ENDOR spectroscopy provides a selective method for measuring angstrom to nanometer distances in structural biology. Here, the performance of 19F ENDOR at fields of 3.4 T and 9.4 T is compared using model compounds containing one to three 19F atoms. CF3 groups are included in two compounds, for which the possible occurrence of uniaxial rotation might affect the distance distribution. At 9.4 T, pronounced asymmetric features are observed in many of the presented 19F ENDOR spectra. Data analysis by spectral simulations shows that these features arise from the chemical shift anisotropy (CSA) of the 19F nuclei. This asymmetry is also observed at 3.4 T, albeit to a much smaller extent, confirming the physical origin of the effect. The CSA parameters are well consistent with DFT predicted values and can be extracted from simulation of the experimental data in favourable cases, thereby providing additional information about the geometrical and electronic structure of the spin system. The feasibility of resolving the CSA at 9.4 T provides important information for the interpretation of line broadening in ENDOR spectra also at lower fields, which is relevant for developing methods to extract distance distributions from 19F ENDOR spectra.

1H high field electron-nuclear double resonance spectroscopy at 263 GHz/9.4 T.

We present and discuss the performance of 1H electron-nuclear double resonance (ENDOR) at 263 GHz/9.4 T by employing a prototype, commercial quasi optical spectrometer. Basic instrumental features of the setup are described alongside a comprehensive characterization of the new ENDOR probe head design. The performance of three different ENDOR pulse sequences (Davies, Mims and CP-ENDOR) is evaluated using the 1H BDPA radical. A key feature of 263 GHz spectroscopy - the increase in orientation selectivity in comparison with 94 GHz experiments - is discussed in detail. For this purpose, the resolution of 1H ENDOR spectra at 263 GHz is verified using a representative protein sample containing approximately 15 picomoles of a tyrosyl radical. Davies ENDOR spectra recorded at 5 K reveal previously obscured spectral features, which are interpreted by spectral simulations aided by DFT calculations. Our analysis shows that seven internal proton couplings are detectable for this specific radical if sufficient orientation selectivity is achieved. The results prove the fidelity of 263 GHz experiments in reporting orientation-selected 1H ENDOR spectra and demonstrate that new significant information can be uncovered in complex molecular systems, owing to the enhanced resolution combined with high absolute sensitivity and no compromise in acquisition time.

Semi-Rigid Nitroxide Spin Label for Long-Range EPR Distance Measurements of Lipid Bilayer Embedded ß-Peptides.

ß-Peptides are an interesting new class of transmembrane model peptides based on their conformationally stable and well-defined secondary structures. Herein, we present the synthesis of the paramagnetic ß-amino acid ß3 -hTOPP (4-(3,3,5,5-tetramethyl-2,6-dioxo-4-oxylpiperazin-1-yl)-d-ß3 -homophenylglycine) that enables investigations of ß-peptides by EPR spectroscopy. This amino acid adds to the, to date, sparse number of ß-peptide spin labels. Its performance was evaluated by investigating the helical turn of a 314 -helical transmembrane model ß-peptide. Nanometer distances between two incorporated ß3 -hTOPP labels in different environments were measured by using pulsed electron/electron double resonance (PELDOR/DEER) spectroscopy. Due to the semi-rigid conformational design, the label delivers reliable distances and sharp (one-peak) distance distributions even in the lipid bilayer. The results indicate that the investigated ß-peptide folds into a 3.2514 helix and maintains this conformation in the lipid bilayer.