Structure and nature of manganese(II) imidazole complexes in frozen aqueous solutions

Simulating Metal-Imidazole Complexes

One commonly observed binding motif in metalloproteins involves the interaction between a metal ion and histidine's imidazole side chains. Although previous imidazole-M(II) parameters established the flexibility and reliability of the 12-6-4 Lennard-Jones (LJ)-type nonbonded model by simply tuning the ligating atom's polarizability, they have not been applied to multiple-imidazole complexes. To fill this gap, we systematically simulate multiple-imidazole complexes (ranging from one to six) for five metal ions (Co(II), Cu(II), Mn(II), Ni(II), and Zn(II)) which commonly appear in metalloproteins. Using extensive (40 ns per PMF window) sampling to assemble free energy association profiles (using OPC water and standard HID imidazole charge models from AMBER) and comparing the equilibrium distances to DFT calculations, a new set of parameters was developed to focus on energetic and geometric features of multiple-imidazole complexes. The obtained free energy profiles agree with the experimental binding free energy and DFT calculated distances. To validate our model, we show that we can close the thermodynamic cycle for metal-imidazole complexes with up to six imidazole molecules in the first solvation shell. Given the success in closing the thermodynamic cycles, we then used the same extended sampling method for six other metal ions (Ag(I), Ca(II), Cd(II), Cu(I), Fe(II), and Mg(II)) to obtain new parameters. Since these new parameters can reproduce the one-imidazole geometry and energy accurately, we hypothesize that they will reasonably predict the binding free energy of higher-level coordination numbers. Hence, we did not extend the analysis of these ions up to six imidazole complexes. Overall, the results shed light on metal-protein interactions by emphasizing the importance of ligand-ligand interaction and metal-π-stacking within metalloproteins.

An Atomically Dispersed Mn-Photocatalyst for Generating Hydrogen Peroxide from Seawater via the Water Oxidation Reaction (WOR)

In this work, we have fabricated an aryl amino-substituted graphitic carbon nitride (g-C₃N₄) catalyst with atomically dispersed Mn capable of generating hydrogen peroxide (H₂O₂) directly from seawater. This new catalyst exhibited excellent reactivity, obtaining up to 2230 μM H₂O₂ in 7 h from alkaline water and up to 1800 μM from seawater under identical conditions. More importantly, the catalyst was quickly recovered for subsequent reuse without appreciable loss in performance. Interestingly, unlike the usual two-electron oxygen reduction reaction pathway, the generation of H₂O₂ was through a less common two-electron water oxidation reaction (WOR) process in which both the direct and indirect WOR processes occurred; namely, photoinduced h⁺ directly oxidized H₂O to H₂O₂ via a one-step 2e^- WOR, and photoinduced h⁺ first oxidized a hydroxide (OH^-) ion to generate a hydroxy radical (^•OH), and H₂O₂ was formed indirectly by the combination of two ^•OH. We have characterized the material, at the catalytic sites, at the atomic level using electron paramagnetic resonance, X-ray absorption near edge structure, extended X-ray absorption fine structure, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, magic-angle spinning solid-state NMR spectroscopy, and multiscale molecular modeling, combining classical reactive molecular dynamics simulations and quantum chemistry calculations.

Role of decomposition products in the oxidation of cyclohexene using a manganese(III) complex

Metal complexes are extensively explored as catalysts for oxidation reactions; molecular-based mechanisms are usually proposed for such reactions. However, the roles of the decomposition products of these materials in the catalytic process have yet to be considered for these reactions. Herein, the cyclohexene oxidation in the presence of manganese(III) 5,10,15,20-tetra(4-pyridyl)-21H,23H-porphine chloride tetrakis(methochloride) (1) in a heterogeneous system via loading the complex on an SBA-15 substrate is performed as a study case. A molecular-based mechanism is usually suggested for such a metal complex. Herein, 1 was selected and investigated under the oxidation reaction by iodosylbenzene or (diacetoxyiodo)benzene (PhI(OAc)₂). In addition to 1, at least one of the decomposition products of 1 formed during the oxidation reaction could be considered a candidate to catalyze the reaction. First-principles calculations show that Mn dissolution is energetically feasible in the presence of iodosylbenzene and trace amounts of water.

Dynamic Polyphosphate Metabolism Coordinating with Manganese Ions Defends against Oxidative Stress in the Extreme Bacterium Deinococcus radiodurans

Deinococcus radiodurans is an extreme bacterium with unparalleled resistance to oxidative stresses. Accumulation of intracellular Mn²⁺ complexing with small metabolites is the key contributor to the tolerance of D. radiodurans against oxidative stress. However, the intracellular reservoir of Mn ions and homeostatic regulation of the Mn complex in D. radiodurans remain unclear. We identified an evolutionarily ancient and negatively charged phosphate polymer (polyphosphate [PolyP]) in D. radiodurans We investigated PolyP metabolism in the response of D. radiodurans to oxidative stress. The genes dr1939, encoding polyphosphatase kinase (PPK_Dr; the subscript "Dr" refers to D. radiodurans), and dra0185, encoding exopolyphosphatase (PPX_Dr), were identified. PPX_Dr is a novel exopolyphosphatase with a cofactor preference to Mn²⁺, which enhances the dimerization and activity of PPX_Dr to allow the effective cleavage of PolyP-Mn. PPK_Dr and PPX_Dr exhibited different dynamic expression profiles under oxidative stress. First, ppk_Dr was upregulated leading to the accumulation of PolyP, which chelated large amounts of intracellular Mn ions. Subsequently, the expression level of ppk_Dr decreased while ppx_Dr was substantially upregulated and effectively hydrolyzed inactive PolyP-Mn to release phosphate (Pi) and Mn²⁺, which could form into Mn-Pi complexes to scavenge O₂ ^- and protect proteins from oxidative damage. Hence, dynamic cellular PolyP metabolites complexed with free Mn ions highlight a defense strategy of D. radiodurans in response to oxidative stress.IMPORTANCE The Mn-phosphate complex (Mn-Pi) plays a key role in the cellular resistance of radioresistant bacteria. The evolutionarily ancient polyphosphate polymers (polyphosphate [PolyP]) could effectively chelate Mn²⁺ and donate phosphates. However, the intracellular reservoir of Mn ions and homeostatic regulation of the Mn-Pi complex remain unclear. Here, we investigated the relationship of PolyP metabolites and Mn²⁺ homeostasis and how they function to defend against oxidative stress in the radioresistant bacterium Deinococcus radiodurans We found that PPX_Dr (the subscript "Dr" refers to D. radiodurans) is a novel exopolyphosphatase with a cofactor preference for Mn²⁺, mediating PolyP-Mn degradation into Pi and Mn ions. The formed Mn-Pi complexes effectively protect proteins. The dynamic PolyP metabolism coordinating with Mn ions is a defense strategy of D. radiodurans in response to oxidative stress. The findings not only provide new insights into the resistance mechanism of the extreme bacterium D. radiodurans but also broaden our understanding of the functions of PolyP metabolism in organisms.

Understanding the g-tensors of perchlorotriphenylmethyl and Finland-type trityl radicals

The 285 GHz EPR spectra of perchlorotriphenylmethyl and tetrathiatriarylmethyl radicals in frozen solution have been accurately measured. The relationship between their molecular structures and their g-tensors has been investigated with the aid of DFT calculations, revealing that the degree of spin density delocalization away from the central methylene carbon is an important determining factor of the g-anisotropy. In particular, the small amount of spin densities on the Cl or S heteroatoms at the 2 and 6 positions with respect to the central carbon have the strongest influence. Furthermore, the amount of spin densities on these heteroatoms and thus the anisotropy can be modulated by the protonation (esterification) state of the carboxylate groups at the 4 position. These results provide unique insights into the g-anisotropy of persistent trityl radicals and how it can be tuned.

Murine Calprotectin Coordinates Mn(II) at a Hexahistidine Site with Ca(II)-Dependent Affinity

Manganese is an essential metal ion that bacterial pathogens need to acquire from the vertebrate host during infection. In the mammalian nutritional immunity strategy to combat bacterial infection, the host restricts bacterial access to Mn(II) by sequestering this metal nutrient using the protein calprotectin (CP). The role of murine calprotectin (mCP) in Mn(II) sequestration has been demonstrated in vivo, but the molecular basis of this function has not been evaluated. Herein, biochemical assays and electron paramagnetic resonance (EPR) spectroscopy are employed to characterize the Mn(II) binding properties of mCP. We report that mCP has one high-affinity Mn(II) binding site. This site is a His₆ site composed of His17 and His27 of mS100A8 and His92, His97, His105, and His107 of mS100A9. Similar to the human ortholog (hCP), Ca(II) binding to the EF-hand domains of mCP enhances the Mn(II) affinity of the protein; however, this effect requires ≈10-fold more Ca(II) than was previously observed for hCP. Mn(II) coordination to the His₆ site also promotes self-association of two mCP heterodimers to form a heterotetramer. Low-temperature X-band EPR spectroscopy revealed a nearly octahedral Mn(II) coordination sphere for the Mn(II)-His₆ site characterized by the zero-field splitting parameters D = 525 MHz and E/D = 0.3. Further electron-nuclear double resonance studies with globally ¹⁵N-labeled mCP provided hyperfine couplings from the coordinating ε-nitrogen atoms of the His ligands (a_iso = 4.3 MHz) as well as the distal δ-nitrogen atoms (a_iso = 0.25 MHz). Mn(II) competition assays between mCP and two bacterial Mn(II) solute-binding proteins, staphylococcal MntC and streptococcal PsaA, showed that mCP outcompetes both proteins for Mn(II) under conditions of excess Ca(II). In total, this work provides the first coordination chemistry study of mCP and reveals striking similarities in the Mn(II) coordination sphere as well as notable differences in the Ca(II) sensitivity and oligomerization behavior between hCP and mCP.

Structural, Spectroscopic, Electrochemical, and Magnetic Properties for Manganese(II) Triazamacrocyclic Complexes

We report the synthesis of [Mn(tacud)2](OTf)2 (1) (tacud = 1,4,8-triazacycloundecane), [Mn(tacd)2](OTf)2 (2) (tacd = 1,4,7-triazacyclodecane), and [Mn(tacn)2](OTf)2 (3) (tacn = 1,4,7-triazacyclononane). Electrochemical measurements on the MnIII/II redox couple show that complex 1 has the largest anodic potential of the set (E 1/2 = 1.16 V vs NHE, ΔE p = 106 mV) compared to 2 (E 1/2 = 0.95 V, ΔE p = 108 mV) and 3 (E 1/2 = 0.93 V, ΔE p = 96 mV). This is due to the fact that 1 has the fewest 5-membered chelate rings and thus is least stabilized. Magnetic studies of 1-3 revealed that all complexes remain high spin throughout the temperature range investigated (2 - 300 K). X-band EPR investigations in methanol glass indicated that the manganese(II) centers for 2 and 3 resided in a more distorted octahedral geometric configuration compared to 1. To ease spectral interpretation and extract ZFS parameters, we performed high-frequency high-field EPR (HFEPR) at frequencies above 200 GHz and a field of 7.5 T. Simulation of the spectral data yielded g = 2.0013 and D = -0.031 cm-1 for 1, g = 2.0008, D = -0.0824 cm-1, |E/D| = 0.12 for 2, and g = 2.00028, D = -0.0884 cm-1 for 3. These results are consistent with 3 possessing the most distorted geometry. Calculations (PBE0/6-31G(d)) were performed on 1-3. Results show that 1 has the largest HOMO-LUMO gap energy (6.37 eV) compared to 2 (6.12 eV) and 3 (6.26 eV). Complex 1 also has the lowest HOMO energies indicating higher stability.

Triple resonance EPR spectroscopy determines the Mn2+ coordination to ATP

Mn²⁺ often serves as a paramagnetic substitute to Mg²⁺, providing means for exploring the close environment of Mg²⁺ in many biological systems where it serves as an essential co-factor. This applies to proteins with ATPase activity, where the ATP hydrolysis requires the binding of Mg²⁺-ATP to the ATPase active site. In this context, it is important to distinguish between the Mn²⁺ coordination mode with free ATP in solution as compared to the protein bound case. In this work, we explore the Mn²⁺ complexes with ATP, the non-hydrolysable ATP analog, AMPPNP, and ADP free in solution. Using W-band ³¹P electron-nuclear double resonance (ENDOR) we obtained information about the coordination to the phosphates, whereas from electron-electron double resonance (ELDOR) - detected NMR (EDNMR) we determined the coordination to an adenosine nitrogen. The coordination to these ligands has been reported earlier, but whether the nitrogen and phosphate coordination is within the same nucleotide molecules or different ones is still under debate. By applying the correlation technique, THYCOS (triple hyperfine correlation spectroscopy), and measuring ¹⁵N-³¹P correlations we establish that in Mn-ATP in solution both phosphates and a nitrogen are coordinated to the Mn²⁺ ion. We also carried out DFT calculations to substantiate this finding. In addition, we expanded the understanding of the THYCOS experiment by comparing it to 2D-EDNMR for ⁵⁵Mn-³¹P correlation experiments and through simulations of THYCOS and 2D-EDNMR spectra with ¹⁵N-³¹P correlations.

A charge polarization model for the metal-specific activity of superoxide dismutases

The pathogenicity of Staphylococcus aureus is enhanced by having two superoxide dismutases (SODs): a Mn-specific SOD and another that can use either Mn or Fe. Using 94 GHz electron-nuclear double resonance (ENDOR) and electron double resonance detected (ELDOR)-NMR we show that, despite their different metal-specificities, their structural and electronic similarities extend down to their active-site ¹H- and ¹⁴N-Mn(ii) hyperfine interactions. However these interactions, and hence the positions of these nuclei, are different in the inactive Mn-reconstituted Escherichia coli Fe-specific SOD. Density functional theory modelling attributes this to a different angular position of the E. coli H171 ligand. This likely disrupts the Mn-H171-E170' triad causing a shift in charge and in metal redox potential, leading to the loss of activity. This is supported by the correlated differences in the Mn(ii) zero-field interactions of the three SOD types and suggests that the triad is important for determining metal specific activity.

Direct Spectroscopic Detection of ATP Turnover Reveals Mechanistic Divergence of ABC Exporters

We have applied high-field (W-band) pulse electron-nuclear double resonance (ENDOR) and electron-electron double resonance (ELDOR)-detected nuclear magnetic resonance (EDNMR) to characterize the coordination sphere of the Mn²⁺ co-factor in the nucleotide binding sites (NBSs) of ABC transporters. MsbA and BmrCD are two efflux transporters hypothesized to represent divergent catalytic mechanisms. Our results reveal distinct coordination of Mn²⁺ to ATP and transporter residues in the consensus and degenerate NBSs of BmrCD. In contrast, the coordination of Mn²⁺ at the two NBSs of MsbA is similar, which provides a mechanistic rationale for its higher rate constant of ATP hydrolysis relative to BmrCD. Direct detection of vanadate ion, trapped in a high-energy post-hydrolysis intermediate, further supports the notion of asymmetric hydrolysis by the two NBSs of BmrCD. The integrated spectroscopic approach presented here, which link energy input to conformational dynamics, can be applied to a variety of systems powered by ATP turnover.

ELDOR-detected NMR: A general and robust method for electron-nuclear hyperfine spectroscopy?

ELDOR-detected NMR (EDNMR) performed at higher magnetic fields is becoming an increasingly popular alternative to conventional ENDOR for the characterization of electron-nuclear hyperfine interactions owing to its enhanced sensitivity. However there are two key problems that limit its widespread adoption, with factors controlling: (i) lineshape distortions and; (ii) overall spectral resolution, still largely understood only at a qualitative level. Indeed highly anisotropic (dipolar) coupled species are particularly problematic in the EDNMR experiment. Nor is it clear as to whether line intensities measured in EDNMR can provide quantitative information. Here we describe how all these problems can be overcome for a nitroxide radical as model system. We introduce a simulation procedure/protocol for the simulation of EDNMR line-shapes collected over a range of high turning angle (HTA) pulse lengths. It is shown that spectral line-shapes can be robustly reproduced and that the intensities of spectral lines and the spin nutation behavior can be quantitatively assessed. This broadens the scope of the EDNMR experiment as a generally applicable, quantitative double resonance method.

Generic tags for Mn(ii) and Gd(iii) spin labels for distance measurements in proteins

The coordination mode of the metal ion in the spin label affects the distance distribution determined by DEER distance measurements.

The Use of Mn(II) Bound to His-tags as Genetically Encodable Spin-Label for Nanometric Distance Determination in Proteins

A genetically encodable paramagnetic spin-label capable of self-assembly from naturally available components would offer a means for studying the in-cell structure and interactions of a protein by electron paramagnetic resonance (EPR). Here, we demonstrate pulse electron-electron double resonance (DEER) measurements on spin-labels consisting of Mn(II) ions coordinated to a sequence of histidines, so-called His-tags, that are ubiquitously added by genetic engineering to facilitate protein purification. Although the affinity of His-tags for Mn(II) was low (800 μM), Mn(II)-bound His-tags yielded readily detectable DEER time traces even at concentrations expected in cells. We were able to determine accurately the distance between two His-tag Mn(II) spin-labels at the ends of a rigid helical polyproline peptide of known structure, as well as at the ends of a completely cell-synthesized 3-helix bundle. This approach not only greatly simplifies the labeling procedure but also represents a first step towards using self-assembling metal spin-labels for in-cell distance measurements.

Peroxidase-like oxidative activity of a manganese-coordinated histidyl bolaamphiphile self-assembly

A peroxidase-like catalyst was constructed through the self-assembly of histidyl bolaamphiphiles coordinated to Mn(2+) ions. The prepared catalyst exhibited oxidation activity for the organic substrate o-phenylenediamine (OPD) in the presence of hydrogen peroxide (H2O2). The histidyl bolaamphiphiles of bis(N-alpha-amido-histidine)-1,7-heptane dicarboxylates self-assembled to make spherical structures in an aqueous solution. Subsequent association of Mn(2+) ions with the histidyl imidazoles in the self-assembly produced catalytic active sites. The optimal Mn(2+) ion concentration was determined and coordination of the Mn(2+) ion with multiple histidine imidazoles was investigated using spectroscopy analysis. The activation energy of the produced catalysts was 55.0 kJ mol(-1), which was comparable to other peroxidase-mimetic catalysts. A detailed kinetics study revealed that the prepared catalyst followed a ping-pong mechanism and that the turnover reaction was promoted by increasing the substrate concentration. Finally, application of the prepared catalyst for glucose detection was demonstrated through cascade enzyme catalysis. This study demonstrated a facile way to prepare an enzyme-mimetic catalyst through the self-assembly of an amphiphilic molecule containing amino acid segments.

A Newly Identified Passive Hyperaccumulator Eucalyptus grandis × E. urophylla under Manganese Stress

Manganese (Mn) is an essential micronutrient needed for plant growth and development, but can be toxic to plants in excess amounts. However, some plant species have detoxification mechanisms that allow them to accumulate Mn to levels that are normally toxic, a phenomenon known as hyperaccumulation. These species are excellent candidates for developing a cost-effective remediation strategy for Mn-polluted soils. In this study, we identified a new passive Mn-hyperaccumulator Eucalyptus grandis × E. urophylla during a field survey in southern China in July 2010. This hybrid can accumulate as much as 13,549 mg/kg DW Mn in its leaves. Our results from Scanning Electron Microscope (SEM) X-ray microanalysis indicate that Mn is distributed in the entire leaf and stem cross-section, especially in photosynthetic palisade, spongy mesophyll tissue, and stem xylem vessels. Results from size-exclusion chromatography coupled with ICP-MS (Inductively coupled plasma mass spectrometry) lead us to speculate that Mn associates with relatively high molecular weight proteins and low molecular weight organic acids, including tartaric acid, to avoid Mn toxicity. Our results provide experimental evidence that both proteins and organic acids play important roles in Mn detoxification in Eucalyptus grandis × E. urophylla. The key characteristics of Eucalyptus grandis × E. urophylla are an increased Mn translocation facilitated by transpiration through the xylem to the leaves and further distribution throughout the leaf tissues. Moreover, the Mn-speciation profile obtained for the first time in different cellular organelles of Eucalyptus grandis × E. urophylla suggested that different organelles have differential accumulating abilities and unique mechanisms for Mn-detoxification.

Pulse Electron Double Resonance Detected Multinuclear NMR Spectra of Distant and Low Sensitivity Nuclei and Its Application to the Structure of Mn(II) Centers in Organisms

The ability to characterize the structure of metal centers beyond their primary ligands is important to understanding their chemistry. High-magnetic-field pulsed electron double resonance detected NMR (ELDOR-NMR) is shown to be a very sensitive approach to measuring the multinuclear NMR spectra of the nuclei surrounding Mn(II) ions. Resolved spectra of intact organisms with resonances arising from (55)Mn, (31)P, (1)H, (39)K, (35)Cl, (23)Na, and (14)N nuclei surrounding Mn(2+) centers were obtained. Naturally abundant cellular (13)C could be routinely measured as well. The amplitudes of the (14)N and (2)H ELDOR-NMR spectra were found to be linearly dependent on the number of nuclei in the ligand sphere. The evolution of the Mn(II) ELDOR-NMR spectra as a function of excitation time was found to be best described by a saturation phenomenon rather than a coherently driven process. Mn(II) ELDOR-NMR revealed details about not only the immediate ligands to the Mn(II) ions but also more distant nuclei, providing a view of their extended structures. This will be important for understanding the speciation and chemistry of the manganese complexes as well as other metals found in organisms.

Manganese binding properties of human calprotectin under conditions of high and low calcium: X-ray crystallographic and advanced electron paramagnetic resonance spectroscopic analysis

The antimicrobial protein calprotectin (CP), a hetero-oligomer of the S100 family members S100A8 and S100A9, is the only identified mammalian Mn(II)-sequestering protein. Human CP uses Ca(II) ions to tune its Mn(II) affinity at a biologically unprecedented hexahistidine site that forms at the S100A8/S100A9 interface, and the molecular basis for this phenomenon requires elucidation. Herein, we investigate the remarkable Mn(II) coordination chemistry of human CP using X-ray crystallography as well as continuous-wave (CW) and pulse electron paramagnetic resonance (EPR) spectroscopies. An X-ray crystallographic structure of Mn(II)-CP containing one Mn(II), two Ca(II), and two Na(I) ions per CP heterodimer is reported. The CW EPR spectrum of Ca(II)- and Mn(II)-bound CP prepared with a 10:0.9:1 Ca(II):Mn(II):CP ratio is characterized by an unusually low zero-field splitting of 485 MHz (E/D = 0.30) for the S = 5/2 Mn(II) ion, consistent with the high symmetry of the His6 binding site observed crystallographically. Results from electron spin-echo envelope modulation and electron-nuclear double resonance experiments reveal that the six Mn(II)-coordinating histidine residues of Ca(II)- and Mn(II)-bound CP are spectroscopically equivalent. The observed (15)N (I = 1/2) hyperfine couplings (A) arise from two distinct classes of nitrogen atoms: the coordinating ε-nitrogen of the imidazole ring of each histidine ligand (A = [3.45, 3.71, 5.91] MHz) and the distal δ-nitrogen (A = [0.11, 0.18, 0.42] MHz). In the absence of Ca(II), the binding affinity of CP for Mn(II) drops by two to three orders of magnitude and coincides with Mn(II) binding at the His6 site as well as other sites. This study demonstrates the role of Ca(II) in enabling high-affinity and specific binding of Mn(II) to the His6 site of human calprotectin.

Nanometric distance measurements between Mn(ii)DOTA centers

The distance between two Mn(ii)DOTA complexes attached to the ends of polyproline helices of varying lengths was measured by 94 GHz PELDOR spectroscopy with good accuracy demonstrating their effectiveness as spin-labels.

Variations in Mn(II) speciation among organisms: what makes D. radiodurans different

The manganese(II) speciation in intact cells of D. radiodurans, E. coli, S. cerevisiae and Arabidopsis thaliana seeds was measured using high-field electron paramagnetic resonance techniques. The majority of the Mn(II) ions in these organisms were six-coordinate, bound predominately by water, phosphates and nitrogen-based molecules. The relative distribution of the different phosphates in bacteria and S. cerevisiae was the same and dominated by monophosphate monoesters. Mn(II) was also found bound to the phosphate backbone of nucleic acids in these organisms. Phosphate ligation in Arabidopsis seeds was dominated by phytate. The extent of nitrogen ligation in the four organisms was also determined. On average, the Mn(II) in D. radiodurans had the most nitrogen ligands followed by E. coli. This was attributed to higher concentrations of Mn(II) bound to proteins in these species. Although constitutively expressed in all four organisms, MnSOD was only detected in D. radiodurans. As previously reported, D. radiodurans also accumulates a second abundant Mn containing protein species. The high concentration of proteinaceous Mn(II) is a unique feature of D. radiodurans.

Correlating nuclear frequencies by two-dimensional ELDOR-detected NMR spectroscopy

ELDOR (Electron Double Resonance)-detected NMR (EDNMR) is a pulse EPR experiment that is used to measure the transition frequencies of nuclear spins coupled to electron spins. These frequencies are further used to determine hyperfine and quadrupolar couplings, which are signatures of the electronic and spatial structures of paramagnetic centers. In recent years, EDNMR has been shown to be particularly useful at high fields/high frequencies, such as W-band (∼95 GHz, ∼3.5 T), for low γ quadrupolar nuclei. Although at high fields the nuclear Larmor frequencies are usually well resolved, the limited resolution of EDNMR still remains a major concern. In this work we introduce a two dimensional, triple resonance, correlation experiment based on the EDNMR pulse sequence, which we term 2D-EDNMR. This experiment allows circumventing the resolution limitation by spreading the signals in two dimensions and the observed correlations help in the assignment of the signals. First we demonstrate the utility of the 2D-EDNMR experiment on a nitroxide spin label, where we observe correlations between (14)N nuclear frequencies. Negative cross-peaks appear between lines belonging to different MS electron spin manifolds. We resolved two independent correlation patterns for nuclear frequencies arising from the EPR transitions corresponding to the (14)N mI=0 and mI=-1 nuclear spin states, which severely overlap in the one dimensional EDNMR spectrum. The observed correlations could be accounted for by considering changes in the populations of energy levels that S=1/2, I=1 spin systems undergo during the pulse sequence. In addition to these negative cross-peaks, positive cross-peaks appear as well. We present a theoretical model based on the Liouville equation and use it to calculate the time evolution of populations of the various energy levels during the 2D-EDNMR experiment and generated simulated 2D-EDMR spectra. These calculations show that the positive cross-peaks appear due to off resonance effects and/or nuclear relaxation effects. These results suggest that the 2D-EDNMR experiment can be also useful for relaxation pathway studies. Finally we present preliminary results demonstrating that 2D-EDNMR can resolve overlapping (33)S and (14)N signals of type 1 Cu(II) center in (33)S enriched Azurin.