Are true scalar proton—proton connectivities ever measured in COSY spectra of paramagnetic macromolecules?

Inversion of Hyperpolarized 13C NMR Signals through Cross-Correlated Cross-Relaxation in Dissolution DNP Experiments

Dissolution dynamic nuclear polarization (DDNP) is a versatile tool to boost signal amplitudes in solution-state nuclear magnetic resonance (NMR) spectroscopy. For DDNP, nuclei are spin-hyperpolarized "ex situ" in a dedicated DNP device and then transferred to an NMR spectrometer for detection. Dramatic signal enhancements can be achieved, enabling shorter acquisition times, real-time monitoring of fast reactions, and reduced sample concentrations. Here, we show how the sample transfer in DDNP experiments can affect NMR spectra through cross-correlated cross-relaxation (CCR), especially in the case of low-field passages. Such processes can selectively invert signals of 13C spins in proton-carrying moieties. For their investigations, we use schemes for simultaneous or "parallel" detection of hyperpolarized 1H and 13C nuclei. We find that 1H → 13C CCR can invert signals of 13C spins if the proton polarization is close to 100%. We deduce that low-field passage in a DDNP experiment, a common occurrence due to the introduction of so-called "ultra-shielded" magnets, accelerates these effects due to field-dependent paramagnetic relaxation enhancements that can influence CCR. The reported effects are demonstrated for various molecules, laboratory layouts, and DDNP systems. As coupled 13C-1H spin systems are ubiquitous, we expect similar effects to be observed in various DDNP experiments. This might be exploited for selective spectroscopic labeling of hydrocarbons.

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. As a result, NMR analysis of paramagnetic complexes is limited in comparison to diamagnetic compounds and often relies on theoretical models. We report a toolbox of 1D (1H, proton-coupled 13C, selective 1H-decoupling 13C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods that enables unprecedented structural characterisation and in some cases, provides more structural information than would be observable for a diamagnetic analogue. We demonstrate the toolbox's broad versatility for fields from coordination chemistry and spin-crossover complexes to supramolecular chemistry through the characterisation of CoII and high-spin FeII mononuclear complexes as well as a Co4L6 cage.

Magnetic susceptibility and paramagnetism-based NMR

The magnetic interactions between the nuclear magnetic moment and the magnetic moment of unpaired electron(s) depend on the structure and dynamics of the molecules where the paramagnetic center is located and of their partners. The long-range nature of the magnetic interactions is thus a reporter of invaluable information for structural biology studies, when other techniques often do not provide enough data for the atomic-level characterization of the system. This precious information explains the flourishing of paramagnetism-assisted NMR studies in recent years. Many paramagnetic effects are related to the magnetic susceptibility of the paramagnetic metal. Although these effects have been known for more than half a century, different theoretical models and new approaches have been proposed in the last decade. In this review, we have summarized the consequences for NMR spectroscopy of magnetic interactions between nuclear and electron magnetic moments, and thus of the presence of a magnetic susceptibility due to metals, and we do so using a unified notation.

Paramagnetic NMR in solution and the solid state

The field of paramagnetic NMR has expanded considerably in recent years. This review addresses both the theoretical description of paramagnetic NMR, and the way in which it is currently practised. We provide a review of the theory of the NMR parameters of systems in both solution and the solid state. Here we unify the different languages used by the NMR, EPR, quantum chemistry/DFT, and magnetism communities to provide a comprehensive and coherent theoretical description. We cover the theory of the paramagnetic shift and shift anisotropy in solution both in the traditional formalism in terms of the magnetic susceptibility tensor, and using a more modern formalism employing the relevant EPR parameters, such as are used in first-principles calculations. In addition we examine the theory first in the simple non-relativistic picture, and then in the presence of spin-orbit coupling. These ideas are then extended to a description of the paramagnetic shift in periodic solids, where it is necessary to include the bulk magnetic properties, such as magnetic ordering at low temperatures. The description of the paramagnetic shift is completed by describing the current understanding of such shifts due to lanthanide and actinide ions. We then examine the paramagnetic relaxation enhancement, using a simple model employing a phenomenological picture of the electronic relaxation, and again using a more complex state-of-the-art theory which incorporates electronic relaxation explicitly. An additional important consideration in the solid state is the impact of bulk magnetic susceptibility effects on the form of the spectrum, where we include some ideas from the field of classical electrodynamics. We then continue by describing in detail the solution and solid-state NMR methods that have been deployed in the study of paramagnetic systems in chemistry, biology, and the materials sciences. Finally we describe a number of case studies in paramagnetic NMR that have been specifically chosen to highlight how the theory in part one, and the methods in part two, can be used in practice. The systems chosen include small organometallic complexes in solution, solid battery electrode materials, metalloproteins in both solution and the solid state, systems containing lanthanide ions, and multi-component materials used in pharmaceutical controlled-release formulations that have been doped with paramagnetic species to measure the component domain sizes.

Iron-molybdenum-oxo complexes as initiators for olefin autoxidation with O2

The reaction between [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) yields the novel tetranuclear complex [(TPA)Fe(μ-Cp*MoO3)]2(OTf)2, 1, with a rectangular [Mo-O-Fe-O-]2 core containing high-spin iron(ii) centres. 1 proved to be an efficient initiator/(pre)catalyst for the autoxidation of cis-cyclooctene with O2 to give cyclooctene epoxide. To test, which features of 1 are essential in this regard, analogues with zinc(ii) and cobalt(ii) central atoms, namely [(TPA)Zn(Cp*MoO3)](OTf), 3, and [(TPA)Co(Cp*MoO3)](OTf), 4, were prepared, which proved to be inactive. The precursor compounds of 1, [(TPA)Fe(MeCN)2](OTf)2 and [nBu4N](Cp*MoO3) as well as Cp2*Mo2O5, were found to be inactive, too. Reactivity studies in the absence of cyclooctene revealed that 1 reacts both with O2 and PhIO via loss of the Cp* ligands to give the triflate salt 2 of the known cation [((TPA)Fe)2(μ-O)(μ-MoO4)](2+). The cobalt analogue 4 reacts with O2 in a different way yielding [((TPA)Co)2(μ-Mo2O8)](OTf)2, 5, featuring a Mo2O8(4-) structural unit which is novel in coordination chemistry. The compound [(TPA)Fe(μ-MoO4)]2, 6, being related to 1, but lacking Cp* ligands failed to trigger autoxidation of cyclooctene. However, initiation of autoxidation by Cp* radicals was excluded via experiments including thermal dissociation of Cp2*.

NMR Spectroscopy of Paramagnetic Metalloproteins

This article deals with the solution structure determination of paramagnetic metalloproteins by NMR spectroscopy. These proteins were believed not to be suitable for NMR investigations for structure determination until a decade ago, but eventually novel experiments and software protocols were developed, with the aim of making the approach suitable for the goal and as user-friendly and safe as possible. In the article, we also give hints for the optimization of experiments with respect to each particular metal ion, with the aim of also providing a handy tool for nonspecialists. Finally, a section is dedicated to the significant progress made on 13C direct detection, which reduces the negative effects of paramagnetism and may constitute a new chapter in the whole field of NMR spectroscopy.

13C direct detected experiments: optimization for paramagnetic signals

To optimize 13C direct detected experiments for the observation of signals close to a paramagnetic center, we have assessed the sensitivity of different sequences based on CO-Cali coherence transfer. Features of CACO experiments were tested for Calbindin D9k, in which one of the two native Ca2+ ions is replaced by the paramagnetic Ce3+ ion. We have studied the comparison of single vs multiple quantum coherence transfer evolution as well as the influence of in-phase vs anti-phase detection of 13CO signals and finally the comparison of a coherence transfer step based on a CyO in plane with respect to a Cy ali in plane. The acquisition of the anti-phase component of the signal, accomplished by the removal of the last refocusing steps, allowed the identification of some signals unobserved with other pathways. The structural dependency of paramagnetism-induced nuclear relaxation is such that the identification of the most suitable coherence transfer pathway is not known "a priori" but it is driven by the relative proximity of Cali and CO to the paramagnetic center.

Measuring 1H-1H and 1H-13C RDCs in methyl groups: example of pulse sequences with numerically optimized coherence transfer schemes

The optimization of coherence-transfer pulse-sequence elements (CTEs) is the most challenging step in the construction of heteronuclear correlation NMR experiments achieving sensitivity close to its theoretical maximum (in the absence of relaxation) in the shortest possible experimental time and featuring active suppression of undesired signals. As reported in the present article, this complex optimization problem in a space of high dimensionality turns out to be numerically tractable. Based on the application of molecular dynamics in the space of pulse-sequence variables, a general method is proposed for constructing optimized CTEs capable of transferring an arbitrary (generally non-Hermitian) spin operator encoding the chemical shift of heteronuclear spins to an arbitrary spin operator suitable for signal detection. The CTEs constructed in this way are evaluated against benchmarks provided by the theoretical unitary bound for coherence transfer and the minimal required transfer time (when available). This approach is used to design a set of NMR experiments enabling direct and selective observation of individual (1)H-transitions in (13)C-labeled methyl spin systems close to optimal sensitivity and using a minimal number of spectra. As an illustrative application of the method, optimized CTEs are used to quantitatively measure (1)H-(1)H and (1)H-(13)C residual dipolar couplings (RDCs) in a 17 kDa protein weakly aligned by means of Pf1 phages.

Modulation of the distance dependence of paramagnetic relaxation enhancements by CSA x DSA cross-correlation

Paramagnetic metal ions with fast-relaxing electronic spin and anisotropic susceptibility tensor provide a rich source of structural information that can be derived from pseudo-contact shifts, residual dipolar couplings, dipole-dipole Curie spin cross-correlation, and paramagnetic relaxation enhancements. The present study draws attention to a cross-correlation effect between nuclear relaxation due to anisotropic chemical shielding (CSA) and due to the anisotropic dipolar shielding (DSA) caused by the electronic Curie spin. This CSA x DSA cross-correlation contribution seems to have been overlooked in previous interpretations of paramagnetic relaxation enhancements. It is shown to be sufficiently large to compromise the 1/r6 distance dependence usually assumed. The effect cannot experimentally be separated from auto-correlated DSA relaxation. It can increase or decrease the observed paramagnetic relaxation enhancement. Under certain conditions, the effect can dominate the entire paramagnetic relaxation, resulting in nuclear resonances narrower than in the absence of the paramagnetic center. CSAxDSA cross-correlation becomes important when paramagnetic relaxation is predominantly due to the Curie rather than the Solomon mechanism. Therefore the effect is most pronounced for relaxation by metal ions with large magnetic susceptibility and fast-relaxing electron spin. It most strongly affects paramagnetic enhancements of transverse relaxation in macromolecules and of longitudinal relaxation in small molecules.

New Routes to the Detection of Relaxation Allowed Coherence Transfer in Paramagnetic Molecules

A new pulse sequence is proposed to measure cross correlation rates between 1H Curie spin relaxation and 1H-15N or 1H-13C dipole-dipole coupling (%@mt;sys@%Gamma%@sx@%H,HX%@be@%CS,DD%@sxx@%%@mx@% ) in paramagnetic systems. The new sequence has been conceived to obtain quantitative measurements of cross correlation rates in the close proximity to the paramagnetic center, preventing the loss of information due to fast transverse relaxation. The approach was tested on the dicalcium protein calbindin D9k in which Ca2+ has been substituted at site II with Ce3+. At variance with previously reported experiments, all peaks observed in HSQC experiments tailored to paramagnetic signals give quantitative estimates of %@mt;sys@%Gamma%@sx@%H%@ital@%,%@rsf@%HX%@be@%CS%@ital@%,%@rsf@%DD%@sxx@%%@mx@% . This is crucial to refine the immediate proximity of the metal ion.

Observation of individual transitions in magnetically equivalent spin systems

Individual transitions of magnetically equivalent spin systems such as methyl groups residing on isotropically tumbling molecules in solution usually cannot be observed as multiplet-split NMR lines. We propose a pair of NMR experiments, 2D [13C,1Halphaalpha]Methyl and [13C,1Hbetabeta]Methyl HSQC, to overcome this limitation and enable direct and selective observation of individual 1H transitions in 13C-labeled methyl spin systems. Immediate applications include quantitative measurements of 1H-1H residual dipolar couplings (RDC) and cross-correlated relaxation between 1H chemical shift anisotropy and 1H-1H dipole-dipole interactions. The use of the experiments for the measurement of RDCs is demonstrated with two proteins, one weakly aligned by means of Pf1 phages and the other using a naturally present paramagnetic heme group.

Two-dimensional NMR study of the heme active site structure of chloroperoxidase

The heme active site structure of chloroperoxidase (CPO), a glycoprotein that displays versatile catalytic activities isolated from the marine mold Caldariomyces fumago, has been characterized by two-dimensional NMR spectroscopic studies. All hyperfine shifted resonances from the heme pocket as well as resonances from catalytically relevant amino acid residues including the heme iron ligand (Cys(29)) attributable to the unique catalytic properties of CPO have been firmly assigned through (a) measurement of nuclear Overhauser effect connectivities, (b) prediction of the Curie intercepts from both one- and two-dimensional variable temperature studies, (c) comparison with assignments made for cyanide derivatives of several well characterized heme proteins such as cytochrome c peroxidase, horseradish peroxidase, and manganese peroxidase, and (d) examination of the crystal structural parameters of CPO. The location of protein modification that differentiates the signatures of the two isozymes of CPO has been postulated. The function of the distal histidine (His(105)) in modulating the catalytic activities of CPO is proposed based on the unique arrangement of this residue within the heme cavity. Contrary to the crystal state, the high affinity Mn(II) binding site in CPO (in solution) is not accessible to externally added Mn(II). The results presented here provide a reasonable explanation for the discrepancies in the literature between spectroscopists and crystallographers concerning the manganese binding site in this unique protein. Our study indicates that results from NMR investigations of the protein in solution can complement the results revealed by x-ray diffraction studies of the crystal form and thus provide a complete and better understanding of the actual structure of the protein.

Long-range magnetization transfer between uncoupled nuclei by dipole-dipole cross-correlated relaxation: a precise probe of beta-sheet geometry in proteins

Interference between dipolar interactions in covalently linked (13)C-(1)H and nonlinked (1)H-(1)H pairs can be used to generate antiphase magnetization between noncoupled spins. The buildup rate of such antiphase terms is highly sensitive to local geometry, in particular the interproton distance and the (13)C-(1)H-(1)H internuclear angle. These rates have been measured for opposing C(alpha)H(alpha) pairs in antiparallel beta-sheets in the third Igg-binding domain of protein G (GB3) and in HIV protease, complexed with the inhibitor DMP323. For GB3, good agreement with the 1.1-A crystal structure is found. However, this agreement rapidly deteriorates with decreasing resolution of the corresponding X-ray structure. For HIV protease, two separate crystal structures that differ by less than 0.2 A from one another exhibit lower agreement in their predicted cross-correlated relaxation rates relative to one another than is found between experimental rates and the average of the rates predicted for the two structures. These data indicate that quantitative measurement of these cross-correlated relaxation rates can provide highly accurate structural information in macromolecules.

(13)C[(13)C] 2D NMR: a novel strategy for the study of paramagnetic proteins with slow electronic relaxation rates

Oxidized human [2Fe-2S] ferredoxin has a notably slow electronic relaxation rate, which precludes the observation of signals from nuclei near the iron-sulfur cluster by conventional 2D or 3D methods that utilize proton detection. We have demonstrated the utility of (13)C[(13)C]CT-COSY in identifying connectivity information from fast relaxing carbon nuclei near the paramagnetic center, including those from residues that ligate the metal center.

Tailored HCCH-TOCSY experiment for resonance assignment in the proximity of a paramagnetic center

The presence of a paramagnetic center may disturb both coherent and incoherent communication between nuclear spins that are affected, to some extent, by the hyperfine interaction. This is a limiting factor to an extensive use of paramagnetic probes in NMR spectroscopy to enhance partial alignment and to exploit cross correlation effects and pseudocontact shifts. We propose here an HCCH-TOCSY experiment tailored to identify spin systems involving resonances that are partly or completely affected by hyperfine interaction. The efficiency of polarization transfer steps when fast relaxing nuclei are involved is discussed. The sequence is tested for the protein Calbindin D(9k), in which one of the two native Ca2+ ions is replaced by the paramagnetic Ce3+ ion as well as for the oxidized form of cytochrome b(562).

Cross-correlation effects involving curie spin relaxation in methyl groups

Cross-correlation effects arising in methyl protons due to the simultaneous presence of dipole-dipole, chemical shift anisotropy, and Curie spin relaxation mechanisms in paramagnetic systems are analyzed. We assess the potential of obtaining structural constraints from the cross-correlation of Curie spin relaxation with dipolar relaxation mechanisms among methyl proton spins. By theoretical analysis and numerical simulations we characterize the transfer functions describing the interconversion processes of different ranks of multispin order. The time dependence of these processes contains a new type of structural information, the orientation of the methyl C(3)-axis with respect to the electron center. Experimental confirmation is found for selected methyl groups in low spin Fe(3+) sperm whale myoglobin.

Cross correlation between the dipole-dipole interaction and the Curie spin relaxation: the effect of anisotropic magnetic susceptibility

Cross-correlated relaxation caused by the interference of nuclear dipole-dipole interaction and the Curie spin relaxation (DD-CSR cross relaxation) is generalized to treat the case of anisotropic magnetic susceptibility, including the important case where the latter originates from zero-field splitting. It is shown that the phenomenon of DD-CSR cross relaxation is absolutely general and to be expected under any electronic configuration. The results of the generalization are presented for a model system, and the consequences for paramagnetic metalloproteins are illustrated with an example of cerium(III)-substituted calbindin. The effects of the magnetic anisotropy are found to be substantial.

Measurement of homonuclear proton couplings based on cross-peak nulling in CT-COSY

A method in which 1H-1H scalar and dipolar couplings are obtained from the cross-peak nulling condition in a series of constant-time (CT) COSY spectra, as a function of the duration of the CT period, is described. The method is best suited for measurement of 1H-1H couplings in the range 5-20 Hz. It is shown, however, that results can be sensitive to cross-correlated relaxation effects. Also, artifactual resonances, resulting from strong coupling, can be quite pronounced in CT-COSY spectra, even when /J(AB)/(deltaA-deltaB)/<0.1. The experiments are demonstrated for the DNA dodecamer d(CGCGAATTCGCG)2, both in isotropic solution and in a liquid crystalline phase.

Interpretation of chemical shifts and coupling constants in macromolecules

Recent developments in NMR spectroscopy, along with advances in computational techniques, have produced new approaches to the interpretation of chemical shifts and spin-spin coupling constants in biomolecules. Quantum chemical studies of useful accuracy are now becoming more routine and are increasingly being used in conjunction with experimental studies to map out expected structural patterns for peptides and oligonucleotides. Topics of recent special interest include spin couplings across hydrogen bonds and patterns of chemical shift anisotropies, in both diamagnetic and paramagnetic proteins.

An 1H-13C-13C-edited 1H NMR experiment for making resonance assignments in the active site of heme proteins

In paramagnetic heme proteins, it is often problematic to make proton resonance assignments for heme substituents that do not have large isotropic shifts and consequently lie under the large envelope of polypeptide resonances. Furthermore, assignments that would normally be performed with the aid of HMBC experiments in diamagnetic molecules can prove difficult in the active site of paramagnetic heme proteins if T2(-1) > 2JCH. To circumvent this problem, a new method is presented to selectively detect 1H in 1Hn-13C-13C fragments biosynthetically introduced into the active site of heme proteins. The pulse sequence combines well-known building blocks such as INEPT to transfer 1H spin magnetization to bonded 13C nuclei, followed by INADEQUATE to generate 13C-13C double-quantum coherence that is selected with pulsed field gradients, and finally reverse-INEPT to transfer magnetization back to 1H nuclei for subsequent observation. The new 1Hn-13C-13C edited experiment takes advantage of the relatively large values of 1JCH and 1JCC, avoiding the long interpulse delays in HMBC that compromise the detectability of rapidly relaxing nuclei. The potential applicability of the pulse sequence is demonstrated by its contribution to the unambiguous assignment of the carbonyl carbons in the heme propionates of ferricytochrome b5.

Electron spin-nuclear spin cross-correlation effects on multiplet splittings in paramagnetic proteins

The effects of cross-correlation between Curie spin-nuclear dipole and nuclear dipole-nuclear dipole interactions on the linewidths and resonance frequencies of the individual lines of an AX multiplet in paramagnetic systems have been calculated. The implication of the relaxation-induced frequency shift of the lines (dynamic frequency shift) for the accurate measurement of residual dipolar couplings in field-oriented systems has been discussed. Our simulations indicate that these effects may play a role in the precise measurement of residual dipolar couplings in systems which belong to the small and intermediate tumbling regime, i.e., correlation times less than 5 ns.

Proton NMR assignments and magnetic axes orientations for wild-type yeast iso-1-ferricytochrome c free in solution and bound to cytochrome c peroxidase

Extensive proton hyperfine-shifted resonance assignments have been made for wild-type yeast iso-1-ferricytochrome c when it is free in solution and when it is noncovalently complexed to resting state cytochrome c peroxidase. Complete heme proton resonance assignments were made for free iso-1-ferricytochrome c, while for CcP-complexed iso-1-ferricytochrome c, 70% of heme proton assignments were made. Additional proton resonance assignments were made for hyperfine-shifted protons of amino acids near the heme. These assignments allowed identification of the most extensive set of complex-induced proton shifts yet reported for CcP/cytochrome c complexes. Several purely dipolar-shifted resonances from heme vicinity amino acid protons were also assigned in both free and complexed iso-1-ferricyt c. Both sets of resonance assignments allowed assessment of the origin of proton complex-induced shifts. Using the assigned dipolar-shifted proton resonances as a basis, the orientations of the principal axis systems of the paramagnetic susceptibility tensors for free and cytochrome c peroxidase-bound iso-1-ferricytochrome c were elucidated. The results indicated that the iso-1-ferricytochrome c magnetic axis system orientation shifts significantly upon complex formation. The direction of the complex-induced shifts for heme proton resonances is largely accounted for by the magnetic anisotropy changes. However, analysis of heme complex-induced shifts also reveals local changes in magnetic environment for two heme substituents, presumably through a specific structure change.

An NMR study of the 7Fe-8S ferredoxin from Rhodopseudomonas palustris and reinterpretation of data on similar systems

The oxidized 7Fe-8S ferredoxin from Rhodopseudomonas palustris is shown to possess a unique 1H NMR spectrum displaying at least one hyperfine-shifted beta-CH2 signal for each cysteine bound to the [3Fe-4S] cluster. COSY and TOCSY spectra and 1- and 2-dimensional NOE experiments, in conjunction with a thorough reexamination of the 1H NMR data on similar systems, permitted the sequential assignment of all of the cysteine beta-CH2 protons even in the absence of the amino acid sequence. The sequential assignment stems on the homology of the hyperfine shift pattern with those of other sequenced 7Fe-8S ferredoxins, which points to a substantial homology in tertiary structure. From the assignment, an analysis of the antiferromagnetic coupling in the [3Fe-4S] system was performed on the basis of a general model of exchange coupling. The NMR signal patterns of [3Fe-4S] clusters in both 3Fe-4S and 7Fe-8S ferredoxins have been discussed, and some correlations are proposed between signal patterns and the primary sequence.

Paramagnetic NMR analysis of the seven-iron ferredoxin from the hyperthermoacidophilic archaeon Desulfurolobus ambivalens reveals structural similarity to other dicluster ferredoxins

The seven-iron ferredoxin from the hyperthermophilic archaeon Desulfurolobus ambivalens has been investigated by one-dimensional and two-dimensional 1H-NMR in its oxidized and dithionite-reduced states. All iron atoms of both the three-iron and the four-iron cluster are bound to cysteine residues whose hyperfine-shifted resonances were characterized. The pattern of these resonances is similar to those from three-iron, four-iron and eight-iron ferredoxins previously described in the literature, but the four-iron cluster has a shift pattern different from that in other seven-iron proteins. A second set of hyperfine-shifted resonances clearly indicates sample heterogeneity, which possibly involves the four-iron cluster. The observation of interresidue NOEs between two different cysteine residues proves the existence of close spatial proximity of the two clusters in D. ambivalens ferredoxin and therefore indicates structural homology to other dicluster ferredoxins. Moreover, this feature is crucial for the sequence-specific assignment of the hyperfine-shifted resonances. The C alpha-C beta-S-Fe dihedral angles of the cysteine residues coordinating the four-iron cluster could be estimated, and the electronic structure of the three-iron cluster is discussed.

1H-NMR study of a cobalt-substituted blue copper protein: Pseudomonas aeruginosa Co(II)-azurin

Substitution of copper by cobalt in blue copper proteins gives a paramagnetic metalloderivative suitable for paramagnetic NMR studies. A thorough analysis of the 1H-NMR spectrum of Pseudomonas aeruginosa Co(II)-azurin is presented here. All the observable contact-shifted signals as well as many other paramagnetic signals from protons placed up to about 1.0 nm around the metal center, including some residues belonging to functionally important parts of the protein like the hydrophobic patch and the His35 region, have been assigned. The results obtained permit the detection and study of structural variations like those originated by the His35 ionization, and allow us to draw a feasible picture of the metal coordination site. Contact-shifted signals correspond to the same five residues which are found in the coordination sphere of the native Cu(II)-azurin, i.e. His46, His117, Cys112, Met121 and Gly45. Among them, the histidine residues present a pattern of resonances typical for histidines coordinated to cobalt in other cobalt protein derivatives, and the cysteine signals clearly indicate a strong interaction with the paramagnetic Co(II) ion. In contrast, the Met121 signals indicate a weak but still existent contact interaction with the metal center. On the other hand, the very weak copper ligand, Gly45, appears here as clearly coordinated to cobalt. Results are consistent with a distorted tetrahedral metal site with the cobalt deviated from the N2S plane towards the Gly45 O axial position and weakly interacting with the Met121 sulfur.

A two-dimensional NMR study of bovine Cu, Co superoxide dismutase. Further assignments in the region surrounding the active site

Bovine Cu, Co superoxide dismutase has been investigated by two-dimensional NMR with regard to the resonances arising from protons that surround the copper site. These protons have magnetic properties that are intermediate between those belonging to residues coordinated to the paramagnetic metal center and the diamagnetic residues. Nuclear Overhauser enhancement spectroscopy spectra have been recorded by using a new procedure and with different mixing times and different spectral widths, in order to observed dipolar connectivities between isotropically shifted and diamagnetic resonances. Scalar-correlated spectra were obtained with both correlation spectroscopy and total-correlation spectroscopy experiments. The original X-ray coordinates of the Cu, Co enzyme were used in order to obtain the appropriate interproton distances. The data allowed us to assign more than 20 new resonances to protons which cover a wide region around the copper ion, including two protons belonging to the catalytically important Arg141 residue. The results represent significant progress in the effort to elucidate the three-dimensional features of the region surrounding the active site of Cu, Zn superoxide dismutase in solution and a tool for a deeper investigation of the reaction mechanism of the enzyme.

NMR investigation of isotopically labeled cyanide derivatives of lignin peroxidase and manganese peroxidase

The 1H NMR spectroscopy was used to study lignin peroxidase (LiP) and manganese peroxidase (MnP) containing deuterated histidines. LiP and MnP were obtained from a histidine auxotroph of the fungus Phanerochaete chrysosporium grown in the presence of deuterated histidines. The derivatives with deuterated histidines have allowed a firm assignment of the protons of the distal and proximal histidines. We have also found that the LiP from this strain exhibits different orientations of the 2-vinyl group compared to the LiP from the strain previously studied. Mobility of the group has also been detected, thus explaining the apparent inconsistency between X-ray solid-state and NMR solution data. The 15N shift values of 15N-enriched CN- in the cyanide derivatives of LiP and MnP have also been measured. The shift patterns, both for 15N and for the proximal histidine protons of several peroxidases, are consistent with predominant contact shift contributions which reflect the bond strength of the metal-axial ligand. Finally, our results confirm a correlation between shift values of 15N and those of proximal histidine protons and the Fe3+/Fe2+ redox potentials.

Two-dimensional 1H NMR spectra of ferricytochrome c551 from Pseudomonas aeruginosa

The full assignment of 1H NMR signals of heme proton resonances of ferricytochrome c551 from Pseudomonas aeruginosa has been performed by means of 2D NMR experiments. This technique allows the complete and unequivocal assignment of all heme resonances, including methylene resonances of the propionic groups, directly implicated in the pH dependence of the redox properties of cytochrome c551.