Metalloenzyme Active-Site Structure and Function through Multifrequency CW and Pulsed ENDOR

Modulation of the Bonding between Copper and a Redox-Active Ligand by Hydrogen Bonds and Its Effect on Electronic Coupling and Spin States

The exchange coupling of electron spins can strongly influence the properties of chemical species. The regulation of this type of electronic coupling has been explored within complexes that have multiple metal ions but to a lesser extent in complexes that pair a redox-active ligand with a single metal ion. To bridge this gap, we investigated the interplay among the structural and magnetic properties of mononuclear Cu complexes and exchange coupling between a Cu center and a redox-active ligand over three oxidation states. The computational analysis of the structural properties established a relationship between the complexes' magnetic properties and a bonding interaction involving a dx2-y2 orbital of the Cu ion and π orbital of the redox-active ligand that are close in energy. The additional bonding interaction affects the geometry around the Cu center and was found to be influenced by intramolecular H-bonds introduced by the external ligands. The ability to synthetically tune the d-π interactions using H-bonds illustrates a new type of control over the structural and magnetic properties of metal complexes.

Primary donor triplet states of Photosystem I and II studied by Q-band pulse ENDOR spectroscopy

The photoexcited triplet state of the "primary donors" in the two photosystems of oxygenic photosynthesis has been investigated by means of electron-nuclear double resonance (ENDOR) at Q-band (34 GHz). The data obtained represent the first set of ¹H hyperfine coupling tensors of the ³P700 triplet state in PSI and expand the existing data set for ³P680. We achieved an extensive assignment of the observed electron-nuclear hyperfine coupling constants (hfcs) corresponding to the methine α-protons and the methyl group β-protons of the chlorophyll (Chl) macrocycle. The data clearly confirm that in both photosystems the primary donor triplet is located on one specific monomeric Chl at cryogenic temperature. In comparison to previous transient ENDOR and pulse ENDOR experiments at standard X-band (9-10 GHz), the pulse Q-band ENDOR spectra demonstrate both improved signal-to-noise ratio and increased resolution. The observed ENDOR spectra for ³P700 and ³P680 differ in terms of the intensity loss of lines from specific methyl group protons, which is explained by hindered methyl group rotation produced by binding site effects. Contact analysis of the methyl groups in the PSI crystal structure in combination with the ENDOR analysis of ³P700 suggests that the triplet is located on the Chl a' (P_A) in PSI. The results also provide additional evidence for the localization of ³P680 on the accessory Chl_D1 in PSII.

Exploring the Role of the Central Carbide of the Nitrogenase Active-Site FeMo-cofactor through Targeted 13C Labeling and ENDOR Spectroscopy

Mo-dependent nitrogenase is a major contributor to global biological N₂ reduction, which sustains life on Earth. Its multi-metallic active-site FeMo-cofactor (Fe₇MoS₉C-homocitrate) contains a carbide (C^4-) centered within a trigonal prismatic CFe₆ core resembling the structural motif of the iron carbide, cementite. The role of the carbide in FeMo-cofactor binding and activation of substrates and inhibitors is unknown. To explore this role, the carbide has been in effect selectively enriched with ¹³C, which enables its detailed examination by ENDOR/ESEEM spectroscopies. ¹³C-carbide ENDOR of the S = 3/2 resting state (E₀) is remarkable, with an extremely small isotropic hyperfine coupling constant, ^Ca = +0.86 MHz. Turnover under high CO partial pressure generates the S = 1/2 hi-CO state, with two CO molecules bound to FeMo-cofactor. This conversion surprisingly leaves the small magnitude of the ¹³C carbide isotropic hyperfine-coupling constant essentially unchanged, ^Ca = -1.30 MHz. This indicates that both the E₀ and hi-CO states exhibit an exchange-coupling scheme with nearly cancelling contributions to ^Ca from three spin-up and three spin-down carbide-bound Fe ions. In contrast, the anisotropic hyperfine coupling constant undergoes a symmetry change upon conversion of E₀ to hi-CO that may be associated with bonding and coordination changes at Fe ions. In combination with the negligible difference between CFe₆ core structures of E₀ and hi-CO, these results suggest that in CO-inhibited hi-CO the dominant role of the FeMo-cofactor carbide is to maintain the core structure, rather than to facilitate inhibitor binding through changes in Fe-carbide covalency or stretching/breaking of carbide-Fe bonds.

Structural and spectroscopic characterization of an Fe(VI) bis(imido) complex

High-valent iron species are key intermediates in oxidative biological processes, but hexavalent complexes apart from the ferrate ion are exceedingly rare. Here, we report the synthesis and structural and spectroscopic characterization of a stable Fe(VI) complex (3) prepared by facile one-electron oxidation of an Fe(V) bis(imido) (2). Single-crystal x-ray diffraction of 2 and 3 revealed four-coordinate Fe centers with an unusual "seesaw" geometry. ⁵⁷Fe Mössbauer, x-ray photoelectron, x-ray absorption, and electron-nuclear double resonance (ENDOR) spectroscopies, supported by electronic structure calculations, support a low-spin (S = 1/2) d³ Fe(V) configuration in 2 and a diamagnetic (S = 0) d² Fe(VI) configuration in 3 Their shared seesaw geometry is electronically dictated by a balance of Fe-imido σ- and π-bonding interactions.

Paramagnetic Resonance of Cobalt(II) Trispyrazolylmethanes and Counterion Association

Paramagnetic resonance studies (EPR, ESEEM, ENDOR, and NMR) of a series of cobalt(II) bis-trispyrazolylmethane tetrafluoroborates are presented. The complexes studied include the parent, unsubstituted ligand (Tpm), two pyrazole-substituted derivatives (4Me and 3,5-diMe), and tris(1-pyrazolyl)ethane (Tpe), which includes a methyl group on the apical carbon atom. NMR and ENDOR establish the magnitude of ¹H hyperfine couplings, while ESEEM provides information on the coordinated ¹⁴N. The data show that the pyrazole 3-position is more electron rich in the Tpm analogues, that the geometry about the apical atom influences the magnetic resonance, and that apical atom geometry appears more fixed in Tpm than in Tp. NMR and ENDOR establish that the BF₄^- counterion remains associated in fluid solution. In the case of the Tpm^3,5Me complex, it appears to associate in solution, in the same position it occupies in the X-ray structure.

Composition and Structure of the Inorganic Core of Relaxed Intermediate X(Y122F) of Escherichia coli Ribonucleotide Reductase

Activation of the diferrous center of the β2 (R2) subunit of the class 1a Escherichia coli ribonucleotide reductases by reaction with O2 followed by one-electron reduction yields a spin-coupled, paramagnetic Fe(III)/Fe(IV) intermediate, denoted X, whose identity has been sought by multiple investigators for over a quarter of a century. To determine the composition and structure of X, the present study has applied (57)Fe, (14,15)N, (17)O, and (1)H electron nuclear double resonance (ENDOR) measurements combined with quantitative measurements of (17)O and (1)H electron paramagnetic resonance line-broadening studies to wild-type X, which is very short-lived, and to X prepared with the Y122F mutant, which has a lifetime of many seconds. Previous studies have established that over several seconds the as-formed X(Y122F) relaxes to an equilibrium structure. The present study focuses on the relaxed structure. It establishes that the inorganic core of relaxed X has the composition [(OH(-))Fe(III)-O-Fe(IV)]: there is no second inorganic oxygenic bridge, neither oxo nor hydroxo. Geometric analysis of the (14)N ENDOR data, together with recent extended X-ray absorption fine structure measurements of the Fe-Fe distance (Dassama, L. M.; et al. J. Am. Chem. Soc. 2013, 135, 16758), supports the view that X contains a "diamond-core" Fe(III)/Fe(IV) center, with the irons bridged by two ligands. One bridging ligand is the oxo bridge (OBr) derived from O2 gas. Given the absence of a second inorganic oxygenic bridge, the second bridging ligand must be protein derived, and is most plausibly assigned as a carboxyl oxygen from E238.

How Formaldehyde Inhibits Hydrogen Evolution by [FeFe]-Hydrogenases: Determination by ¹³C ENDOR of Direct Fe-C Coordination and Order of Electron and Proton Transfers

Formaldehyde (HCHO), a strong electrophile and a rapid and reversible inhibitor of hydrogen production by [FeFe]-hydrogenases, is used to identify the point in the catalytic cycle at which a highly reactive metal-hydrido species is formed. Investigations of the reaction of Chlamydomonas reinhardtii [FeFe]-hydrogenase with formaldehyde using pulsed-EPR techniques including electron-nuclear double resonance spectroscopy establish that formaldehyde binds close to the active site. Density functional theory calculations support an inhibited super-reduced state having a short Fe-(13)C bond in the 2Fe subsite. The adduct forms when HCHO is available to compete with H(+) transfer to a vacant, nucleophilic Fe site: had H(+) transfer already occurred, the reaction of HCHO with the Fe-hydrido species would lead to methanol, release of which is not detected. Instead, Fe-bound formaldehyde is a metal-hydrido mimic, a locked, inhibited form analogous to that in which two electrons and only one proton have transferred to the H-cluster. The results provide strong support for a mechanism in which the fastest pathway for H2 evolution involves two consecutive proton transfer steps to the H-cluster following transfer of a second electron to the active site.

Modeling the signatures of hydrides in metalloenzymes: ENDOR analysis of a Di-iron Fe(μ-NH)(μ-H)Fe core

The application of 35 GHz pulsed EPR and ENDOR spectroscopies has established that the biomimetic model complex L(3)Fe(μ-NH)(μ-H)FeL(3) (L(3) = [PhB(CH(2)PPh(2))(3)](-)) complex, 3, is a novel S = (1)/(2) type-III mixed-valence di-iron II/III species, in which the unpaired electron is shared equally between the two iron centers. (1,2)H and (14,15)N ENDOR measurements of the bridging imide are consistent with an allyl radical molecular orbital model for the two bridging ligands. Both the (μ-H) and the proton of the (μ-NH) of the crystallographically characterized 3 show the proposed signature of a 'bridging' hydride that is essentially equidistant between two 'anchor' metal ions: a rhombic dipolar interaction tensor, T ≈ [T, -T, 0]. The point-dipole model for describing the anisotropic interaction of a bridging H as the sum of the point-dipole couplings to the 'anchor' metal ions reproduces this signature with high accuracy, as well as the axial tensor of a terminal hydride, T ≈ [-T, -T, 2T], thus validating both the model and the signatures. This validation in turn lends strong support to the assignment, based on such a point-dipole analysis, that the molybdenum-iron cofactor of nitrogenase contains two [Fe-H(-)-Fe] bridging-hydride fragments in the catalytic intermediate that has accumulated four reducing equivalents (E(4)). Analysis further reveals a complementary similarity between the isotropic hyperfine couplings for the bridging hydrides in 3 and E(4). This study provides a foundation for spectroscopic study of hydrides in a variety of reducing metalloenzymes in addition to nitrogenase.

Simulating suppression effects in Pulsed ENDOR, and the 'hole in the middle' of Mims and Davies ENDOR Spectra

All pulsed ENDOR techniques, and in particular the Mims and Davies sequences, suffer from detectability biases ('blindspots') that are directly correlated to the size of the hyperfine interactions of coupled nuclei. Our efforts at ENDOR 'crystallography' and 'mechanism determination' with these techniques has led our group to refine our simulations of pulsed ENDOR spectra to take into account these biases, and we here describe the process and illustrate it with several examples. We first focus on an issue whose major significance is not widely appreciated, the 'hole in the middle' of pulsed ENDOR spectra caused by the n = 0 suppression hole in Mims ENDOR and by the analogous A→0 suppression in Davies ENDOR (Section I). This section discusses the issue for nuclei with I = ½ and also for (2)H (I = 1), using the treatment of Section II. In Section II we discuss the general treatment of suppression effects for I = 1, illustrating it with a treatment of Mims suppression for (14)N (I = 1) (Section II).

Frequency Dependence of Pulsed EPR Experiments

The frequency dependence of the signal-to-noise ratio (S/N) that is theoretically possible for pulsed EPR experiments is the same as for continuous wave experiments. To select the optimum resonance frequency or frequencies for pulsed EPR experiments it is important to consider not only S/N, but also orientation selection, depth of spin echo modulation, and intensities of forbidden transitions. Evaluation of factors involved in selecting the optimum frequency for pulsed EPR measurements of distances between spins is discussed.

ENDOR spectroscopy shows that guanine N1 binds to [4Fe-4S] cluster II of the S-adenosylmethionine-dependent enzyme MoaA: mechanistic implications

The S-adenosylmethionine-dependent enzyme MoaA, in concert with MoaC, catalyzes the first step of molybdenum cofactor biosynthesis, the conversion of guanosine 5'-triphosphate (5'-GTP) into precursor Z. A published X-ray crystal structure of MoaA with the substrate 5'-GTP revealed that the substrate might be bound to the unique iron of one of two 4Fe-4S clusters through either or both the amino and N1 nitrogen nuclei. Use of 35 GHz continuous-wave ENDOR spectroscopy of MoaA with unlabeled and (15)N-labeled substrate and a reduced [4Fe-4S](+) cluster now demonstrates that only one nitrogen nucleus is bound to the cluster. Experiments with the substrate analogue inosine 5'-triphosphate further demonstrate that it is the N1 nitrogen that binds. Two of the more distant nitrogen nuclei have also been detected by 35 GHz pulsed ENDOR spectroscopy, allowing a rough approximation of their distances from the cluster to be calculated. Combining this information with the crystal structure, we propose that the guanine base adopts the enol tautomer as N1 binds to Fe4 and the O6-H hydroxyl group forms a hydrogen bond with S4 of the 4Fe-4S cluster, and that this binding-induced tautomerization may have important mechanistic ramifications.

Climbing nitrogenase: toward a mechanism of enzymatic nitrogen fixation

"Nitrogen fixation", the reduction of dinitrogen (N2) to two ammonia (NH3) molecules, by the Mo-dependent nitrogenase is essential for all life. Despite four decades of research, a daunting number of unanswered questions about the mechanism of nitrogenase activity make it the "Everest of enzymes". This Account describes our efforts to climb one "face" of this mountain by meeting two interdependent challenges central to determining the mechanism of biological N2 reduction. The first challenge is to determine the reaction pathway: the composition and structure of each of the substrate-derived moieties bound to the catalytic FeMo cofactor (FeMo-co) of the molybdenum-iron (MoFe) protein of nitrogenase. To overcome this challenge, it is necessary to discriminate between the two classes of potential reaction pathways: (1) a "distal" (D) pathway, in which H atoms add sequentially at a single N or (2) an "alternating" (A) pathway, in which H atoms add alternately to the two N atoms of N2. Second, it is necessary to characterize the dynamics of conversion among intermediates within the accepted Lowe-Thorneley kinetic scheme for N2 reduction. That goal requires an experimental determination of the number of electrons and protons delivered to the MoFe protein as well as their "inventory", a partition into those residing on each of the reaction components and released as H2 or NH3. The principal obstacle to this "climb" has been the inability to generate N2 reduction intermediates for characterization. A combination of genetic, biochemical, and spectroscopic approaches recently overcame this obstacle. These experiments identified one of the four-iron Fe-S faces of the active-site FeMo-co as the specific site of reactivity, indicated that the side chain of residue alpha70V controls access to this face, and supported the involvement of the side chain of residue alpha195H in proton delivery. We can now freeze-quench trap N2 reduction pathway intermediates and use electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) spectroscopies to characterize them. However, even successful trapping of a N2 reduction intermediate occurs without synchronous electron delivery to the MoFe protein. As a result, the number of electrons and protons, n, delivered to MoFe during its formation is unknown. To determine n and the electron inventory, we initially employed ENDOR spectroscopy to analyze the substrate moiety bound to the FeMo-co and 57Fe within the cofactor. Difficulties in using that approach led us to devise a robust kinetic protocol for determining n of a trapped intermediate. This Account describes strategies that we have formulated to bring this "face" of the nitrogenase mechanism into view and afford approaches to its climb. Although the summit remains distant, we look forward to continued progress in the ascent.

Identification of protonated oxygenic ligands of ribonucleotide reductase intermediate X

We previously used a combination of continuous-wave (CW) and pulsed electron-nuclear double resonance (ENDOR) protocols to identify the types of protonated oxygen (OH(x)) species and their disposition within the Fe(III)/Fe(IV) cluster of intermediate X, the direct precursor of the essential diferric-tyrosyl radical cofactor of the beta2 subunit of Escherichia coli ribonucleotide reductase (RNR). We concluded that X contains the [(H(x)O)Fe(III)OFe(IV)] fragment (T model), and does not contain a mu-hydroxo bridge. When combined with a subsequent (17)O ENDOR study of X prepared with H(2)(17)O and with (17)O(2), the results led us to suggest that this fragment is the entire inorganic core of X. This has been questioned by recent reports, but these reports do not themselves agree on the core of X. One concluded that X possesses a di-mu-oxo Fe(III)/Fe(IV) core plus a terminal (H(2)O) bound to Fe(III) [e.g., Han, W.-G.; Liu, T.; Lovell, T.; Noodleman, L. J. Am. Chem. Soc. 2005, 127, 15778-15790]. The other [Mitic, N.; Clay, M. D.; Saleh, L.; Bollinger, J. M.; Solomon, E. I. J. Am. Chem. Soc. 2007, 129, 9049-9065] concluded that X contains only a single oxo bridge and postulated the presence of an additional hydroxo bridge plus a terminal hydroxyl bound to Fe(III). In this report we take advantage of improvements in 35 GHz pulsed ENDOR performance to reexamine the protonation state of oxygenic ligands of the inorganic core of X by directly probing the exchangeable proton(s) with (2)H pulsed ENDOR spectroscopy. These (2)H ENDOR measurements confirm that X contains an Fe(III)-bound terminal aqua ligand (H(x)O), but the spectra contain none of the features that would be required for the proton of a bridging hydroxyl. Thus, we confirm that X contains a terminal aqua (most likely hydroxo) ligand to Fe(III) in addition to one or two mu-oxo bridges but does not contain a mu-hydroxo bridge. The (2)H ENDOR measurements further demonstrate that this conclusion is applicable to both wild type and Y122F-beta2 mutant, and in fact we detect no difference between the properties of protons on the terminal oxygens in the two variants; likewise, (14)N ENDOR measurements of histidyl ligands bound to Fe show no difference between the two variants.

Active site reactant center geometry in the Co(II)-product radical pair state of coenzyme B12-dependent ethanolamine deaminase determined by using orientation-selection electron spin-echo envelope modulation spectroscopy

The distances and orientations among reactant centers in the active site of coenzyme B12-dependent ethanolamine deaminase from Salmonella typhimurium have been characterized in the Co(II)-product radical pair state by using X-band electron paramagnetic resonance (EPR) and two-pulse electron spin-echo envelope modulation (ESEEM) spectroscopies in the disordered solid state. The unpaired electron spin in the product radical is localized on C2. Our approach is based on the orientation-selection created in the EPR spectrum of the biradical by the axial electron-electron dipolar interaction. Simulation of the EPR line shape yielded a best-fit Co(II)-C2 distance of 9.3 A. ESEEM spectroscopy performed at four magnetic field values addressed the hyperfine coupling of the unpaired electron spin on C2 with 2H in the C5' methyl group of 5'-deoxyadenosine and in the beta-2H position at C1 of the radical. Global ESEEM simulations (over the four magnetic fields) were weighted by the orientation dependence of the EPR line shape. A Nelder-Mead direct search fitting algorithm was used to optimize the simulations. The results lead to a partial model of the active site, in which C5' is located a perpendicular distance of 1.6 A from the Co(II)-C2 axis, at distances of 6.3 and 3.5 A from Co(II) and C2, respectively. The van der Waals contact of the C5'-methyl group and C2 indicates that C5' remains close to the radical species during the rearrangement step. The C2-Hs-C5' angle including the strongly coupled hydrogen, Hs, and the C5'-Hs orientation relative to the C1-C2 axis are consistent with a linear hydrogen atom transfer coordinate and an in-line acceptor p-orbital orientation. The trigonal plane of the C2 atom defines sub-spaces within the active site for C5' radical migration and hydrogen atom transfers (side of the plane facing Co(II)) and amine migration (side of the plane facing away from Co(II)).

Comparing the electronic properties of the low-spin cyano-ferric [Fe(N4)(Cys)] active sites of superoxide reductase and p450cam using ENDOR spectroscopy and DFT calculations

Superoxide reductase (SOR) and P450 enzymes contain similar [Fe(N)4(SCys)] active sites and, although they catalyze very different reactions, are proposed to involve analogous low-spin (hydro)peroxo-Fe(III) intermediates in their respective mechanisms that can be modeled by cyanide binding. The equatorial FeN4 ligation by four histidine ligands in CN-SOR and the heme in CN-P450cam is directly compared by 14N ENDOR, while the axial Fe-CN and Fe-S bonding is probed by 13C ENDOR of the cyanide ligand and 1Hbeta ENDOR measurements to determine the spin density delocalization onto the cysteine sulfur. There are small, but notable, differences in the bonding between Fe(III) and its ligands in the two enzymes. The ENDOR measurements are complemented by DFT computations that support the semiempirical equation used to compute spin densities on metal-coordinated cysteinyl and shed light on bonding changes as the Fe-C-N linkage bends. They further indicate that H bonds to the cysteinyl thiolate sulfur ligand reduce the spin density on the sulfur in both active sites to a degree that exceeds the difference induced by the alternative sets of "in-plane" nitrogen ligands.

Electron inventory, kinetic assignment (E(n)), structure, and bonding of nitrogenase turnover intermediates with C2H2 and CO

Improved 1H ENDOR data from the S(EPR1) intermediate formed during turnover of the nitrogenase alpha-195Gln MoFe protein with C2(1,2)H2 in (1,2)H2O buffers, taken in context with the recent study of the intermediate formed from propargyl alcohol, indicate that S(EPR1) is a product complex, likely with C2H4 bound as a ferracycle to a single Fe of the FeMo-cofactor active site. 35 GHz CW and Mims pulsed 57Fe ENDOR of 57Fe-enriched S(EPR1) cofactor indicates that it exhibits the same valencies as those of the CO-bound cofactor of the lo-CO intermediate formed during turnover with CO, [Mo4+, Fe3+, Fe6(2+), S9(2-)(d43)](+1), reduced by m = 2 electrons relative to the resting-state cofactor. Consideration of 57Fe hyperfine coupling in S(EPR1) and lo-CO leads to a picture in which CO bridges two Fe of lo-CO, while the C2H4 of S(EPR1) binds to one of these. To correlate these and other intermediates with Lowe-Thorneley (LT) kinetic schemes for substrate reduction, we introduce the concept of an "electron inventory". It partitions the number of electrons a MoFe protein intermediate has accepted from the Fe protein (n) into the number transmitted to the substrate (s), the number that remain on the intermediate cofactor (m), and the additional number delivered to the cofactor from the P clusters (p): n = m + s - p (with p = 0 here). The cofactors of lo-CO and S(EPR1) both are reduced by m = 2 electrons, but the intermediates are not at the same LT reduction stage (E(n)): (n = 2; m = 2, s = 0) for lo-CO; (n = 4; s = 2, m = 2) for S(EPR1). This is the first proposed correlation of an LT E(n) kinetic state with a well-defined chemical state of the enzyme.

The interstitial atom of the nitrogenase FeMo-cofactor: ENDOR and ESEEM evidence that it is not a nitrogen

X-ray crystallographic study of the nitrogenase MoFe protein revealed electron density from an atom (denoted X) inside the active-site metal cluster, the [MoFe7S9:homocitrate] FeMo-cofactor. The electron density associated with X is consistent with a single N, O, or C atom. We now have tested whether X is an N or not by comparing the Q-band ENDOR and ESEEM signals from resting-state (S = 3/2) MoFe protein and NMF-extracted FeMo-co from bacteria grown with either 14N or 15N as the exclusive N source. All of the 14N or 15N signals associated with the protein are lost upon extraction of the FeMo-co. We interpret this as strong evidence that X is not an N.

Influence of the anisotropic hyperfine interaction on the 14N ENDOR and the ESEEM orientation-disordered spectra

The influence of the anisotropic hyperfine interaction on the 14N electron-nuclear double resonance/electron spin echo envelope modulation spectra is studied by approximate analytical and graphical methods for the case of the isotropic g-factor. The suggested determination of the modified characteristic directions of the magnetic field due to anisotropy enhances the insight in the structural details of the system and analytical solutions of the secular equation for these conditions are derived. The graphical method, previously used for the analysis of the orientation dependence of the 14N nuclear-transition frequencies in orientation-disordered samples for isotropic hyperfine interaction is extended to the case of arbitrary anisotropic hyperfine tensor. The above analytical and graphical methods are illustrated and tested against exact simulations in two practically important cases: (i) isotropic hyperfine interaction (hfi) exceeding other nuclear interactions in nuclear spin Hamiltonian. (ii) Cancellation of the isotropic part of the hfi.

ENDOR Studies of the Ligation and Structure of the Non-Heme Iron Site in ACC Oxidase

Ethylene is a plant hormone involved in all stages of growth and development, including regulation of germination, responses to environmental stress, and fruit ripening. The final step in ethylene biosynthesis, oxidation of 1-aminocyclopropane-1-carboxylic acid (ACC) to yield ethylene, is catalyzed by ACC oxidase (ACCO). In a previous EPR and ENDOR study of the EPR-active Fe(II)-nitrosyl, [FeNO],(7) complex of ACCO, we demonstrated that both the amino and the carboxyl moieties of the inhibitor d,l-alanine, and the substrate ACC by analogy, coordinate to the Fe(II) ion in the Fe(II)-NO-ACC ternary complex. In this report, we use 35 GHz pulsed and CW ENDOR spectroscopy to examine the coordination of Fe by ACCO in more detail. ENDOR data for selectively (15)N-labeled derivatives of substrate-free ACCO-NO (E-NO) and substrate/inhibitor-bound ACCO-NO (E-NO-S) have identified two histidines as protein-derived ligands to Fe; (1,2)H and (17)O ENDOR of samples in D(2)O and H(2)(17)O solvent have confirmed the presence of water in the substrate-free Fe(II) coordination sphere (E-NO). Analysis of orientation-selective (14,15)N and (17)O ENDOR data is interpreted in terms of a structural model of the ACCO active site, both in the presence (E-NO-S) and in the absence (E-NO) of substrate. Evidence is also given that substrate binding dictates the orientation of bound O(2).

Substrate modulation of the properties and reactivity of the oxy-ferrous and hydroperoxo-ferric intermediates of cytochrome P450cam as shown by cryoreduction-EPR/ENDOR spectroscopy

EPR/ENDOR studies have been carried out on oxyferrous cytochrome P450cam one-electron cryoreduced by gamma-irradiation at 77 K in the absence of substrate and in the presence of a variety of substrates including its native hydroxylation substrate, camphor (a), and the alternate substrates, 5-methylenyl-camphor (b), 5,5-difluorocamphor (c), norcamphor (d), and adamantanone (e); the equivalent experiments have been performed on the T252A mutant complexed with a and b. The present study shows that the properties and reactivity of the oxyheme and of both the primary and the annealed intermediates are modulated by a bound substrate. This includes alterations in the properties of the heme center itself (g tensor; (14)N, (1)H, hyperfine couplings). It also includes dramatic changes in reactivity: the presence of any substrate increases the lifetime of hydroperoxoferri-P450cam (2) no less than ca. 20-fold. Among the substrates, b stands out as having an exceptionally strong influence on the properties and reactivity of the P450cam intermediates, especially in the T252A mutant. The intermediate, 2(T252A)-b, does not lose H(2)O(2), as occurs with 2(T252A)-a, but decays with formation of the epoxide of b. Thus, these observations show that substrate can modulate the properties of both the monoxygenase active-oxygen intermediates and the proton-delivery network that encompasses them.

Cryoreduction EPR and 13C, 19F ENDOR study of substrate-bound substates and solvent kinetic isotope effects in the catalytic cycle of cytochrome P450cam and its T252A mutant

We recently used cryoreduction EPR/ENDOR techniques to show that a substrate can modulate the properties of both the monooxygenase active-oxygen intermediates and of the proton-delivery network which encompasses them. In the present report we use Q-band pulsed 19F ENDOR (Mims 3-pulse sequence) to examine the substrate binding geometries of camphor, through use of the 5,5'--difluorocamphor, and 13C ENDOR to examine the binding of 5-methylenyl camphor labeled with 13C at C11. These probes are examined in multiple states of the catalytic cycle of P450cam and its T252A mutant. As part of this investigation we further report a new cryoreduction reaction, the reduction of a ferroheme to the EPR-visible Fe(I) state, and use it to probe the substrate binding to the EPR-silent ferroheme state. Finally we report the solvent kinetic isotope effect on the decay of the camphor complex of the hydroperoxo-ferric intermediate, the first such measurement on an individual step within the P450cam reaction cycle. Following reduction of oxyferrous-P450cam, this step is the rate-limiting step in camphor hydroxylation, and its solv-KIE of 1.8 at 190 K establishes that it involves activation of the hydroperoxo moiety by transfer of the 'second' proton of catalysis. We suggest that the finding that the heme pocket can exist in multiple substates, including multiple substrate binding locations, even in P450cam, along with the established possibility that the hydroperoxo-ferriheme intermediate can react with substrate, may explain the formation of multiple products by P450s.

An organometallic intermediate during alkyne reduction by nitrogenase

Nitrogenase is the metalloenzyme that catalyzes the nucleotide-dependent reduction of N(2), as well as reduction of a variety of other triply bonded substrates, including the alkyne, acetylene. Substitution of the alpha-70(Val) residue in the nitrogenase MoFe protein by alanine expands the range of substrates to include short-chain alkynes not reduced by the unaltered protein. Rapid freezing of the alpha-70(Ala) nitrogenase MoFe protein during reduction of the alkyne propargyl alcohol (HC triple bond CH(2)OH; PA) traps an S = (1)/(2) intermediate state of the active-site metal cluster, the FeMo-cofactor. We have combined CW and pulsed (13)C ENDOR (electron-nuclear double resonance) with two quantitative 35 GHz (1,2)H ENDOR techniques, Mims pulsed ENDOR and the newly devised "stochastic field-modulated" ENDOR, to study this intermediate prepared with isotopically substituted ((13)C, (1,2)H) propargyl alcohol in H(2)O and D(2)O buffers. These measurements allow the first description of a trapped nitrogenase reduction intermediate. The S = (1)/(2) turnover intermediate generated during the reduction of PA contains the 3-carbon chain of PA and exhibits resolved (1,2)H ENDOR signals from three protons, two strongly coupled (H(a)) and one weakly coupled (H(b)); H(a)(c) originates as the C3 proton of PA, while H(a)(s) and H(b) are solvent-derived. The two H(a) protons have identical hyperfine tensors, despite having different origins. The equality of the (H(a)(s), H(a)(c)) hyperfine tensors strongly constrains proposals for the structure of the cluster-bound reduced PA. Through consideration of model structures found in the Cambridge Structural Database, we propose that the intermediate contains a novel bio-organometallic complex in which a reduction product of propargyl alcohol binds as a metalla-cyclopropane ring to a single Fe atom of the Fe-S face of the FeMo-cofactor that is composed of Fe atoms 2, 3, 6, and 7. Of the two most attractive structures, one singly reduced at C3 (4), the other being the doubly reduced allyl alcohol product (6), we tentatively favor 6 because of the "natural" assignment it affords for H(b).

Investigation of the calcium-binding site of the oxygen evolving complex of photosystem II using 87Sr ESEEM spectroscopy

The proximity of the calcium/strontium binding site of the oxygen evolving complex (OEC) of photosystem II (PSII) to the paramagnetic Mn cluster is explored with (87)Sr three-pulse electron spin-echo envelope modulation (ESEEM) spectroscopy. CW-EPR spectra of Sr(2+)-substituted Ca(2+)-depleted PSII membranes show the modified g = 2 multiline EPR signal as previously reported. We performed three-pulse ESEEM on this modified multiline signal of the Mn cluster using natural abundance Sr and (87)Sr, respectively. Three-pulse ESEEM of the natural abundance Sr sample exhibits no detectable modulation by the 7% abundance (87)Sr. On the other hand, that of the (87)Sr enriched (93%) sample clearly reveals modulation arising from the I = (9)/(2) (87)Sr nucleus weakly magnetically coupled to the Mn cluster. Using a simple point dipole approximation for the electron spin, analysis of the (87)Sr ESEEM modulation depth via an analytic expression suggests a Mn-Ca (Sr) distance of 4.5 A. Simulation of three-pulse ESEEM with a numerical matrix diagonalization procedure gave good agreement with this analytical result. A more appropriate tetranuclear magnetic/structural model for the Mn cluster converts the 4.5 A point dipole distance to a 3.8-5.0 A range of distances. DFT calculations of (43)Ca and (87)Sr quadrupolar interactions on Ca (and Sr substituted) binding sites in various proteins suggest that the lack of the nuclear quadrupole induced splitting in the ESEEM spectrum of (87)Sr enriched PSII samples is related to a very high degree of symmetry of the ligands surrounding the Sr(2+) ion in the substituted Ca site. Numerical simulations show that moderate (87)Sr quadrupolar couplings decrease the envelope modulation relative to the zero quadrupole case, and therefore we consider that the 3.8-5.0 A range obtained without quadrupolar coupling included in the simulation represents an upper limit to the actual manganese-calcium distance. This (87)Sr pulsed EPR spectroscopy provides independent direct evidence that the calcium/strontium binding site is close to the Mn cluster in the OEC of PSII.

Direct Detection of a Hydrogen Ligand in the [NiFe] Center of the Regulatory H2-Sensing Hydrogenase from Ralstonia eutropha in Its Reduced State by HYSCORE and ENDOR Spectroscopy

The regulatory H2-sensing [NiFe] hydrogenase of the beta-proteobacterium Ralstonia eutropha displays an Ni-C "active" state after reduction with H2 that is very similar to the reduced Ni-C state of standard [NiFe] hydrogenases. Pulse electron nuclear double resonance (ENDOR) and four-pulse ESEEM (hyperfine sublevel correlation, HYSCORE) spectroscopy are applied to obtain structural information on this state via detection of the electron-nuclear hyperfine coupling constants. Two proton hyperfine couplings are determined by analysis of ENDOR spectra recorded over the full magnetic field range of the EPR spectrum. These are associated with nonexchangeable protons and belong to the beta-CH(2) protons of a bridging cysteine of the NiFe center. The signals of a third proton exhibit a large anisotropic coupling (Ax = 18.4 MHz, Ay = -10.8 MHz, Az = -18 MHz). They disappear from the 1H region of the ENDOR spectra after exchange of H2O with 2H2O and activation with 2H2 instead of H2 gas. They reappear in the 2H region of the ENDOR and HYSCORE spectra. Based on a comparison with the spectroscopically similar [NiFe] hydrogenase of Desulfovibrio vulgaris Miyazaki F, for which the g-tensor orientation of the Ni-C state with respect to the crystal structure is known (Foerster et al. J. Am. Chem. Soc. 2003, 125, 83-93), an assignment of the 1H hyperfine couplings is proposed. The exchangeable proton resides in a bridging position between the Ni and Fe and is assigned to a formal hydride ion. After illumination at low temperature (T = 10 K), the Ni-L state is formed. For the Ni-L state, the strong hyperfine coupling observed for the exchangeable hydrogen in Ni-C is lost, indicating a cleavage of the metal-hydride bond(s). These experiments give first direct information on the position of hydrogen binding in the active NiFe center of the regulatory hydrogenase. It is proposed that such a binding situation is also present in the active Ni-C state of standard hydrogenases.

The interstitial atom of the nitrogenase FeMo-cofactor: ENDOR and ESEEM show it is not an exchangeable nitrogen

A recent high-resolution X-ray crystallographic study (1.16 A) of the Azotobacter vinelandii nitrogenase MoFe protein revealed a previously undetected electron density associated with the active site FeMo-cofactor. The density is located inside the cluster at the center of the "trigonal prism" of six irons and is assigned to a species "X". The identity of species X was not resolved, although the electron density is consistent with a single N, O, or C atom. One proposal is that X is an N atom that derives from and exchanges with N from N2 during catalysis. In the present study, we have examined this possibility by employing 14N and 15N isotopes of N2 along with ENDOR and ESEEM spectroscopies. The WT MoFe protein and alpha-359Arg-->Lys and alpha-381Phe-->Leu variants were allowed to turn over in the presence of 14N2 or 15N2, and then were examined as resting enzymes by ENDOR and ESEEM at X- and Q-bands to look for all 14N and 15N signals coupled to the electron spin of the FeMo-cofactor and to determine if any exchanged during turnover. We have found five peaks in Q-band pulsed ENDOR spectra that appear to arise not only from previously reported N1/N2, which give rise to the ESEEM, but also from one or two additional coupled nitrogens. None of the ENDOR and ESEEM signals vanish or are altered by catalytic turnover with 15N2, and no new 15N signal is detected, leading to the conclusion that if species X is a nitrogen atom, it does not exchange during dinitrogen reduction.

Electron-nuclear double resonance spectroscopy (and electron spin-echo envelope modulation spectroscopy) in bioinorganic chemistry

This perspective discusses the ways that advanced paramagnetic resonance techniques, namely electron-nuclear double resonance (ENDOR) and electron spin-echo envelope modulation (ESEEM) spectroscopies, can help us understand how metal ions function in biological systems.

Effects of substrates (methyl isocyanide, C2H2) and inhibitor (CO) on resting-state wild-type and NifV(-)Klebsiella pneumoniae MoFe proteins

We report the use of electron nuclear double resonance (ENDOR) spectroscopy to examine how the metal sites in the FeMo-cofactor cluster of the resting nitrogenase MoFe protein respond to addition of the substrates acetylene and methyl isocyanide and the inhibitor carbon monoxide. 1H, 57Fe and 95Mo ENDOR measurements were performed on the wild-type and the NifV(-)proteins from Klebsiella pneumoniae. Among the molecules tested, only the addition of acetylene to either protein induced widespread changes in the 57Fe ENDOR spectra. Acetylene also induced increases in intensity from unresolved protons in the proton ENDOR spectra. Thus we conclude that acetylene may bind to the resting-state MoFe protein to perturb the FeMo-cofactor environment. On the other hand, the present results show that methyl isocyanide and carbon monoxide do not substantially alter the FeMo cofactor's geometric and electronic structures. We interpret this as lack of interaction between those two molecules and the FeMo cofactor in the resting state MoFe protein. Thus, although it is generally accepted that substrates or inhibitors bind to the FeMo-cofactor only under turnover condition, this work provides evidence that at least one substrate can perturb the active site of nitrogenase under non-catalytic conditions.

Simulations of the (1)H electron spin echo-electron nuclear double resonance and (2)H electron spin echo envelope modulation spectra of exchangeable hydrogen nuclei coupled to the S(2)-state photosystem II manganese cluster

The pulsed EPR methods of electron spin echo envelope modulation (ESEEM) and electron spin echo-electron nuclear double resonance (ESE-ENDOR) are used to investigate the proximity of exchangeable hydrogens around the paramagnetic S(2)-state Mn cluster of the photosystem II oxygen-evolving complex. Although ESEEM and ESE-ENDOR are both pulsed electron paramagnetic resonance techniques, the specific mechanisms by which nuclear spin transitions are observed are quite different. We are able to generate good simulations of both (1)H ESE-ENDOR and (2)H ESEEM signatures of exchangeable hydrogens at the S(2)-state cluster. The convergence of simulation parameters for both methods provides a high degree of confidence in the simulations. Several exchangeable protons-deuterons with strong dipolar couplings are observed. In the simulations, two of the close ( approximately 2.5 A) hydrogen nuclei exhibit strong isotropic couplings and are therefore most probably associated with direct substrate ligation to paramagnetic Mn. Another two of the close ( approximately 2.7 A) hydrogen nuclei show no isotropic couplings and are therefore most probably not contained in Mn ligands. We suggest that these proximal hydrogens may be associated with a Ca(2+)-bound substrate, as indicated in recent mechanistic proposals for O(2) formation.

Electro-nuclear double resonance spectroscopic evidence for a hydroxo-bridge nucleophile involved in catalysis by a dinuclear hydrolase

Despite the current availability of several crystal structures of purple acid phosphatases, to date there is no direct evidence for solvent-derived ligands occupying terminal positions in the active enzyme. This is of central importance, because catalysis has been shown to proceed through the direct attack on a metal-bound phosphate ester by a metal-activated solvent-derived moiety, which has been proposed to be either (i) a hydroxide ligand terminally bound to the ferric center or (ii) a bridging hydroxide. In this work we use (2)H Q-band (35 GHz) pulsed electron-nuclear double resonance (ENDOR) spectroscopy to identify solvent molecules coordinated to the active mixed-valence (Fe(3+)Fe(2+)) form of the dimetal center of uteroferrin (Uf), as well as to its complexes with the anions MoO(4), AsO(4), and PO(4). The solvent-derived coordination of the dinuclear center of Uf as deduced from ENDOR data includes a bridging hydroxide and a terminal water/hydroxide bound to Fe(2+) but no terminal water/hydroxide bound to Fe(3+). The terminal water is lost upon anion binding while the hydroxyl bridge remains. These results are not compatible with a hydrolysis mechanism involving a terminal Fe(3+)-bound nucleophile, but they are consistent with a mechanism that relies on the bridging hydroxide as the nucleophile.

Determination of hyperfine tensor components from nuclear frequencies at canonical orientations of the g-tensor

The analytical procedure for the determination of all components of the symmetric hyperfine tensor of the I=1/2 nucleus in the g-tensor coordinate system is described, assuming that nuclear frequencies corresponding to the principal directions of the g-tensor and exact values of the external magnetic field (or nuclear Zeeman frequencies) are experimentally available.

Catalytic mechanism of heme oxygenase through EPR and ENDOR of cryoreduced oxy-heme oxygenase and its Asp 140 mutants

Heme oxygenase (HO) catalyzes the O(2)- and NADPH-cytochrome P450 reductase-dependent conversion of heme to biliverdin, Fe, and CO through a process in which the heme participates both as a prosthetic group and as a substrate. In the present study, we have generated a detailed reaction cycle for the first monooxygenation step of HO catalysis, conversion of the heme to alpha-meso-hydroxyheme. We employed EPR (using both (16)O(2) and (17)O(2)) and (1)H, (14)N ENDOR spectroscopies to characterize the intermediates generated by 77 K radiolytic cryoreduction and subsequent annealing of wild-type oxy-HO and D140A, F mutants. One-electron cryoreduction of oxy-HO yields a hydroperoxoferri-HO with g-tensor, g = [2.37, 2.187, 1.924]. Annealing of this species to 200 K is accompanied by spectroscopic changes that include the appearance of a new (1)H ENDOR signal, reflecting rearrangements in the active site. Kinetic measurements at 214 K reveal that the annealed hydroperoxoferri-HO species, denoted R, generates the ferri-alpha-meso-hydroxyheme product in a first-order reaction. Disruption of the H-bonding network within the distal pocket of HO by the alanine and phenylalanine mutations of residue D140 prevents product formation. The hydroperoxoferri-HO (D140A) instead undergoes heterolytic cleavage of the O-O bond, ultimately yielding an EPR-silent compound II-like species that does not form product. These results, which agree with earlier suggestions, establish that hydroperoxoferri-HO is indeed the reactive species, directly forming the alpha-meso-hydroxyheme product by attack of the distal OH of the hydroperoxo moiety at the heme alpha-carbon.

17O ENDOR detection of a solvent-derived Ni-(OH(x))-Fe bridge that is lost upon activation of the hydrogenase from Desulfovibrio gigas

Crystallographic studies of the hydrogenases (Hases) from Desulfovibrio gigas (Dg) and Desulfovibrio vulgaris Miyazaki (DvM) have revealed heterodinuclear nickel-iron active centers in both enzymes. The structures, which represent the as-isolated (unready) Ni-A (S = (1)/(2)) enzyme state, disclose a nonprotein ligand (labeled as X) bridging the two metals. The bridging atom was suggested to be an oxygenic (O(2)(-) or OH(-)) species in Dg Hase and an inorganic sulfide in DvM Hase. To determine the nature and chemical characteristics of the Ni-X-Fe bridging ligand in Dg Hase, we have performed 35 GHz CW (17)O ENDOR measurements on the Ni-A form of the enzyme, exchanged into H(2)(17)O, on the active Ni-C (S = (1)/(2)) form prepared by H(2)-reduction of Ni-A in H(2)(17)O, and also on Ni-A formed by reoxidation of Ni-C in H(2)(17)O. In the native state of the protein (Ni-A), the bridging ligand does not exchange with the H(2)(17)O solvent. However, after a reduction/reoxidation cycle (Ni-A --> Ni-C --> Ni-A), an (17)O label is introduced at the active site, as seen by ENDOR. Detailed analysis of a 2-D field-frequency plot of ENDOR spectra taken across the EPR envelope of Ni-A((17)O) shows that the incorporated (17)O has a roughly axial hyperfine tensor, A((17)O) approximately [5, 7, 20] MHz, discloses its orientation relative to the g tensor, and also yields an estimate of the quadrupole tensor. The substantial isotropic component (a(iso)((17)O) approximately 11 MHz) of the hyperfine interaction indicates that a solvent-derived (17)O is indeed a ligand to Ni and thus that the bridging ligand X in the Ni-A state of Dg Hase is indeed an oxygenic (O(2)(-) or OH(-)) species; comparison with earlier EPR results by others indicates that the same holds for Ni-B. The small (57)Fe hyperfine coupling seen previously for Ni-A (A((57)Fe) approximately 0.9 MHz) is now shown to persist in Ni-C, A((57)Fe) approximately 0.8 MHz. However, the (17)O signal is lost upon reductive activation to the Ni-C state; reoxidation to Ni-A leads to the reappearance of the signal. Consideration of the electronic structure of the EPR-active states of the dinuclear center leads us to suggest that the oxygenic bridge in Ni-A(B) is lost in Ni-C and is re-formed from solvent upon reoxidation to Ni-A. This implies that the reductive activation to Ni-C opens Ni/Fe coordination sites which may play a central role in the enzyme's activity.

Generation and electron paramagnetic resonance spin trapping detection of thiyl radicals in model proteins and in the R1 subunit of Escherichia coli ribonucleotide reductase

In the Escherichia coli class Ia ribonucleotide reductase (RNR), the best characterized RNR, there is no spectroscopic evidence for the existence of the postulated catalytically essential thiyl radical (R-S(*)) in the substrate binding subunit R1. We report first results on artificially generated thiyl radicals in R1 using two different methods: chemical oxidation by Ce(IV)/nitrilotriacetate (NTA) and laser photolysis of nitric oxide from nitrosylated cysteines. In both cases, EPR spin trapping at room temperature using phenyl-N-t-butylnitrone, and controls with chemically blocked cysteines, has shown that the observed spin adduct originates from thiyl radicals. The EPR line shape of the protein-bound spin adduct is typical for slow motion of the nitroxide moiety, which indicates that the majority of trapped thiyl radicals are localized in a folded region of R1. In aerobic R1 samples without spin trap that were frozen after treatment with Ce(IV)/NTA or laser photolysis, we observed sulfinyl radicals (R-S(*)=O) assigned via their g-tensor components 2.0213, 2.0094, and 2.0018 and the hyperfine tensor components 1.0, 1.1, and 0.9 mT of one beta-proton. Sulfinyl radicals are the reaction products of thiyl radicals and oxygen and give additional evidence for generation of thiyl radicals in R1 by the procedures used.

31P NMR probes of chemical dynamics: paramagnetic relaxation enhancement of the (1)H and (31)P NMR resonances of methyl phosphite and methylethyl phosphate anions by selected metal complexes

Methyl phosphite ((CH(3)O)P(H)(O)(2)(-); MeOPH) and methylethyl phosphate ((CH(3)O)P(OCH(2)CH(3))(O)(2)(-); MEP) are two members of a class of anionic ligands whose (31)P T(2) relaxation rates are remarkably sensitive to paramagnetic metal ions. The temperature dependence of the (31)P NMR line broadenings caused by the Mn(H(2)O)(6)(2+) ion and a water-soluble manganese(III) porphyrin (Mn(III)TMPyP(5+)) indicates that the extent of paramagnetic relaxation enhancement is a measure of the rate at which the anionic probes come into physical contact with the paramagnetic center (i.e., enter the inner coordination shell); that is, piDeltanu(par) = k(assn)[M], where Deltanu(par) is the difference between the line widths of the resonance in paramagnetic and diamagnetic solutions, and k(assn) is the second-order rate constant for association of the phosphorus ligand with the metal, M. Comparison of the (31)P T(1) and T(2) relaxation enhancements shows that rapid T(2) relaxation by the metal ion is caused by scalar interaction with the electronic spin. Relaxation of the phosphorus-bound proton of MeOPH ((1)H-P) by Mn(III)TMPyP(5+) displayed intermediate exchange kinetics over much of the observable temperature range. The field strength dependence of (1)H-P T(2) enhancement and the independence of the (31)P T(2) support these assertions. As in the case of the (31)P T(2), the (1)H-P T(2) relaxation enhancement results from scalar interaction with the electronic spin. The scalar coupling interpretation of the NMR data is supported by a pulsed EPR study of the interactions of Mn(H(2)O)(6)(2+) with the P-deuterated analogue of methyl phosphite, CH(3)OP((2)H)(O)(2)(-). The electron to (31)P and (2)H nuclear scalar coupling constants were found to be 4.6 and 0.10 MHz, respectively. In contrast, the effects of paramagnetic ions on the methoxy and ethoxy (1)H resonances of MeOPH and MEP are weak, and the evidence suggests that relaxation of these nuclei occurs by a dipolar mechanism. The wide variation in the relaxation sensitivities of the (1)H and (31)P nuclei of MeOPH and MEP permits us to study how differences in the strengths of the interactions between an observed nucleus and a paramagnetic center affect NMR T(2) relaxations. We propose that these anion ligand probes may be used to study ligand-exchange reactivities of manganese complexes without requiring variable temperature studies. The (31)P T(2) is determined by chemical association kinetics when the following condition is met: (T(2M,P)/T(2M,H))(Deltanu(P)/Deltanu(HP) - 1) < 0.2 where T(2M,P) and T(2M,H) are the transverse relaxation times of the (31)P and (1)H nuclei when the probe is bound to the metal, and Deltanu(P) and Deltanu(HP) are the paramagnetic line broadenings of the (31)P and (1)H-P nuclei, respectively. We assert that the ratio T(2M,P)/T(2M,H) can be estimated for a general metal complex using the results of EPR and NMR experiments.

Multifrequency probe for pulsed EPR and ENDOR spectroscopy

The design, construction, and performance of a multifrequency pulsed EPR and ENDOR probe for use at cryogenic temperatures are described. Interchangeable resonators based on a folded strip line design allow variation of the resonance frequency over a range of 5-11 GHz. Variable coupling to the resonator is achieved capacitively via a simple mechanical adjustment which is thermally and mechanically stable. The entire assembly is robust and easily fabricated. Common methods of analyzing the resonator parameters such as the Q-factor and coupling coefficient are discussed quantitatively. Probe performance data and multifrequency pulsed ENDOR spectra are presented.

Cryoreduction of methyl-coenzyme M reductase: EPR characterization of forms, MCR(ox1) and MCR (red1)

Methyl-coenzyme M reductase (MCR) catalyzes the formation of methyl-coenzyme M (CH(3)S-CH(2)CH(2)SO(3)) from methane. The active site is a nickel tetrahydrocorphinoid cofactor, factor 430, which in inactive form contains EPR-silent Ni(II). Two such forms, denoted MCR(silent) and MCR(ox1)(-)(silent), were previously structurally characterized by X-ray crystallography. We describe here the cryoreduction of both of these MCR forms by gamma-irradiation at 77 K, which yields reduced protein maintaining the structure of the oxidized starting material. Cryoreduction of MCR(silent) yields an EPR signal that strongly resembles that of MCR(red1), the active form of MCR; and stepwise annealing to 260-270 K leads to formation of MCR(red1). Cryoreduction of MCR(ox1)(-)(silent) solutions shows that our preparative method for this state yields enzyme that contains two major forms. One behaves similarly to MCR(silent), as shown by the observation that both of these forms give essentially the same redlike EPR signals upon cryoreduction, both of which give MCR(red1) upon annealing. The other form is assigned to the crystallographically characterized MCR(ox1)(-)(silent) and directly gives MCR(ox1) upon cryoreduction. X-band spectra of these cryoreduced samples, and of conventionally prepared MCR(red1) and MCR(ox1), all show resolved hyperfine splitting from four equivalent nitrogen ligands with coupling constants in agreement with those determined in previous EPR studies and from (14)N ENDOR of MCR(red1) and MCR(ox1). These experiments have confirmed that all EPR-visible forms of MCR contain Ni(I) and for the first time generated in vitro the EPR-visible, enzymatically active MCR(red1) and the activate-able "ready" MCR(ox1) from "silent" precursors. Because the solution Ni(II) species we assign as MCR(ox1)(-)(silent) gives as its primary cryoreduction product the Ni(I) state MCR(ox1), previous crystallographic data on MCR(ox1)(-)(silent) allow us to identify the exogenous axial ligand in MCR(ox1) as the thiolate from CoM; the cryoreduction experiments further allow us to propose possible axial ligands in MCR(red1). The availability of model compounds for MCR(red1) and MCR(ox1) also is discussed.

Pulsed and parallel-polarization EPR characterization of the photosystem II oxygen-evolving complex

Photosystem II uses visible light to drive the oxidation of water, resulting in bioactivated electrons and protons, with the production of molecular oxygen as a byproduct. This water-splitting reaction is carried out by a manganese cluster/tyrosine radial ensemble, the oxygen -evolving complex. Although conventional continuous-wave, perpendicular -polarization electron paramagnetic resonance (EPR) spectroscopy has significantly advanced our knowledge of the structure and function of the oxygen-evolving complex, significant additional information can be obtained with the application of additional EPR methodologies. Specifically, parallel-polarization EPR spectroscopy can be use to obtain highly resolved EPR spectra of integer spin Mn species, and pulsed EPR spectroscopy with electron spin echo-based sequences, such as electron spin echo envelope modulation and electron spin echo-electron nuclear double resonance, can be used to measure weak interactions obscured in continuous-wave spectroscopy by inhomogeneous broadening.

Hydroxylation of camphor by reduced oxy-cytochrome P450cam: mechanistic implications of EPR and ENDOR studies of catalytic intermediates in native and mutant enzymes

We have employed gamma-irradiation at cryogenic temperatures (77 K and also approximately 6 K) of the ternary complexes of camphor, dioxygen, and ferro-cytochrome P450cam to inject the "second" electron of the catalytic process. We have used EPR and ENDOR spectroscopies to characterize the primary product of reduction as well as subsequent states created by annealing reduced oxyP450, both the WT enzyme and the D251N and T252A mutants, at progressively higher temperatures. (i) The primary product upon reduction of oxyP450 4 is the end-on, "H-bonded peroxo" intermediate 5A. (ii) This converts even at cryogenic temperatures to the hydroperoxo-ferriheme species, 5B, in a step that is sensitive to these mutations. Yields of 5B are as high as 40%. (iii) In WT and D251N P450s, brief annealing in a narrow temperature range around 200 K causes 5B to convert to a product state, 7A, in which the product 5-exo-hydroxycamphor is coordinated to the ferriheme in a nonequilibrium configuration. Chemical and EPR quantitations indicate the reaction pathway involving 5B yields 5-exo-hydroxycamphor quantitatively. Analogous (but less extensive) results are seen for the alternate substrate, adamantane. (iv) Although the T252A mutation does not interfere with the formation of 5B, the cryoreduced oxyT252A does not yield product, which suggests that 5B is a key intermediate at or near the branch-point that leads either to product formation or to nonproductive "uncoupling" and H(2)O(2) production. The D251N mutation appears to perturb multiple stages in the catalytic cycle. (v) There is no spectroscopic evidence for the buildup of a high-valence oxyferryl/porphyrin pi-cation radical intermediate, 6. However, ENDOR spectroscopy of 7A in H(2)O and D(2)O buffers shows that 7A contains hydroxycamphor, rather than water, bound to Fe(3+), and that the proton removed from the C(5) carbon of substrate during hydroxylation is trapped as the hydroxyl proton. This demonstrates that hydroxylation of substrates by P450cam in fact occurs by the formation and reaction of 6. (vi) Annealing at > or = 220 K converts the initial product state 7A to the equilibrium product state 7, with the transition occurring via a second nonequilibrium product state, 7B, in the D251N mutant; in states 7B and 7 the hydroxycamphor hydroxyl proton no longer is trapped. (vii) The present results are discussed in the context of other efforts to detect intermediates in the P450 catalytic cycle.

Probing the Molecular Geometry of Five-Coordinate Vanadyl Complexes with Pulsed ENDOR

The pulsed electron paramagnetic resonance (EPR) technique of (51)V electron spin echo-electron nuclear double resonance (ESE-ENDOR) has been used to measure the nuclear quadrupole coupling constants of a series of five-coordinate vanadyl complexes containing Schiff base ligands with geometries ranging from distorted square pyramidal to distorted trigonal bipyramidal. Vanadium nuclear quadrupole coupling constants are sensitive to the coordination geometry of the vanadyl ion, and thus sensitive to the structural distortions within this series of complexes. (51)V ESE-ENDOR has been shown to be a probe of the coordination geometry of vanadyl complexes. Such a spectroscopic probe should prove useful in the investigation of vanadyl of unknown coordination geometry, such as may be found in the interaction of the vanadyl ion with biomolecules.