Pulsed Electron Paramagnetic Resonance Studies Research Articles

Encapsulated Atomic Hydrogen in Octamethyl-POSS Cages: A Pulsed EPR Study.

Encapsulated atomic hydrogen in polyhedral oligomeric silsesquioxane (POSS) cages is a promising candidate for spin-based quantum technologies. Key parameters such as spin relaxation times and magnetic interactions with surrounding electron and nuclear spins can be typically probed with advanced electron paramagnetic resonance (EPR) methods. Here we present a detailed pulsed EPR study of the species H@Si8O12R8 with R=CH3, namely encapsulated atomic hydrogen in the octamethyl POSS derivative. The temperature dependence of the spin-lattice relaxation rate 1/ is analyzed in terms of a Raman process with a Debye temperature of K and a thermally activated process with (552 cm-1), whereas, the phase memory time shows the typical shortening behaviour at K observed for all methyl-containing derivatives. The hyperfine coupling of the cage Si nuclei is measured by hyperfine sublevel correlation (HYSCORE) spectroscopy and is found to fulfil the so-called "matching condition" at the low-field EPR transition. The potential of this paramagnetic molecule to perform one-qubit quantum operations is probed by room-temperature Rabi oscillations.

Ligand Effects on the Spin Relaxation Dynamics and Coherent Manipulation of Organometallic La(II) Potential Qudits.

We present pulsed electron paramagnetic resonance (EPR) studies on three La(II) complexes, [K(2.2.2-cryptand)][La(Cp')3] (1), [K(2.2.2-cryptand)][La(Cp″)3] (2), and [K(2.2.2-cryptand)][La(Cptt)3] (3), which feature cyclopentadienyl derivatives as ligands [Cp' = C5H4SiMe3; Cp″ = C5H3(SiMe3)2; Cptt = C5H3(CMe3)2] and display a C3 symmetry. Long spin-lattice relaxation (T1) and phase memory (Tm) times are observed for all three compounds, but with significant variation in T1 among 1-3, with 3 being the slowest relaxing due to higher s-character of the SOMO. The dephasing times can be extended by more than an order of magnitude via dynamical decoupling experiments using a Carr-Purcell-Meiboom-Gill (CPMG) sequence, reaching 161 μs (5 K) for 3. Coherent spin manipulation is performed by the observation of Rabi quantum oscillations up to 80 K in this nuclear spin-rich environment (1H, 13C, and 29Si). The high nuclear spin of 139La (I = 7/2), and the ability to coherently manipulate all eight hyperfine transitions, makes these molecules promising candidates for application as qudits (multilevel quantum systems featuring d quantum states; d >2) for performing quantum operations within a single molecule. Application of HYSCORE techniques allows us to quantify the electron spin density at ligand nuclei and interrogate the role of functional groups to the electron spin relaxation properties.

Continuous-wave and pulsed EPR studies of glass-like carbon with high spin concentration: Evidence for triplet states

Glass-like carbons (GLCs) are a large group of disordered carbon materials which have applications in various fields, but not all their properties are well characterized. In particular, they have high concentration of paramagnetic centers (PCs) which are easily observed by electron paramagnetic resonance (EPR). However, an apparently simple EPR spectrum, together with its peculiarities caused by high electrical conductivity, make it difficult to identify the PCs and their origin remains unknown. To resolve these unsettled questions, in this work we use the vacuum Rabi splitting and Rabi oscillations which so far have not been applied to study these materials by the continuous wave (CW) and pulsed EPR. GLC produced by pyrolysis of phenol-formaldehyde resin at 1120 K was studied in the temperature range of 4.3–300 K. Using the Rabi splitting we found conditions for undistorted registration of CW EPR spectra and determined spin concentration (1×1020 spin/g). The temperature dependence of the EPR susceptibility shows the coexistence of Pauli paramagnetism due to conduction electrons and the Curie-type paramagnetism of thermally activated PCs with the activation energy of 5.7 cm-1. These PCs may be excited triplet states, the presence of which is revealed by Rabi oscillations in the pulsed EPR.

How to Build a Metalloenzyme: Lessons from a Protein-Based Model of Acetyl Coenzyme A Synthase

"What I cannot create, I do not understand"─Richard Feynman. This sentiment motivates the entire field of artificial metalloenzymes. Naturally occurring enzymes catalyze reactions with efficiencies, rates, and selectivity that generally cannot be achieved in synthetic systems. Many of these processes represent vital building blocks for a sustainable society, including CO2 conversion, nitrogen fixation, water oxidation, and liquid fuel synthesis. Our inability as chemists to fully reproduce the functionality of naturally occurring enzymes implicates yet-unknown contributors to reactivity. To identify these properties, it is necessary to consider all of the components of naturally occurring metalloenzymes, from the active site metal(s) to large-scale dynamics. In this Account, we describe the holistic development of a metalloprotein-based model that functionally reproduces the acetyl coenzyme A synthase (ACS) enzyme.ACS catalyzes the synthesis of a thioester, acetyl coenzyme A, from gaseous carbon monoxide, a methyl group donated by a cobalt corrinoid protein, and coenzyme A. The active site of ACS contains a bimetallic nickel site coupled to a [4Fe-4S] cluster. This reaction mimics Monsanto's acetic acid synthesis and represents an ancient process for incorporating inorganic carbon into cellular biomass through the primordial Wood-Ljungdahl metabolic pathway. From a sustainability standpoint, the reversible conversion of C1 substrates into an acetyl group and selective downstream transfer to a thiolate nucleophile offer opportunities to expand this reactivity to the anthropogenic synthesis of liquid fuels. However, substantial gaps in our understanding of the ACS catalytic mechanism coupled with the enzyme's oxygen sensitivity and general instability have limited these applications. It is our hope that development of an artificial metalloenzyme that carries out ACS-like reactions will advance our mechanistic understanding and enable synthesis of robust compounds with the capacity for similar reactivity.To construct this model, we first focused on the catalytic proximal nickel (NiP) site, which has a single metal center bound by three bridging cysteine residues in a "Y"-shaped arrangement. With an initial emphasis on reproducing the general structure of a low-coordinate metal binding site, the type I cupredoxin, azurin, was selected as the protein scaffold, and a nickel center was incorporated into the mononuclear site. Using numerous spectroscopic and computational techniques, including electron paramagnetic resonance (EPR) spectroscopy, nickel-substituted azurin was shown to have similar electronic and geometric structures to the NiP center in ACS. A substrate access channel was installed, and both carbon monoxide and a methyl group were shown to bind individually to the reduced NiI center. The elusive EPR-active S = 1/2 Ni-CH3 species, which has never been detected in native ACS, was observed in the azurin-based model, establishing the capacity of a biological NiI species to support two-electron organometallic reactions. Pulsed EPR studies on the S = 1/2 Ni-CH3 species in azurin suggested a noncanonical electronic structure with an inverted ligand field, which was proposed to prevent irreversible site degradation. This model azurin protein was ultimately shown to perform carbon-carbon and carbon-sulfur bond formation using sequential, ordered substrate addition for selective, stoichiometric thioester synthesis. X-ray spectroscopic methods were used to provide characterization of the remaining catalytic intermediates, resolving some debate over key mechanistic details.The overall approach and strategies that we employed for the successful construction of a functional protein-based model of ACS are described in this Account. We anticipate that these principles can be adapted across diverse metalloenzyme classes, providing essential mechanistic details and guiding the development of next-generation, functional artificial metalloenzymes.

Continuous wave and pulsed EPR study of Cd1-xMnxTe crystals with different Mn content

The cadmium manganese telluride (Cd1–xMnxTe) crystals (x < 0.001 and x = 0.02, 0.04, 0.1) grown using the Bridgman method were studied by applying continuous wave and pulsed electron paramagnetic resonance (EPR) spectroscopy in the wide temperature range. The Cd1–xMnxTe crystals with x < 0.001 revealed the EPR spectrum from isolated Mn2+ with g⊥ = g∥ = 2.0074(3), |A⊥| = |A∥| = 56.97 ⋅ 10–4 cm–1, |a| = 30.02⋅10–4 cm–1, while Cd1–xMnxTe crystals with x = 0.02…0.04 are characterized by two single broad isotropic EPR lines of Lorentzian shape (g ~ 2.009 and g ~ 1.99) due to Mn clusters of different sizes. The EPR spectrum of Cd1–xMnxTe crystals with x = 0.01 consists of the single broad line at g ~ 2.01 due to higher level of homogeneity inherent to these crystals. The temperature dependence of spin relaxation times for the isolated Mn2+ center in the Cd1–xMnxTe crystals with x < 0.001 has been described using the conceptions of Orbach process for TM–1 and two-phonon Raman process for T1–1.

Pulsed EPR Study of the Initial Steps of Radical-Scavenging Reactions of C70 with Diphenylphosphine Oxide, Hydroxycyclohexyl, and 2-Hydroxypropyl Radicals

The radical scavenging reactions of C70 were investigated using time-resolved (TR-) and pulsed electron paramagnetic resonance (EPR) methods. In the diphenylphosphine oxide (DPPO) radical and C70 system, structural isomers of the adduct radicals of C70 appeared in the TR-EPR spectrum with clear hyperfine structures due to the phosphorus atom. Four isomers were identified through the analysis of the hyperfine coupling constants. The assignment of the adduct radicals was confirmed by the semiempirical calculation of the relative addition reaction rate constants that produce each isomer. The radical scavenging rate constants, ksca, of C70 in toluene were determined for DPPO, hydroxylcyclohexyl, and 2-hydroxypropyl radicals through electron spin echo observations using the pulsed-EPR method. The ksca values were nearly 109 mol-1 dm3 s-1 and were almost equal to the diffusion-controlled rate constant in toluene. This proves that C70 acts as an excellent radical scavenger. In addition, the radical addition rate constants, kadd, of C70 for varying carbon atoms in C70 were obtained by considering the peak intensity ratio of the adduct radicals in the TR-EPR spectrum. In this study, we demonstrated that the large number of carbon atoms in pentagons (five-membered rings) is responsible for the high reactivity of fullerenes.

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal $4H$-, $6H$-, and rhombic $15R$-silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of ${\mathrm{M}}_{\mathrm{S}}$. The signs of these interactions and, as a result, the signs of oscillating spin density at $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with $^{29}\mathrm{Si}$ and $^{13}\mathrm{C}$ nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory.

Conformational landscape of non-B variants of HIV-1 protease: A pulsed EPR study

HIV infection is a global health epidemic with current FDA-approved HIV-1 Protease inhibitors (PIs) designed against subtype B protease, yet they are used in HIV treatment world-wide regardless of patient HIV classification. In this study, double electron-electron resonance (DEER) electron paramagnetic resonance (EPR) spectroscopy was utilized to gain insights in how natural polymorphisms in several African and Brazilian protease (PR) variants affect the conformational landscape both in the absence and presence of inhibitors. Findings show that Subtypes F and H HIV-1 PR adopt a primarily closed conformation in the unbound state with two secondary mutations, D60E and I62V, postulated to be responsible for the increased probability for closed conformation. In contrast, subtype D, CRF_AG, and CRF_BF HIV-1 PR adopt a primarily semi-open conformation, as observed for PI-naïve-subtype B when unbound by substrate or inhibitor. The impact that inhibitor binding has on shifting the conformational land scape of these variants is also characterized, where analysis provides classification of inhibitor induced shifts away from the semi-open state into weak, moderate and strong effects. The findings are compared to those for prior studies of inhibitor induced conformational shifts in PI-naïve Subtype B, C and CRF_AE.

Electron paramagnetic resonance study of ferroelectric phase transition and dynamic effects in a Mn2+ doped [NH4][Zn(HCOO)3] hybrid formate framework.

We present an X- and Q-band continuous wave (CW) and pulse electron paramagnetic resonance (EPR) study of a manganese doped [NH4][Zn(HCOO)3] hybrid framework, which exhibits a ferroelectric structural phase transition at 190 K. The CW EPR spectra obtained at different temperatures exhibit clear changes at the phase transition temperature. This suggests a successful substitution of the Zn2+ ions by the paramagnetic Mn2+ centers, which is further confirmed by the pulse EPR and 1H ENDOR experiments. Spectral simulations of the CW EPR spectra are used to obtain the temperature dependence of the Mn2+ zero-field splitting, which indicates a gradual deformation of the MnO6 octahedra indicating a continuous character of the transition. The determined data allow us to extract the critical exponent of the order parameter (β = 0.12), which suggests a quasi two-dimensional ordering in [NH4][Zn(HCOO)3]. The experimental EPR results are supported by the density functional theory calculations of the zero-field splitting parameters. Relaxation time measurements of the Mn2+ centers indicate that the longitudinal relaxation is mainly driven by the optical phonons, which correspond to the vibrations of the metal-oxygen octahedra. The temperature behavior of the transverse relaxation indicates a dynamic process in the ordered ferroelectric phase.

Identifying exchangeable protons in the QA‐site of photosystem II

Through the use of photosystem II (PS II), plants possess the ability to convert light energy from the sun into chemical energy that may be used for cellular functions such as carbon fixation (1). PS II has been used in the creation of a solar cell that oxidizes water to oxygen at the anode and reduces oxygen to water at the cathode thus forming an oxidation‐reduction cycle that does not produce a byproduct. Generation of such a solar cell requires electrically linking PS II to a charge transporting polymer matrix at the binding site of plastoquinone B (QB), an electron carrier found within PS II (2). Understanding more about the binding site for the redox mediators found in PS II may assist in the future development of solar cells utilizing PS II directly or modeled after PS II. Quinones play a central role in many biological electron transport chains due to their ability to participate in one electron or two electron oxidation reduction reactions (3).Interestingly, the quinones that occupy the primary quinone binding site (QA) and the secondary quinone binding site (QB−−) in PS II both have the same structure; yet, QA participates in one electron transfers while QB can be fully reduced forming a quinol that is capable of diffusing away from PS II. When QA is reduced, an anionic semiquinone free radical is formed (QA−•). Their difference in redox properties has been attributed to differences in the protein environment of the two binding sites, including hydrogen bonding interactions which can redistribute electron density stabilizing the anionic semiquinone state of QA (4). Pulsed electron paramagnetic resonance (EPR) has established the presence of hydrogen bonds between an amide nitrogen from the protein backbone and an amino nitrogen from an imidazole with the carbonyl oxygens on QA−• (5;6). Two‐dimensional (2D) pulsed EPR has resolved five‐cross peaks from the interaction of nearby protons with the unpaired electron in QA−•. Three of the cross peaks have been assigned to protons endogenous to the structure of QA−• itself–to methyl protons on the ring, β‐methylene protons, and the ring proton. Two cross peaks have been attributed to the hydrogen bond protons (5). However, the assignment of determined coupling constants to particular protons are not consistent with corresponding values that were obtained using DFT calculations (4). We have performed 2D pulsed EPR of PS II samples with QA−−• in 1H2O and 2H2O buffer. These experiments have allowed us to identify unambiguously the cross‐features from exchangeable and nonexchangeable protons and to determine more precisely the hyperfine coupling constants for them. These results provide the data set more suitable for a comparison with calculated hyperfine couplings.Support or Funding InformationPulsed EPR studies were supported by DE‐FG02‐08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US DOE (S.A.D.), and NCRR/NIH Grants S10‐RR15878 and S10‐RR025438 for pulsed EPR instrumentation. Continuous wave EPR studies were supported by NSF Grant CHE‐1229498.This abstract is from the Experimental Biology 2018 Meeting. There is no full text article associated with this abstract published in The FASEB Journal.

CW EPR parameters reveal cytochrome P450 ligand binding modes

Cytochrome P450 (CYP) monoxygenses utilize heme cofactors to catalyze oxidation reactions. They play a critical role in metabolism of many classes of drugs, are an attractive target for drug development, and mediate several prominent drug interactions. Many substrates and inhibitors alter the spin state of the ferric heme by displacing the heme's axial water ligand in the resting enzyme to yield a five-coordinate iron complex, or they replace the axial water to yield a nitrogen-ligated six-coordinate iron complex, which are traditionally assigned by UV–vis spectroscopy. However, crystal structures and recent pulsed electron paramagnetic resonance (EPR) studies find a few cases where molecules hydrogen bond to the axial water. The water-bridged drug-H2O-heme has UV–vis spectra similar to nitrogen-ligated, six-coordinate complexes, but are closer to “reverse type I” complexes described in older liteature. Here, pulsed and continuous wave (CW) EPR demonstrate that water-bridged complexes are remarkably common among a range of nitrogenous drugs or drug fragments that bind to CYP3A4 or CYP2C9. Principal component analysis reveals a distinct clustering of CW EPR spectral parameters for water-bridged complexes. CW EPR reveals heterogeneous mixtures of ligated states, including multiple directly-coordinated complexes and water-bridged complexes. These results suggest that water-bridged complexes are under-represented in CYP structural databases and can have energies similar to other ligation modes. The data indicates that water-bridged binding modes can be identified and distinguished from directly-coordinated binding by CW EPR.

Pulse EPR Study of Gas Adsorption in Cu2+-Doped Metal–Organic Framework [Zn2(1,4-bdc)2(dabco)

Gas separation and storage are the hot topics for addressing current challenges in energy and environmental science, including the air pollution problems and alternative fuels, and metal–organic frameworks (MOFs) have a great potential in these fields. Herewith, we present the electron paramagnetic resonance (EPR) study of the adsorption of several gases (hydrogen D2, methane 13CH4 and CD4, and carbon dioxide 13CO2) in Cu2+-doped MOF [Zn2(1,4-bdc)2(dabco)]. The obtained compound of composition [Zn1.993Cu0.007(1,4-bdc)2(dabco)] is suitable for studying adsorption geometries at Cu2+ ions and in their closest environments using pulse EPR. In attempt to characterize D2, 13CH4, CD4, and 13CO2 adsorption sites, we applied echo-detected EPR along with hyperfine sublevel correlation spectroscopy and pulse electron-nuclear double resonance spectroscopy. Altogether, these methods demonstrated the preferred location of gas molecules in the framework being at least 6 A away from the copper ions. In addition, EPR spectroscopy allowed determination of the proton environment of copper and confirmed its incorporation into the MOF lattice, which is hard to establish using other techniques.

The structural flexibility of the human copper chaperone Atox1: Insights from combined pulsed EPR studies and computations.

Metallochaperones are responsible for shuttling metal ions to target proteins. Thus, a metallochaperone's structure must be sufficiently flexible both to hold onto its ion while traversing the cytoplasm and to transfer the ion to or from a partner protein. Here, we sought to shed light on the structure of Atox1, a metallochaperone involved in the human copper regulation system. Atox1 shuttles copper ions from the main copper transporter, Ctr1, to the ATP7b transporter in the Golgi apparatus. Conventional biophysical tools such as X-ray or NMR cannot always target the various conformational states of metallochaperones, owing to a requirement for crystallography or low sensitivity and resolution. Electron paramagnetic resonance (EPR) spectroscopy has recently emerged as a powerful tool for resolving biological reactions and mechanisms in solution. When coupled with computational methods, EPR with site-directed spin labeling and nanoscale distance measurements can provide structural information on a protein or protein complex in solution. We use these methods to show that Atox1 can accommodate at least four different conformations in the apo state (unbound to copper), and two different conformations in the holo state (bound to copper). We also demonstrate that the structure of Atox1 in the holo form is more compact than in the apo form. Our data provide insight regarding the structural mechanisms through which Atox1 can fulfill its dual role of copper binding and transfer.

A study of the exchangeable protons in the QA site of photosystem II

Plants have developed the only biological system capable of oxidizing water. Through the oxidation of water plants are able to convert light energy into chemical energy. This water oxidation is catalyzed by photosystem II (PSII) at a site called the oxygen‐evolving complex (OEC). The OEC consists of a tetranuclear manganese cluster and is the only known biological system capable of oxidizing water (Blakenship 2001).Removal of the manganese cluster allows for study of the mechanism called photoactivation, a light‐and dark‐dependent process by which the OEC assembles. Further study of this process can provide great detail as to repair of PSII following inhibition (G. Dismukes 2008). Removal of the manganese cluster and associated extrinsic proteins also allows unhindered exploration of electron transfer during photochemical events. Plastoquinone A (QA) and plastoquinone B (QB) are electron carriers found in PSII. Despite sharing the same molecular structure, QA and QB exhibit unique redox properties determined by their protein environment. QA participates only in one‐electron transfers, transiently forming paramagnetic semiquinone QA−•(SQA), whereas QB function as a two‐electron gate. Hydrogen bonding from nearby amino acids residues is believed to be partly responsible for such differences in redox properties due to redistribution of electron density. High‐resolution electron paramagnetic resonance (EPR) spectroscopy has the ability to measure magnetic interactions between unpaired electrons and nearby nuclei and provide information about the strength of these interactions (Rutherford, A. W. 1999).Previous pulsed EPR studies established the formation of the H‐bonds between carbonyl oxygens of the QA−• and an amide nitrogen from the protein backbone, and the amino nitrogen, Nd, of an imidazole (Rutherford; Lakshmi). Two‐dimensional pulsed EPR of the SQA in Mn depleted, CN‐treated PSII at pH 9.8, resolved five pairs of cross‐peaks from different types of protons (Lakshmi). They were assigned to the methyl, β‐methylene, and ring protons of the SQA and two protons of the H‐bonds. Unfortunately, these assignments were not verified by the influence of 1H2O/2H2O exchange on the spectra. As a consequence, the assignment of the couplings is not consistent with the values obtained in the DFT calculations (T.‐J. Lin and P. J. O'Malley 2011). In this work we performed 2D pulsed EPR with the SQA in the PS II samples in 1H2O and 2H2O buffered solution in order to unambiguously identify contribution of the exchangeable protons and to determine corresponding hyperfine couplings. These results form a more supported basis for comparison with the results of theoretical computations.Support or Funding InformationPulsed EPR studies were supported by DE‐FG02‐08ER15960 Grant from Chemical Sciences, Geosciences and Biosciences Division, Office of Basic Energy Sciences, Office of Sciences, US DOE (S.A.D.), and NCRR/NIH Grants S10‐RR15878 and S10‐RR025438 for pulsed EPR instrumentation. Continuous wave EPR studies were supported by NSF‐MRI CHE‐1229498.

Pulsed electron paramagnetic resonance study of domain docking in neuronal nitric oxide synthase: the calmodulin and output state perspective.

The binding of calmodulin (CaM) to neuronal nitric oxide synthase (nNOS) enables formation of the output state of nNOS for nitric oxide production. Essential to NOS function is the geometry and dynamics of CaM docking to the NOS oxygenase domain, but little is known about these details. In the present work, the domain docking in a CaM-bound oxygenase/FMN (oxyFMN) construct of nNOS was investigated using the relaxation-induced dipolar modulation enhancement (RIDME) technique, which is a pulsed electron paramagnetic resonance technique sensitive to the magnetic dipole interaction between the electron spins. A cysteine was introduced at position 110 of CaM, after which a nitroxide spin label was attached at the position. The RIDME study of the magnetic dipole interaction between the spin label and the ferric heme centers in the oxygenase domain of nNOS revealed that, with increasing [Ca2+], the concentration of nNOS·CaM complexes increases and reaches a maximum at [Ca2+]/[CaM] ≥ 4. The RIDME kinetics of CaM-bound nNOS represented monotonous decays without well-defined oscillations. The analysis of these kinetics based on the structural models for the open and docked states has shown that only about 15 ± 3% of the CaM-bound nNOS is in the docked state at any given time, while the remaining 85 ± 3% of the protein is in the open conformations characterized by a wide distribution of distances between the bound CaM and the oxygenase domain. The results of this investigation are consistent with a model that the Ca2+–CaM interaction causes CaM docking with the oxygenase domain. The low population of the docked state indicates that the CaM-controlled docking between the FMN and heme domains is highly dynamic.

Rabi oscillation and electron-spin-echo envelope modulation of the photoexcited triplet spin system in silicon

We report on a pulsed electron paramagnetic resonance (EPR) study of the photoexcited triplet state ($S=1$) of oxygen-vacancy centers in silicon. Rabi oscillations between the triplet sublevels are observed using coherent manipulation with a resonant microwave pulse. The Hahn echo and stimulated echo decay profiles are superimposed with strong modulations known as electron-spin-echo envelope modulation (ESEEM). The ESEEM spectra reveal a weak but anisotropic hyperfine coupling between the triplet electron spin and a ${}^{29}$Si nuclear spin ($I=1/2$) residing at a nearby lattice site, that cannot be resolved in conventional field-swept EPR spectra.

NH3 and O2interaction with tetrahedral Ti3+ions isomorphously substituted in the framework of TiAlPO-5. A combined pulse EPR, pulse ENDOR, UV-Vis and FT-IR study

Continuous Wave (CW), pulse Electron Paramagnetic Resonance (EPR) and pulse Electron Nuclear Double Resonance (ENDOR) spectroscopies, in conjunction with UV-Vis and Infrared (IR) spectroscopies, are used to investigate the chemical reactivity of tetrahedrally coordinated Ti(3+) ions isomorphously substituted in the framework of AlPO-5 towards NH(3) and O(2). The coordination of ammonia to Ti(3+) centres is followed in detail by complementary vibrational and electron magnetic resonance techniques. In particular HYSCORE spectra allow identifying the coordination of two ammonia molecules to Ti(3+) centres resolving the full hyperfine and quadrupole (14)N coupling tensors. The reactivity of the reduced TiAlPO sample towards molecular oxygen is detailed by means of CW-EPR and pulse ENDOR spectroscopy. (17)O(2) is employed, allowing to establish the formation of a "side-on" η(2) O(2)(-)-Ti(4+) electrostatic complex. Pulse ENDOR spectra provide detailed information on the local environment of the formed superoxide radical anion which acts as a paramagnetic probe, providing evidence for Ti-O-Ti oligomeric species.

Time-Resolved and Pulse EPR Study of Triplet States of Alkylketones in β-Cyclodextrin

The triplet states of deoxybenzoin (DOB) and benzophenone (BP) molecules in randomly methylated β-cyclodextrin (CD) cavity are studied by time-resolved (TR) and pulse electron paramagnetic resonance (EPR). The observed TR EPR spectrum of DOB in β-CD at 30 K is close to the spectrum measured in polar solvent trifluoroethanol, revealing strong hydration by water molecules. At the same time, TR EPR spectrum of BP in β-CD corresponds to nonpolar surrounding of the CO-group. The electron spin relaxation times T1 and T2 of triplet BP at 30 K measured by pulse EPR are found to be different in β-CD compared to nonpolar toluene glass. The observed increase of T2 by up to a factor of four in β-CD is caused by the lower vibration amplitude of CO-bond of BP due to the confinement in β-CD. The influence of β-CD with covalently attached nitroxide on the triplet states of DOB and BP is principally different: the excited triplet states could not be observed by TR EPR due to the efficient quenching of the excited states by nitroxide.

Pulse electron spin resonance investigation of bismuth-doped silicon: Relaxation and electron spin echo envelope modulation

Donors in silicon represent good candidates for the realization of several innovative devices exploiting classical as well as quantum information processing functionalities. Although most of the current work in the field is focused on the technological donors (P, As), the more exotic Bi offers interesting properties due to its nuclear spin and electronic structure. We present a detailed pulsed electron paramagnetic resonance study of the spin-spin and spin-lattice relaxation rates along with a full angular analysis of electron spin echo envelope modulation (ESEEM) spectra in bismuth-doped silicon samples having two different donor concentrations. The measurements were performed up to $40$ K. ESEEM spectra provide information on the donor wave function via the leading superhyperfine interactions between donor nuclei and nearby $^{29}\mathrm{Si}$ coordination shells. The assignment of the most intense contributions was made on the basis of the angular variation of ESEEM resonances and elementary considerations on the deep donor states.

Fast motion in molecular solids at low temperatures: Evidence from a pulsed electron paramagnetic resonance study of nitroxyl radical relaxation

We have investigated the electron phase-memory relaxation time of the nitroxyl radical 2,2,6,6-tetramethylpiperidine-1-oxyl at temperatures between 5 and 80 K in crystalline and glassy states of ethanol using pulsed X-band electron paramagnetic resonance spectroscopy. The results indicate that the transition from the slow to fast motion regimes of the paramagnetic center occurs upon further cooling of the sample below ∼20 K. We provide experimental evidence that this phenomenon cannot be ascribed to the impact of hyperfine interactions with methyl protons in the system, but it can be instead a signature of the coupling of the electron spin with the boson peak excitations of the lattice.