Electron Nuclear Double Resonance Experiments Research Articles

Structure and Flexibility of Copper-Modified DNA G-Quadruplexes Investigated by 19 F ENDOR Experiments at 34 GHz.

DNA G-quadruplexes (GQs) are of great interest due to their involvement in crucial biological processes such as telomerase maintenance and gene expression. Furthermore, they are reported as catalytically active DNAzymes and building blocks in bio-nanotechnology. GQs exhibit remarkable structural diversity and conformational heterogeneity, necessitating precise and reliable tools to unravel their structure-function relationships. Here, we present insights into the structure and conformational flexibility of a unimolecular GQ with high spatial resolution via electron-nuclear double resonance (ENDOR) experiments combined with Cu(II) and fluorine labeling. These findings showcase the successful application of the 19 F-ENDOR methodology at 34 GHz, overcoming the limitations posed by the complexity and scarcity of higher-frequency spectrometers. Importantly, our approach retains both sensitivity and orientational resolution. This integrated study not only enhances our understanding of GQs but also expands the methodological toolbox for studying other macromolecules.

19F electron nuclear double resonance (ENDOR) spectroscopy for distance measurements using trityl spin labels in DNA duplexes.

The combination of fluorine labeling and pulsed electron-nuclear double resonance (ENDOR) is emerging as a powerful technique for obtaining structural information about proteins and nucleic acids. In this work, we explored the capability of Mims 19F ENDOR experiments on reporting intermolecular distances in trityl- and 19F-labeled DNA duplexes at three electron paramagnetic resonance (EPR) frequencies (34, 94, and 263 GHz). For spin labeling, we used the hydrophobic Finland trityl radical and hydrophilic OX063 trityl radical. Fluorine labels were introduced into two positions of a DNA oligonucleotide. The results indicated that hyperfine splittings visible in the ENDOR spectra are consistent with the most populated interspin distances between 19F and the trityl radical predicted from molecular dynamic (MD) simulations. Moreover, for some cases, ENDOR spectral simulations based on MD results were able to reproduce the conformational distribution reflected in the experimental ENDOR line broadening. Additionally, MD simulations provided more detailed information about the melting of terminal base pairs of the oligonucleotides and about the configuration of the trityls relative to a DNA end.

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 aʹ (PA) in PSI. The results also provide additional evidence for the localization of 3P680 on the accessory ChlD1 in PSII.

Cu2+ and Cu3+ acceptors in β-Ga2O3 crystals: A magnetic resonance and optical absorption study

Electron paramagnetic resonance (EPR) and optical absorption are used to characterize Cu2+ (3d9) and Cu3+ (3d8) ions in Cu-doped β-Ga2O3. These Cu ions are singly ionized acceptors and neutral acceptors, respectively (in semiconductor notation, they are Cu− and Cu0 acceptors). Two distinct Cu2+ EPR spectra are observed in the as-grown crystals. We refer to them as Cu2+(A) and Cu2+(B). Spin-Hamiltonian parameters (a g matrix and a 63,65Cu hyperfine matrix) are obtained from the angular dependence of each spectrum. Additional electron-nuclear double resonance (ENDOR) experiments on Cu2+(A) ions give refined 63Cu and 65Cu hyperfine matrices and provide information about the nuclear electric quadrupole interactions. Our EPR results show that the Cu2+(A) ions occupy octahedral Ga sites with no nearby defect. The Cu2+(B) ions, also at octahedral Ga sites, have an adjacent defect, possibly an OH− ion, an oxygen vacancy, or an H− ion trapped within an oxygen vacancy. Exposing the crystals at room temperature to 275 nm light produces Cu3+ ions and reduces the number of Cu2+(A) and Cu2+(B) ions. The Cu3+ ions have an S = 1 EPR spectrum and are responsible for broad optical absorption bands peaking near 365, 422, 486, 599, and 696 nm. An analysis of loops observed in the Cu3+ EPR angular dependence gives 2.086 for the g value and 22.18, 3.31, and −25.49 GHz for the principal values of D (the fine-structure matrix). Thermal anneal studies above room temperature show that the Cu3+ ions decay and the Cu2+ ions recover between 75 and 375 °C.

Electron Paramagnetic Resonance and DFT Analysis of the Effects of Bulky Perfluoroalkyl Substituents on a Vanadyl Perfluoro Phthalocyanine

Abstract F64PcVO, the vanadyl complex of the perfluoro-isopropyl-substituted perfluorophthalocyanine ligand F64PcH2, exhibits interesting electronic properties compared to related vanadyl phthalocyanine complexes. X- and W-band continuous-wave electron paramagnetic resonance (CW EPR) of undiluted F64PcVO powders illustrate the absence of π-π stacking in these materials due to the bulky substituents. Furthermore, pulsed EPR and electron nuclear double resonance (ENDOR) experiments in combination with density functional theory (DFT) computations were used to determine the principal g values and the 51V, 14N, 1H and 19F hyperfine couplings of a frozen ethanol solution of F64PcVO. The axial ligation of ethanol to vanadium was proven. Axial accessibility of the vanadyl center combined with the high solubility of the complex in different solvents make F64PcVO an interesting molecule for catalytic applications.

Role of surface water molecules in stabilizing trapped hole centres in titanium dioxide (anatase) as monitored by electron paramagnetic resonance

Abstract A key factor affecting the photo-efficiency of TiO2 is strictly related to the fate of charge carriers, electrons (e−) and holes (h+), generated upon band gap excitation. In the present paper we point our attention to the nature of the hole trapping sites in the anatase polymorph monitored coupling the conventional continuous wave EPR (CW-EPR) technique with pulse electron nuclear double resonance (ENDOR) experiments. The attention is focused on the role of surface adsorbed water (both in molecular and in dissociated form) in the stabilization of photogenerated hole centres. CW-EPR results indicate that two distinct O − hole centres can be identified in Anatase (O −surf. and O −subsurf.) and that the quantitative ratio (measured in terms of spectral intensity) of these two species is markedly conditioned by the presence of surface physisorbed water. For the first time a h+-proton distance, evaluated via ENDOR measurement, is reported.

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

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

High field electron paramagnetic resonance characterization of electronic and structural environments for paramagnetic metal ions and organic free radicals in Deepwater Horizon oil spill tar balls.

In the first use of high-field electron paramagnetic resonance (EPR) spectroscopy to characterize paramagnetic metal-organic and free radical species from tar balls and weathered crude oil samples from the Gulf of Mexico (collected after the Deepwater Horizon oil spill) and an asphalt volcano sample collected off the coast of Santa Barbara, CA, we are able to identify for the first time the various paramagnetic species present in the native state of these samples and understand their molecular structures and bonding. The two tar ball and one asphalt volcano samples contain three distinct paramagnetic species: (i) an organic free radical, (ii) a [VO](2+) containing porphyrin, and (iii) a Mn(2+) containing complex. The organic free radical was found to have a disc-shaped or flat structure, based on its axially symmetric spectrum. The characteristic spectral features of the vanadyl species closely resemble those of pure vanadyl porphyrin; hence, its nuclear framework around the vanadyl ion must be similar to that of vanadyl octaethyl porphyrin (VOOEP). The Mn(2+) ion, essentially undetected by low-field EPR, yields a high-field EPR spectrum with well-resolved hyperfine features devoid of zero-field splitting, characteristic of tetrahedral or octahedral Mn-O bonding. Although the lower-field EPR signals from the organic free radicals in fossil fuel samples have been investigated over the last 5 decades, the observed signal was featureless. In contrast, high-field EPR (up to 240 GHz) reveals that the species is a disc-shaped hydrocarbon molecule in which the unpaired electron is extensively delocalized. We envisage that the measured g-value components will serve as a sensitive basis for electronic structure calculations. High-field electron nuclear double resonance experiments should provide an accurate picture of the spin density distribution for both the vanadyl-porphyrin and Mn(2+) complexes, as well as the organic free radical, and will be the focus of follow-up studies.

Electron Paramagnetic Resonance and Electron Nuclear Double Resonance Study of the Paramagnetic Complexes of Anthraquinone on the Surface of γ-Al2O3

Progress in the synthesis and applications of nanomaterials including nanocatalysts demands a use of precise analytical tools for their surface characterization. Continuous wave (cw) and pulsed electron paramagnetic resonance (EPR) techniques, including electron–nuclear double resonance (ENDOR) have been applied to study paramagnetic complexes formed by adsorption of 9,10-anthraquinone (AQ) as probe molecule by the surface of γ-Al2O3. Up to three different paramagnetic complexes (11-line pattern and two single EPR lines) could be separated in our experiments. Their spectroscopic characteristics are extracted. It is shown that at very high concentration (ca. 10 wt %) of AQ, the obtained EPR signal is close to the single line and can be incorrectly interpreted as due to the EPR signal of AQ itself or due to the lower catalytic activity of the investigated surface. That fact should be taken into account by using AQ as a probe of the surface catalytic activity. Mims and Davies ENDOR experiments confirm the re...

55Mn pulsed ENDOR spectroscopy of Mn2+ ions in ZnO thin films and single crystal

55Mn pulsed electron nuclear double resonance (ENDOR) experiments were performed at X-band on high spin S=5/2 Mn2+ ions incorporated at zinc lattice sites in heteroepitaxial ZnO thin films. The films have been prepared by pulsed laser deposition and the manganese ions were doped during the growth process. We examine how the c/a lattice axes ratio of the ZnO films influences the 55Mn hyperfine (hf) and nuclear quadrupole (nq) coupling parameters of the Mn2+ probe ions. The results are compared with those obtained for Mn2+ ions present as impurities in ZnO single crystals and revealed that the 55Mn nq coupling monitors sensitively the structural distortions in the bonding environment of the Mn2+ ions. The experiments provided the full axially symmetric 55Mn hf and nq interaction tensors. The latter is found to be very sensitive to small axial distortions of the MnO4 tetrahedrons. In particular, the 55Mn pulsed ENDOR spectra of the ZnO:Mn thin films are strongly subjected to strain effects in the nq coupling parameter indicating a variation of the local structural parameters for the heteroepitaxial films. In the analysis of the 55Mn pulsed ENDOR spectra the axial and cubic zero field splitting of the Mn2+ ions was taken into account and intensity effects in the ENDOR spectra due to the zero field splitting effects were discussed.

Enhanced sensitivity of electron-nuclear double resonance (ENDOR) by cross polarisation and relaxation

Electron-nuclear double resonance (ENDOR) is a method of choice to detect magnetic nuclei in the coordination sphere of paramagnetic molecules, but its sensitivity substantially suffers from saturation effects. Recently we introduced a new pulsed ENDOR experiment based on electron-nuclear cross polarisation (CP) transfer. Here we analyse the time evolution of the spin polarization in CP-ENDOR and show that CP combined with inherent fast relaxation leads to enhanced sensitivity as compared to Davies ENDOR.

Ground state of the singly ionized oxygen vacancy in rutile TiO2

Results from electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) experiments are used to establish the model for the ground state of the singly ionized oxygen vacancy in the interior of bulk rutile TiO2 crystals. Hyperfine from 47Ti and 49Ti nuclei show that the unpaired electron in this S = 1/2 defect is localized on one titanium ion adjacent to the oxygen vacancy (i.e., the spin is not shared by two titanium ions). These defects are formed at low temperature (∼35 K) in as-grown oxidized crystals when sub-band-gap 442 nm laser light converts doubly ionized nonparamagnetic oxygen vacancies to the singly ionized paramagnetic charge state. The g matrix is obtained from EPR spectra and the 47Ti and 49Ti hyperfine and nuclear electric quadrupole matrices (A and Q) are obtained from ENDOR spectra. Principal values of the 47Ti and 49Ti hyperfine matrices are 64.54, 11.57, and 33.34 MHz. All the matrices have a principal axis along the [001] direction. In the basal plane, principal axes of the hyperfine and quadrupole matrices also coincide. The principal axes of the g matrix in the basal plane, however, deviate significantly from those of the A and Q matrices, thus indicating mixing of d orbitals due to the low symmetry at the Ti3+ ion site and participation of excited-state orbitals.

Anomalous Phases in Cavity-Free 240 GHz Pulsed ENDOR Spectra of 1.44 GeV Xe-Irradiated LiF

Paramagnetic centers generated by swift heavy ion irradiation of LiF crystals could be identified as electrons trapped at regular anion vacancy sites (F centers). Well-resolved electron-nuclear double resonance (ENDOR) spectra resulting from the hyperfine interaction with 7Li and 19F nuclei located in six different shells could be recorded. In order to preserve the millimeter-sized crystals, a cavity-free setup was used for the ENDOR experiments at an electronic Larmor frequency of 240 GHz. Apparently even under conditions of extremely high local energy loss in the ion track, the local density of persistent F centers is still sufficiently low to prevent distortions of the ionic crystal. The spread of hyperfine coupling constants was less than 5 %. Neither in electron paramagnetic resonance (EPR) nor in ENDOR spectra there was evidence for different types of paramagnetic centers. When performing ENDOR by applying the radiofrequency pulse directly after the 3-pulse Mims-type microwave sequence, an anomalous ENDOR effect was observed. The observed “positive” and “negative” ENDOR response can be attributed to efficient hole and anti-hole formation in the inhomogeneously broadened EPR spectrum and can be used to determine the sign of hyperfine coupling constants.

Measuring central-spin interaction with a spin bath by pulsed ENDOR: Towards suppression of spin diffusion decoherence

We present pulsed electron-nuclear double resonance (ENDOR) experiments which enable us to characterize the coupling between bismuth donor spin-qubits in Si and the surrounding spin-bath of (29)Si impurities which provides the dominant decoherence mechanism (nuclear spin diffusion) at low temperatures (< 16 K). Decoupling from the spin-bath is predicted and cluster correlation expansion simulations show near-complete suppression of spin diffusion, at optimal working points. The suppression takes the form of sharply peaked divergences of the spin diffusion coherence time, in contrast with previously identified broader regions of insensitivity to classical fluctuations. ENDOR data shows anisotropic contributions are comparatively weak, so the form of the divergences is independent of crystal orientation.

A tunable general purpose Q-band resonator for CW and pulse EPR/ENDOR experiments with large sample access and optical excitation

We describe a frequency tunable Q-band cavity (34 GHz) designed for CW and pulse Electron Paramagnetic Resonance (EPR) as well as Electron Nuclear Double Resonance (ENDOR) and Electron Electron Double Resonance (ELDOR) experiments. The TE 011 cylindrical resonator is machined either from brass or from graphite (which is subsequently gold plated), to improve the penetration of the 100 kHz field modulation signal. The (self-supporting) ENDOR coil consists of four 0.8 mm silver posts at 2.67 mm distance from the cavity center axis, penetrating through the plunger heads. It is very robust and immune to mechanical vibrations. The coil is electrically shielded to enable CW ENDOR experiments with high RF power (500 W). The top plunger of the cavity is movable and allows a frequency tuning of ±2 GHz. In our setup the standard operation frequency is 34.0 GHz. The microwaves are coupled into the resonator through an iris in the cylinder wall and matching is accomplished by a sliding short in the coupling waveguide. Optical excitation of the sample is enabled through slits in the cavity wall (transmission ∼60%). The resonator accepts 3 mm o.d. sample tubes. This leads to a favorable sensitivity especially for pulse EPR experiments of low concentration biological samples. The probehead dimensions are compatible with that of Bruker flexline Q-band resonators and it fits perfectly into an Oxford CF935 Helium flow cryostat (4–300 K). It is demonstrated that, due to the relatively large active sample volume (20–30 μl), the described resonator has superior concentration sensitivity as compared to commercial pulse Q-band resonators. The quality factor ( Q L ) of the resonator can be varied between 2600 (critical coupling) and 1300 (over-coupling). The shortest achieved π/2-pulse durations are 20 ns using a 3 W microwave amplifier. ENDOR (RF) π-pulses of 20 μs ( 1H @ 51 MHz) were obtained for a 300 W amplifier and 7 μs using a 2500 W amplifier. Selected applications of the resonator are presented.

Photo-Induced Electron Spin Polarization in Chemical and Biological Reactions: Probing Structure and Dynamics of Transient Intermediates by Multifrequency EPR Spectroscopy

In this minireview, modern multifrequency electron paramagnetic resonance (EPR) spectroscopy, in particular, at high magnetic fields, is shown to provide detailed information about structure, motional dynamics and spin chemistry of transient radicals and radical pairs occurring in photochemical reactions. Examples discussed comprise spin-polarized radicals and radical pairs in disordered systems, such as ultraviolet-irradiated quinone and ketone compounds in fluid alcohol solutions, green-light initiated electron transfer in biomimetic porphyrin–quinone donor–acceptor model systems in frozen solution, aiming at artificial photosynthesis, and red-light initiated electron transfer in natural photosynthetic reaction-center protein complexes. The transient paramagnetic states exhibit characteristic electron polarization (CIDEP) effects originating from a triplet mechanism, a radical-pair mechanism or a correlated coupled radical-pair mechanism. They contain valuable information about structure and dynamics of the short-lived reaction intermediates. Moreover, the CIDEP effects can be exploited for signal enhancement. Continuous-wave and pulsed versions of time-resolved high-field EPR spectroscopy, such as transient EPR and electron spin-echo experiments, are compared with respect to their advantages and limitations for the specific photoreaction under study. Furthermore, orientation resolving W-band pulsed electron-electron double resonance (PELDOR) experiments on the spin-correlated coupled radical pair $$ {\text{P}}_{865}^{ \cdot + } $$ $$ {\text{Q}}_{\text{A}}^{ \cdot - } $$ in frozen solution reaction centers from the purple photosynthetic bacterium Rb. sphaeroides reveal details of distance and orientation of the pair partners in their charge-separated transient state. The results are compared with those of the ground-state P865QA. In conjunction with Q-band proton electron-nuclear double resonance (ENDOR) experiments the W-band PELDOR results provide decisive evidence that the local structure of the QA binding site does not change under photoreduction of the quinone—in agreement with earlier FTIR studies. The examples given demonstrate that multifrequency EPR experiments on disordered systems add heavily to the capabilities of “classical” spectroscopic and diffraction techniques for determining structure–dynamics–function relations of biochemical processes, since short-lived intermediates can be observed in real time while staying in their working states at biologically relevant time scales.

Structural Characterization of a High Affinity Mononuclear Site in the Copper(II)-α-Synuclein Complex

Human α-Synuclein (aS), a 140 amino acid protein, is the main constituent of Lewy bodies, the cytoplasmatic deposits found in the brains of Parkinson's disease patients, where it is present in an aggregated, fibrillar form. Recent studies have shown that aS is a metal binding protein. Moreover, heavy metal ions, in particular divalent copper, accelerate the aggregation process of the protein. In this work, we investigated the high affinity binding mode of truncated aS (1-99) (aS99) with Cu(II), in a stoichiometric ratio, to elucidate the residues involved in the binding site and the role of copper ions in the protein oligomerization. We used Electron Paramagnetic Resonance spectroscopy on the Cu(II)-aS99 complex at pH 6.5, performing both multifrequency continuous wave experiments and pulsed experiments at X-band. The comparison of 9.5 and 95 GHz data showed that at this pH only one binding mode is present. To identify the nature of the ligands, we performed Electron Spin Echo Envelope Modulation, Hyperfine Sublevel Correlation Spectroscopy, and pulsed Davies Electron-Nuclear Double Resonance (Davies-ENDOR) experiments. We determined that the EPR parameters are typical of a type-II copper complex, in a slightly distorted square planar geometry. Combining the results from the different pulsed techniques, we obtained that the equatorial coordination is {N(Im), N(-), H(2)O, O}, where N(im) is the imino nitrogen of His50, N(-) a deprotonated amido backbone nitrogen that we attribute to His50, H(2)O an exchangeable water molecule, and O an unidentified oxygen ligand. Moreover, we propose that the free amino terminus (Met1) participates in the complex as an axial ligand. The MXAN analysis of the XAS k-edge absorption data allowed us to independently validate the structural features proposed on the basis of the magnetic parameters of the Cu(II)-aS99 complex and then to further refine the quality of the proposed structural model.

High-Frequency EPR and ENDOR Spectroscopy on Semiconductor Quantum Dots

It is shown that high-frequency electron paramagnetic resonance (EPR) and electron-nuclear double resonance (ENDOR) spectroscopy are excellent tools for the investigation of the electronic properties of semiconductor quantum dots (QDs). The great attractions of these techniques are that, in contrast to optical methods, they allow the identification of the dopants and provide information about the spatial distribution of the electronic wave function. This latter aspect is particularly attractive because it allows for a quantitative measurement of the effect of confinement on the shape and properties of the wave function. In this contribution EPR and ENDOR results are presented on doped ZnO QDs. Shallow donors (SDs), related to interstitial Li and Na and substitutional Al atoms, have been identified in this material by pulsed high-frequency EPR and ENDOR spectroscopy. The shallow character of the wave function of the donors is evidenced by the multitude of ENDOR transitions of the 67Zn nuclear spins and by the hyperfine interaction of the 7Li, 23Na and 27Al nuclear spins that are much smaller than for atomic lithium, sodium and aluminium. The EPR signal of an exchange-coupled pair consisting of a shallow donor and a deep Na-related acceptor has been identified in ZnO nanocrystals with radii smaller than 1.5 nm. From ENDOR experiments it is concluded that the deep Na-related acceptor is located at the interface of the ZnO core and the Zn(OH)2 capping layer, while the shallow donor is in the ZnO core. The spatial distribution of the electronic wave function of a shallow donor in ZnO semiconductor QDs has been determined in the regime of quantum confinement by using the nuclear spins as probes. Hyperfine interactions as monitored by ENDOR spectroscopy quantitatively reveal the transition from semiconductor to molecular properties upon reduction of the size of the nanoparticles. In addition, the effect of confinement on the g-factor of SDs in ZnO as well as in CdS QDs is observed. Finally, it is shown that an almost complete dynamic nuclear polarization (DNP) of the 67Zn nuclear spins in the core of ZnO QDs and of the 1H nuclear spins in the Zn(OH)2 capping layer can be obtained. This DNP is achieved by saturating the EPR transition of SDs present in the QDs with resonant high-frequency microwaves at low temperatures. This nuclear polarization manifests itself as a hole and an antihole in the EPR absorption line of the SD in the QDs and a shift of the hole (antihole). The enhancement of the nuclear polarization opens the possibility to study semiconductor nanostructures with nuclear magnetic resonance techniques.

Pulsed EPR and ENDOR on the Peridinin Triplet State Involved in the Photoprotective Mechanism in Peridinin–Chlorophyll a–Proteins

The photoexcited triplet state of the carotenoid peridinin in the peridinin–chlorophyll a–protein (PCP) of the dinoflagellate Heterocapsa pygmaea has been investigated by pulsed electron paramagnetic resonance (EPR) and pulsed electron-nuclear double resonance (ENDOR) spectroscopies. The α- and β-protons hyperfine couplings of the peridinin-conjugated chain have been derived from Davies and Mims ENDOR experiments. The spectroscopic results have been compared to those obtained for the main form of the PCP complex and for the high-salt PCP form from Amphidinium carterae. The EPR features of the peridinin triplet state are very similar in the antenna complexes belonging to the two different dinoflagellate species, proving that the triplet formation pathway and the triplet localization on one specific peridinin per subcluster are common features of different PCP antennas. No significant variation of the hyperfine couplings of the peridinin triplet state has been detected between the main form of the PCP complex from A. carterae and H. pygmaea. The spectroscopic results confirm the close relationship between the Amphidinium PCP and the corresponding Heterocapsa complex at least in terms of mutual arrangement of the chlorophyll a–peridinin pair involved in photoprotection and in terms of conformation of the peridinin-conjugated chain.

Tyrosyl Radical in the W164Y Mutant of P. eryngii Versatile Peroxidase: an EPR and DFT/PCM Study

Versatile peroxidases (VP) constitute a new class of high redox potential fungal enzymes that are able to degrade lignin and large substrate molecules. These enzymes catalyze the oxidation of substrates at an exposed tryptophan radical formed by a long-range electron transfer mechanism to heme following the activation by H2O2. In a previous paper, it was demonstrated using electron paramagnetic resonance (EPR) and electron-nuclear double resonance experiments on wild-type VP that Trp164 was the radical site and that it was in a hydrogen-bonded neutral form. In this paper, the W164Y variant is analyzed and it is shown that also the variant is able to form the so-called Compound I (VPI) in the form of protein radical, although in different yields with respect to the wild type. The X-band EPR experiments in combination with density functional theory/polarizable continuum model calculations show that the W164Y mutant is able to form a neutral radical on Tyr164 residue, after activation by H2O2, but in contrast to Trp164, tyrosine is not expected to be hydrogen bonded.