Electron Spin Echo Research Articles

Epr Spectroscopy of Adsorbed Spin-Labeled Peptides: Immobilization of Trichogin on the Titanium Oxide

The TOAC-spin-labeled peptide Trichogin GA IV adsorbed on the TiO2 surface is studied. It is shown that the continuous wave (CW) electron paramagnetic resonance (EPR) spectrum does not depend on temperature in a wide range of 77–300 K. A pulsed EPR method of electron spin echo (ESE) utilizing a two-pulse sequence (π/2-τ-π) is used to study temperature dependence of the phase relaxation time, TF. The TF values are found to change from 750 ns to 100 ns in the interval of 77–300 K. The pulsed electronelectron double resonance (PELDOR) measurements utilizing the pulse sequence((π/2)A,-T-πB,-(τ-T)-πA) show that the space distribution of spin labels on the surface remains uniform irrespective of the temperature, and provide the fractal dimension of the surface of 2.7±0.1. The obtained results testify that EPR pulse experiments can be used to study adsorbed spin-labeled molecules at room temperatures, i.e. not only at cryogenic temperatures.

Low-Temperature Dynamical Transition in Lipid Bilayers Detected by Spin-Label ESE Spectroscopy

Data on neutron scattering in biological systems show low-temperature dynamical transition between 170 and 230 K manifesting itself as a drastic increase of the atomic mean-squared displacement, 〈x2〉, detected for hydrogen atoms in the nano- to picosecond time scale. For spin-labeled systems, electron spin echo (ESE) spectroscopy—a pulsed version of electron paramagnetic resonance—is also capable of detection of dynamical transition. A two-pulse ESE decay in frozen matrixes is induced by spin relaxation arising from stochastic molecular librations, and allows to obtain the 〈α2〉τc parameter, where 〈α2〉 is a mean-squared angular amplitude of the motion and τc is the correlation time lying in the sub- and nanosecond time ranges. In this work, the ESE technique was applied to spin-labeled amphiphilic molecules of three different kinds embedded in bilayers of fully saturated 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and mono-unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) lipids. Two-pulse ESE data revealed the appearance of stochastic librations above 130 K, with the parameter 〈α2〉τc obeying the Arrhenius type of temperature dependence and increasing remarkably above 170–180 K. A comparison with a dry sample suggests that onset of motions is not related with lipid internal motions. Three-pulse ESE experiments (resulting in stimulated echos) in DPPC bilayers showed the appearance of slow molecular rotations above 170–180 K. For D2O-hydrated bilayers, ESE envelope modulation experiments indicate that isotropic water molecular motions in the nearest hydration shell of the bilayer appear with a rate of ~ 105 s−1 in the narrow temperature range between 175 and 179 K. The similarity of the experimental data found for three different spin-labeled compounds suggests a cooperative character for the ESE-detected molecular motions. The data were interpreted within a model suggesting that dynamical transition is related with overcoming barriers, of 10–20 kJ/mol height, existing in the system for the molecular reorientations.

Clustering of spin-labeled cholesterol analog diluted in bilayers of saturated and unsaturated phospholipids

Clustering of spin-labeled cholesterol analog, 3β-doxyl-5α-cholestane (DChl), diluted in bilayers comprised of either saturated dipalmitoyl-glycero-phosphocholine (DPPC) or unsaturated dioleoyl-glycero-phosphocholine (DOPC) phospholipids was studied. DChl molar fraction X varied between 0.005 and 0.04. EPR spectroscopy applied at low temperatures (200 K) enabled exploring magnetic dipole-dipole (d-d) interaction between spin labels. For DOPC bilayers, EPR spectra were found to broaden remarkably with X increase. The broadening was simulated for the models of 2-dimentional (2-D) clusters with enhanced local concentration, Xloc, which was several times larger than X, and for 1-dimensional (1-D) DChl clusters. The distance of closest approach in these simulations attained the intermolecular lateral distance in the membrane (~0.7 nm). For DPPC bilayers, EPR spectra showed only small broadening, which in these simulations could not be reproduced even if Xloc was taken as small as X. However strong concentration dependence was found for electron spin echo (ESE) decays. Both the EPR and ESE data for DPPC bilayers were explained within the model assuming encapsulation of DChl molecules in lipid shells so preventing them to approach each other closer than a certain distance, Rmin. The Rmin value was found to vary between ~2.5 nm and 5 nm, for X varying between 0.04 and 0.005; Xloc in these simulations was several times larger than X. So the DChl clustering in DOPC bilayers is driven by attractive lipid-mediated forces, while in DPPC bilayers long-range nanoscale lipid-mediated repulsive/attractive forces take place for distances smaller and larger Rmin, correspondingly.

Feedback control optimisation of ESR experiments

Numerically optimised microwave pulses are used to increase excitation efficiency and modulation depth in electron spin resonance experiments performed on a spectrometer equipped with an arbitrary waveform generator. The optimisation procedure is sample-specific and reminiscent of the magnet shimming process used in the early days of nuclear magnetic resonance – an objective function (for example, echo integral in a spin echo experiment) is defined and optimised numerically as a function of the pulse waveform vector using noise-resilient gradient-free methods. We found that the resulting shaped microwave pulses achieve higher excitation bandwidth and better echo modulation depth than the pulse shapes used as the initial guess. Although the method is theoretically less sophisticated than simulation based quantum optimal control techniques, it has the advantage of being free of the linear response approximation; rapid electron spin relaxation also means that the optimisation takes only a few seconds. This makes the procedure fast, convenient, and easy to use. An important application of this method is at the final stage of the implementation of theoretically designed pulse shapes: compensation of pulse distortions introduced by the instrument. The performance is illustrated using spin echo and out-of-phase electron spin echo envelope modulation experiments. Interface code between Bruker SpinJet arbitrary waveform generator and Matlab is included in versions 2.2 and later of the Spinach library.

Measurement of T1e, T1N, T1HE, T2e, and T2HE by Pulse EPR at X-Band for Nitroxides at Concentrations Relevant to Solution DNP

Relaxation times were measured at X-band (9.5 GHz) at concentrations up to 20.8 mM for two nitroxides that are widely used in Overhauser dynamic nuclear polarization (ODNP) experiments. Carboxy proxyl (CP, 3-carboxy-2,2,5,5-tetramethyl-1-pyrrolidinyloxy) in water and tempol (TP, 4-hydroxy-2,2,6,6-tetramethylpiperidin-1-oxyl) in toluene have been studied with oxygen removed. For comparison, solutions of CP were also studied in air-saturated solution, which is the typical preparation for ODNP experiments. T2 and T2HE (HE = Heisenberg exchange) were measured by electron spin echo. The spin magnetization recovery time constant from inversion recovery experiments, which we denote as T1e*, includes contributions from T1e and T1HE. In the absence of oxygen, values of T1e* for both radicals decrease with increasing concentration up to ~ 1 mM, then increase again towards 10 mM. The concentration dependence results from changes in the relative contributions from T1e and T1HE. In air-saturated solutions of CP T1e* decreases with concentration to about 200 ns at 1 mM, and then remains independent of further concentration increases. T1e and T1N were also measured with a digital saturation recovery spectrometer. Using the combined results from spin echo, inversion recovery, and saturation recovery we could extract the values of T1e, T1HE, T1N, T2e, and T2HE for both radicals in this fast tumbling regime.

Cu(II) EPR Reveals Two Distinct Binding Sites and Oligomerization of Innate Immune Protein Calgranulin C

S100A12 or Calgranulin C is a homodimeric antimicrobial protein of the S100 family of EF-hand calcium-modulated proteins. S100A12 is involved in many diseases such as inflammation, tumor invasion, cancer and neurological disorders such as Alzheimer’s disease. The binding of transition metal ions to the protein is important as the sequestering of the metal ion induces conformational changes in the protein, inhibiting the growth of various pathogenic microorganisms. In this work, we probe the Cu2+ binding properties of Calgranulin C. We demonstrate that the two Cu2+ binding sites in Calgranulin C show different coordination environments in solution. Continuous wave-electron spin resonance (CW-ESR) spectra of Cu2+-bound protein clearly show two distinct components at higher Cu2+:protein ratios, which is indicative of the two different binding environments for the Cu2+ ions. The g|| and A|| values are also different for the two components, indicating that the number of directly coordinated nitrogen in each site differs. Furthermore, we perform CW-ESR titrations to obtain the binding affinity of the Ca2+-loaded protein to Cu2+ ions. We observe a positive cooperativity in binding of the two Cu2+ ions. To further probe the Cu2+ coordination, we also perform electron spin echo envelope modulation (ESEEM) experiment. We perform ESEEM at two different fields where one Cu2+ binding site dominates the other. At both sites we see distinct signatures of Cu2+–histidine coordination. However, we clearly see that the ESEEM spectra corresponding to the two Cu2+ binding sites are significantly different. There is clear change in the intensity of the double quantum peak with respect to the nuclear quadrupole interaction peak at the two different fields. Furthermore, ESEEM along with hyperfine sublevel correlation show that only one of the two Cu2+ binding sites has backbone coordination, confirming our previous observation. Finally, we perform double electron–electron resonance spectroscopy to probe if the difference in binding environment is due to the Cu2+ binding to different sites in the protein. We obtain a distance distribution with a sharp peak at ~ 3 nm and a broad peak at ~ 4 nm. The shorter distance agrees with the Cu2+–Cu2+ distance expected for a dimer from the crystal structure. The longer distance is consistent with the Cu2+–Cu2+ distance when oligomerization occurs.

Relationship investigation of structure and properties of Nd3+: Ga2O3-Al2O3-PbO-CaO via Raman, infrared, NMR and EPR spectroscopy

A detailed study structure and luminescence properties of Nd3+: Ga2O3-Al2O3-PbO-CaO laser glasses was conducted. The network structure was mainly dominated by Al (IV) or Ga (IV). With the increase of the Al2O3 content, the Al (IV) content increases, whereas the Ga (IV) content decreases. Hyperfine sublevel correlation (HYSCORE) spectra reveal 27Al and 207Pb weak hyperfine coupling, which indicates that these nuclei are in the second- or higher-order Nd3+ coordination sphere. No 69Ga or 71Ga signals can be detected in the three-pulse electron spin echo envelope modulation (3P-ESEEM) and HYSCORE spectra. For germanate glass, the local environment of the Nd3+ ions is characterized by the Pb ligand, for aluminum gallium glasses, the Nd3+ ions are surrounded by Al3+ and Pb2+ ions, and for aluminum glass, Nd3+ ions are mainly surrounded by 27Al cations. The change in the structure and in the local environment of Nd3+ ions results in a change in the luminescence properties of glasses.

Rescaling of 2D ESEEM Data as a Tool for Inverse Problem Solving

A rescaling procedure is proposed for electron spin echo envelope modulation spectra observed at several electron paramagnetic resonance transitions. Analytical expressions describing the relations between the rescaled frequencies and hyperfine and quadrupole parameters of the remote nucleus are obtained. The dependences of the rescaled data on the external magnetic field and spin projections of the ion nucleus and the remote nuclei are used to derive the parameters of the nuclear state in the crystals Y2SiO5 and YVO4 doped by ion Nd3+.

Abstract 668: Electron paramagnetic resonance (EPR) pO2 image-guided tumor biopsies to analyze hypoxia-induced proteins

Abstract Stereotactic MCa4 and FSa tumor tissue biopsies registered with electron paramagnetic resonance (EPR) O2 imaging were used to identify sampling sites suitable to measure protein levels of three hypoxic cell biomarkers, hypoxia-induced factor 1-alpha (HIF-1α), carbonic anhydrase IX (CA9), and vascular endothelial growth factor (VEGF). EPR imaging provided quantitative localization of pO2 levels in tumors grown in the legs of C3H mice. Since hypoxic cells (pO2 <10 torr) are up to three times more resistant than normoxic cells to radiation lethality, identification of hypoxic populations to predict radiotherapeutic outcome is important. The oxygen broadening of narrow EPR spectral lines, or, equivalently, the increase in relaxation rates of electron magnetization, displays pO2 with 1-2 torr resolution in image voxels less than 1 mm3. The pO2 reporter molecule OX063 was used to image hypoxic areas in tumors. The molecule is selectively retained in tumors with a half-life of ~30 minutes. We used inversion recovery electron spin echo (IRESE) to measure the T1 rate of the trityl inside the tumor-bearing leg of mice. Voxel maps with O2 levels less than 10 torr (HF10) were used to guide biopsy cannulas to obtain tumor cell samples. Each of these biopsy cores were further subdivided into samples from which protein amounts could be determined via ELISA. Results from MCa4 and FSa tumor cells indicate a strong correlation between EPR pO2-identified hypoxic areas (<10 torr) and HIF-1α, CA9, and VEGF protein. The activation of various genes, including stem-cell activation, angiogenesis, vasodilation, glucose metabolism, reduced apoptotic activity, and cell cycle changes, contributes to poor radiotherapeutic outcome. EPR pO2 imaging may be used to precisely identify hypoxic areas harboring cells that protect tumor tissue from the toxicity of conventional radiation leading to radiotherapeutic failure. Grant Support: This work was supported by grants from the NIH, including R01 CA98575 and P41 EB002034. Citation Format: Richard Miller, Boris Epel, Martyna Elas, Martyna Krzykawska-Serda, Matthew Maggio, Eugene Barth, Mihai Giurcanu, Howard Halpern. Electron paramagnetic resonance (EPR) pO2 image-guided tumor biopsies to analyze hypoxia-induced proteins [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 668.

Rare‐Earth Ion Local Environments in Nd:Al2O3‐P2O5‐K2O Glass Studied by Electron Paramagnetic Resonance Spectroscopies

The rare‐earth local environments of Nd:Al2O3‐P2O5‐K2O glass with different P/Al ratios are analyzed using electron paramagnetic resonance (EPR). The echo‐detected field sweep (EDFS) line‐shape change hints at a modification of the local environment around Nd3+ ion by various P/Al ratios. The coordination sphere structure of Nd3+ ion is determined by three and four pulse electron spin echo envelope modulation (ESEEM) spectra. The off‐diagonal signal corresponding to 31P Larmor frequency confirms that the second‐order coordination sphere of Nd3+ ion is almost occupied by P atoms. Al is located at the higher‐order Nd3+ coordination sphere as only diagonal resonance corresponding to the 27Al was detected, the content of which rapidly increases with decreasing P/Al ratio.

Measurements for Addition Reaction Rate Constants of Organic Free Radicals to Maleic Anhydride by Means of Pulsed EPR Spectroscopy with Laser Excitation

Addition reaction of photo-generated organic radicals to maleic anhydride (MA), that is a common monomer in polymerization, was studied by time-resolved (TR)- and pulsed electron paramagnetic resonance (EPR) spectroscopic methods. The radicals examined are 2-hydroxypropyl (2Hy-Pr), diphenylphosphine oxide (DPPO) and hydroxycyclohexyl (Hy-CyH). Analysis of TR-EPR spectra gave the structures of adduct radicals. The addition rate constants of the radicals generated by the photolysis of initiators were measured by electron spin echo detection method. It was found that the addition reaction rate constants of 2Hy-Pr, DPPO and Hy-CyH radicals to MA show exceptionally smaller values than those estimated by the theoretical reaction model based on enthalpy and polar effects.

Utilization of 13C-labeled amino acids to probe the α-helical local secondary structure of a membrane peptide using electron spin echo envelope modulation (ESEEM) spectroscopy

Electron spin echo envelope modulation (ESEEM) spectroscopy in combination with site-directed spin labeling (SDSL) has been established as a valuable biophysical technique to provide site-specific local secondary structure of membrane proteins. This pulsed electron paramagnetic resonance (EPR) method can successfully distinguish between α-helices, β-sheets, and 310-helices by strategically using 2H-labeled amino acids and SDSL. In this study, we have explored the use of 13C-labeled residues as the NMR active nuclei for this approach for the first time. 13C-labeled d5-valine (Val) or 13C-labeled d6-leucine (Leu) were substituted at a specific Val or Leu residue (i), and a nitroxide spin label was positioned 2 or 3 residues away (denoted i-2 and i-3) on the acetylcholine receptor M2δ (AChR M2δ) in a lipid bilayer. The 13C ESEEM peaks in the FT frequency domain data were observed for the i-3 samples, and no 13C peaks were observed in the i-2 samples. The resulting spectra were indicative of the α-helical local secondary structure of AChR M2δ in bicelles. This study provides more versatility and alternative options when using this ESEEM approach to study the more challenging recombinant membrane protein secondary structures.

Investigating the Secondary Structure of Membrane Peptides Utilizing Multiple 2H-Labeled Hydrophobic Amino Acids via Electron Spin Echo Envelope Modulation (ESEEM) Spectroscopy.

An electron spin echo envelope modulation (ESEEM) approach was used to probe local secondary structures of membrane proteins and peptides. This ESEEM method detects dipolar couplings between 2H-labeled nuclei on the side chains of an amino acid (Leu or Val) and a strategically placed nitroxide spin-label in the proximity up to 8 Å. ESEEM spectra patterns for different samples correlate directly to the periodic structural feature of different secondary structures. Since this pattern can be affected by the side chain length and flexibility of the 2H-labeled amino acid used in the experiment, it is important to examine several different hydrophobic amino acids (d3 Ala, d8 Val, d8 Phe) utilizing this ESEEM approach. In this work, a series of ESEEM data were collected on the AChR M2δ membrane peptide to build a reference for the future application of this approach for various biological systems. The results indicate that, despite the relative intensity and signal-to-noise level, all amino acids share a similar ESEEM modulation pattern for α-helical structures. Thus, all commercially available 2H-labeled hydrophobic amino acids can be utilized as probes for the further application of this ESEEM approach. Also, the ESEEM signal intensities increase as the side chain length gets longer or less rigid. In addition, longer side chain amino acids had a larger 2H ESEEM FT peak centered at the 2H Larmor frequency for the i ± 4 sample when compared to the corresponding i ± 3 sample. For shorter side chain amino acids, the 2H ESEEM FT peak intensity ratio between i ± 4 and i ± 3 was not well-defined.

Electron paramagnetic resonance of a 10B-containing heterocyclic radical

Electron paramagnetic resonance measurements for a 10B-containing heterocyclic phenanthrenedione radical, (C6F5)2B(O2C14H8), were made at X-band in 9:1 toluene:dichloromethane from 10 to 293 K and in toluene from 180 to 293 K. In well-deoxygenated 0.1 mM toluene solution at room temperature hyperfine couplings to 10B, four pairs of protons and five pairs of fluorines contribute to a continuous wave spectrum with many resolved lines. Hyperfine couplings were adjusted to provide the best fit for spectra of the radical enriched in 10B and the analogous radical synthesized with 10,11B in natural abundance, resulting in small refinements of the hyperfine coupling constants previously reported for the natural abundance sample. Electron spin relaxation rates at temperatures between 15 and 293 K were similar for samples containing 10B and natural isotope abundance. Analysis of electron spin echo envelope modulation and hyperfine correlation spectroscopy data at 80 K found Axx = −7.5 ± 0.3, Ayy = −8.5 ± 0.3, and Azz = −10.8 ± 0.3 MHz for 11B, which indicates small spin density on the boron. The spin echo and hyperfine spectroscopy data for the 10B –containing radical are consistent with the factor of 2.99 smaller hyperfine values for 10B than for 11B.

Aluminum and gallium nuclei as microscopic probes for pulsed electron-nuclear double resonance diagnostics of electric-field gradient and spin density in garnet ceramics doped with paramagnetic ions

The presence of aluminum and gallium isotopes with large nuclear magnetic and quadrupole moments in the nearest environment of impurity ions Mn2+ and Ce3+ in garnets made it possible to use hyperfine and quadrupole interactions with these ions to determine the spatial distribution of the unpaired electron and the gradient of the electric field at the sites of aluminum and gallium in the garnet lattice. High-frequency (94 GHz) electron spin echo detected electron paramagnetic resonance and electron-nuclear double resonance measurements have been performed. Large difference in the electric field gradient and quadrupole splitting at octahedral and tetrahedral sites allowed identifying the positions of aluminum and gallium ions in the garnet lattice and proving that gallium first fills tetrahedral positions in mixed aluminum-gallium garnets. This should be taken into account in the development of garnet-based scintillators and lasers. It is shown that the electric field gradient at aluminum nuclei near Mn2+ possessing an excess negative charge in the garnet lattice is ca. 2.5 times larger than on aluminum nuclei near Ce3+.

Localization of Guest Molecules in Nanopores by Pulsed EPR Spectroscopy

The localization of guest molecules at the molecular scale in mesoporous host materials is crucial for applications in heterogeneous catalysis, chromatography, drug delivery, and in different biomedical applications. Here, we present for the first time the precise localization of different guest molecules inside the mesoporous organosilica material UKON2a with a pore size of 6 nm. We exploited paramagnetic probe molecules 4-amino-2,2,6,6-tetramethylpiperidine-1-oxyl (4-amino-TEMPO) and 4-carboxy-TEMPO in combination with a deuteration strategy. Applying a complementary set of different pulsed electron paramagnetic resonance methods, we obtained information about the dimensionality of the spatial distribution and local concentration via double electron–electron resonance experiments, orientation of the guest molecules with respect to the pore walls via electron spin echo envelope modulation spectroscopy, and about the distance between guest molecules and pore walls via electron nuclear double resonance spe...

Ether-linked lipids: Spin-label EPR and spin echoes

Electron spin echo envelope modulation (ESEEM) and conventional electron paramagnetic resonance (EPR) of site-specifically spin-labelled phospholipids are used to investigate the effect of ether-linked chains on the water-penetration and polarity profiles, as well as the phase behaviour and chain flexibility profiles, of phospholipid membranes. D2O-ESEEM reveals that water exposure of the terminal methyl groups in the interdigitated phase of dihexadecyl phosphatidylcholine (DHPC) is comparable to that of the methylene groups at the polar head-group end of the chains. Similarly, an uniform transmembrane polarity profile is obtained from the dependence of the outer 14N-hyperfine splitting on the spin-label position along the chain in frozen interdigitated DHPC dispersions. Two-component conventional EPR spectra of spin labels at the terminal methyl end of the chain reveal that the intermediate gel phase above the pretransition of DHPC contains components in which the lipid chains are interdigitated. The polarity and chain-flexibility profiles in the fluid Lα-phase of DHPC with ether-linked chains are shifted outwards, towards the polar-apolar interface, as compared with that of dihexadecanoyl phosphatidylcholine (DPPC) with ester-linked chains. Also, the polarity profile of DHPC is shifted upwards, to higher polarities. These differences reflect those in hydrocarbon thickness and area/lipid molecule reported by x-ray diffraction for the Lα-phases of the two lipids.

Structural Effects of Ammonia Binding to the Mn4CaO5 Cluster of Photosystem II.

The Mn4CaO5 oxygen-evolving complex (OEC) of photosystem II catalyzes the light-driven oxidation of two substrate waters to molecular oxygen. ELDOR-detected NMR along with computational studies indicated that ammonia, a substrate analogue, binds as a terminal ligand to the Mn4A ion trans to the O5 μ4 oxido bridge. Results from electron spin echo envelope modulation (ESEEM) spectroscopy confirmed this and showed that ammonia hydrogen bonds to the carboxylate side chain of D1-Asp61. Here we further probe the environment of OEC with an emphasis on the proximity of exchangeable protons, comparing ammonia-bound and unbound forms. Our ESEEM and electron nuclear double resonance (ENDOR) results indicate that ammonia substitutes for the W1 terminal water ligand without significantly altering the electronic structure of the OEC.

Alamethicin self-assembling in lipid membranes: concentration dependence from pulsed EPR of spin labels.

The antimicrobial action of the peptide antibiotic alamethicin (Alm) is commonly related to peptide self-assembling resulting in the formation of voltage-dependent channels in bacterial membranes, which induces ion permeation. To obtain a deeper insight into the mechanism of channel formation, it is useful to know the dependence of self-assembling on peptide concentration. With this aim, we studied Alm F50/5 spin-labeled analogs in a model 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) membrane, for peptide-to-lipid (P/L) ratios varying between 1/1500 and 1/100. Pulsed electron-electron double resonance (PELDOR) spectroscopy reveals that even at the lowest concentration investigated, the Alm molecules assemble into dimers. Moreover, under these conditions, electron spin echo envelope modulation (ESEEM) spectroscopy of D2O-hydrated membranes shows an abrupt change from the in-plane to the trans-membrane orientation of the peptide. Therefore, we hypothesize that dimer formation and peptide reorientation are concurrent processes and represent the initial step of peptide self-assembling. By increasing peptide concentration, higher oligomers are formed. A simple kinetic model of equilibrium among monomers, dimers, and pentamers allows for satisfactorily describing the experimental PELDOR data. The inter-label distances in the oligomers obtained from PELDOR experiments become better resolved with increasing P/L ratio, thus suggesting that the supramolecular organization of the higher-order oligomers becomes more defined.

Pulse EPR and ENDOR Study of Manganese Doped [(CH3)2NH2][Zn(HCOO)3] Hybrid Perovskite Framework

We present a pulse electron paramagnetic resonance (EPR) and electron–nuclear double resonance (ENDOR) study of a manganese-doped [(CH3)2NH2][Zn(HCOO)3] dense metal–organic framework which exhibits a structural phase transition at 163 K. The echo-detected field sweep Mn2+ EPR spectrum of the low-temperature phase is in a perfect agreement with the previous continuous-wave EPR results, while the spectrum of the disordered phase reveals a significant EPR transition-dependent relaxation. The 1H ENDOR pattern indicates several protons in the vicinity of the Mn2+ ion. The experimental ENDOR spectrum is successfully simulated using the proton hyperfine tensors calculated by the density functional theory. A multifrequency electron spin echo envelope modulation (ESEEM) spectroscopy shows a peculiar signal which is unaffected by the external magnetic field. The modulation depth of this signal starts to decrease above 40 K, coinciding with the temperature at which the methyl groups of the (CH3)2NH2+ cations start t...