Echo Envelope Research Articles

Paramagnetic resonance investigation of mono- and di-manganese-containing systems in biochemistry.

Although not as important as iron, manganese is one of the key transition metal (d block) elements in biochemistry. The role of Mn in photosynthesis is perhaps its "starring role," but has been extensively covered elsewhere and will not be discussed in this chapter. In contrast to iron, for which 57Fe Mӧssbauer effect (nuclear gamma resonance) spectroscopy is available, 55Mn (100% abundant) lacks this ability. Instead, the defining technique for Mn is electron paramagnetic resonance (EPR) spectroscopy, as Mn(II), Mn(IV), and even Mn(III), as well as certain di-Mn systems are generally EPR active. This chapter describes how EPR and its advanced derivatives, electron spin echo envelope modulation (ESEEM) and electron-nuclear/electron-electron double resonance (ENDOR/ELDOR) spectroscopies, provide valuable biochemical information on many Mn-containing systems. These include endogenous Mn in mono- and di-Mn (protein) enzymes as well as brief mention of applications to Mn in non-protein biological systems.

Probing the local secondary structure of bacteriophage S21 pinholin membrane protein using electron spin echo envelope modulation spectroscopy

There have recently been advances in methods for detecting local secondary structures of membrane protein using electron paramagnetic resonance (EPR). A three pulsed electron spin echo envelope modulation (ESEEM) approach was used to determine the local helical secondary structure of the small hole forming membrane protein, S21 pinholin. This ESEEM approach uses a combination of site-directed spin labeling and 2H-labeled side chains. Pinholin S21 is responsible for the permeabilization of the inner cytosolic membrane of double stranded DNA bacteriophage host cells. In this study, we report on the overall global helical structure using circular dichroism (CD) spectroscopy for the active form and the negative-dominant inactive mutant form of S21 pinholin. The local helical secondary structure was confirmed for both transmembrane domains (TMDs) for the active and inactive S21 pinholin using the ESEEM spectroscopic technique. Comparison of the ESEEM normalized frequency domain intensity for each transmembrane domain gives an insight into the α-helical folding nature of these domains as opposed to a π or 310-helix which have been observed in other channel forming proteins.

Electron Spin Echo Envelope Modulation Spectroscopy Reveals How Adenosylcobalamin-Dependent Lysine 5,6-Aminomutase Positions the Radical Pair Intermediates and Modulates Their Stabilities for Efficient Catalysis

Electron spin echo envelope modulation (ESEEM) spectroscopy reveals several interactions that serve to weaken the Co–C5′ bond, through stabilization of the cleaved state, thereby facilitating the 5′-deoxyadenosyl (5′-dAdo•) radical formation in unlabeled and 15N-labeled lysine 5,6-aminomutase. In the first place, we show that the CoII–His133β interaction enhances the spin delocalization from CoII to the α-axial ligand, reducing the 5′-dAdo•–CoII recombination probability in situ and allowing nascent 5′-dAdo• to migrate toward the substrate. Next, using [5′-2H]-deoxyadenosylcobalamin, we show that the C5′ methyl group of 5′-dAdoH is at closest contact with the substrate radical. In the case of 5′-dAdo•, this would allow the 5′-methylene radical to be rapidly quenched by H-transfer from the substrate. Finally, we show that the spin density is transferred noncovalently from the substrate radical to the adenine N7 of 5′-dAdoH. In the one-radical substrate–5′-dAdo•–product H-exchange triad, which is central in the mechanism of action of B12 enzymes, such transfer would promote the forward H-transfer reaction by dynamically lowering the free energy of the substrate radical, thereby facilitating its formation and quenching of 5′-dAdo•. In the reverse H-transfer reaction following the 1,2-rearrangement, the reversibility of the noncovalent spin transfer allows the product radical to restore full radical character to reform 5′-dAdo• and a diamagnetic product. The role of dynamical and reversible spin partitioning as a mechanism for modulating the radical stability is likely common in B12 enzymes. This study leads to a better comprehension about how an enzyme controls the reaction trajectory of 5′-dAdo• exquisitely with specificity. The ESEEM results support the positioning of the interacting partners in the catalytic form of lysine 5,6-aminomutase obtained by modeling.

Statistical modeling of ultrasound signals related to the packing factor of wave scattering phenomena for structural characterization

The two-dimensional homodyned K-distribution has been widely used to model the echo envelope of ultrasound radio frequency (RF) signals in the field of medical ultrasonics. The main contribution of this work is to present a theoretical framework for supporting this model of the echo envelope and statistical models of the RF signals and their Hilbert transform in the case in which the scatterers' positions may be dependent. In doing so, the law of large numbers, Lyapounov's central limit theorem, and the Berry-Esseen theorem are being used. In particular, the proposed theoretical framework supports a previous conjecture relating the scatterer clustering parameter of the homodyned K-distribution to the packing factor W, which is related to the spatial organization of the scatterers, appearing in statistical physics or backscatter coefficient modeling. Simulations showed that the proposed modeling is valid for a number of scatterers and packing factors varying by steps of 2 from 1 to 21 and 1 to 11, respectively. The proposed framework allows, in principle, the detection of the structural information taking place at a scale smaller than the wavelength based solely on the statistical analysis of the RF signals or their echo envelope, although this goal was previously achieved based on the spectral analysis of ultrasound signals.

Shear Wave Elastography and Quantitative Ultrasound as Biomarkers to Characterize Deep Vein Thrombosis In Vivo.

Investigate shear wave elastography (SWE) and quantitative ultrasound (QUS) parameters in patients hospitalized for lower limb deep vein thrombosis (DVT). Sixteen patients with DVT were recruited and underwent SWE and radiofrequency data acquisitions for QUS on day 0, day 7, and day 30 after the beginning of symptoms, in both proximal and distal zones of the clot identified on B-mode scan. SWE and QUS features were computed to differentiate between thrombi at day 0, day 7, and day 30 following treatment with heparin or oral anticoagulant. The Young's modulus from SWE was computed, as well as QUS homodyned K-distribution (HKD) parameters reflecting blood clot structure. Median and interquartile range of SWE and QUS parameters within clot were taken as features. In the proximal zone of the clot, the HKD ratio of coherent-to-diffuse backscatter median showed a significant decrease from day 7 to day 30 (P=.036), while the HKD ratio of diffuse-to-total backscatter median presented a significant increase from day 7 to day 30 (P=.0491). In the distal zone of the clot, the HKD normalized intensity of the echo envelope median showed a significant increase from day 0 to day 30 (P=.0062). No SWE features showed statistically significant differences over time. Nonetheless, a trend of lower median of Young's modulus within clot for patients who developed a pulmonary embolism was observed. QUS features may be relevant to characterize clot's evolution over time. Further analysis of their clinical interpretation and validation on a larger dataset would deserve to be studied.

Insight into the Partitioning and Clustering Mechanism of Rare-Earth Cations in Alkali Aluminoborosilicate Glasses

Rare-earth (RE) containing alkali aluminoborosilicate glasses find increasingly broad technological applications, with their further development only impeded by yet-poor understanding of coordination environment and structural role of RE ions in glasses. In this work we combine free induction decay (FID)-detected electron paramagnetic resonance (EPR), electron spin echo envelope modulation (ESEEM), and MAS NMR spectroscopies, to examine the coordination environment and the clustering tendencies of RE3+ in a series of peralkaline aluminoborosilicate glasses co-doped with Nd2O3 (0.001-0.1 mol%) and 5 mol% La2O3. Quantitative EPR spectral analysis reveals three different Nd3+ forms coexisting in the glasses: isolated Nd3+ centers, dipole-coupled Nd clusters (Nd-O-X-O-Nd, where X = Si/B/Al), and spin-exchange-coupled Nd clusters, (Nd-O-Nd) and (Nd-O-La-O-Nd). Extensive RE clustering is observed at high RE2O3 concentrations, with more than 90% REs converting to dipole- and exchange-coupled Nd clusters already at [RE2O3] = 0.01 mol%. ESEEM analysis of the EPR-detectable Nd centers indicates a Na/Si-rich environment (four Na+ per Nd3+) for the isolated Nd3+ centers and the Na/Si/B-rich environment (2-3 Na+ and 1-2 boron per each Nd3+) for the dipole-coupled Nd clusters, while the EPR-undetectable exchanged-coupled RE clusters are predicted to exist in a Na/B-rich environment. The RE clustering induces nano-scale glass phase separation, while the Na/B-rich environment of the RE clusters implies a depletion of the same elements from the remaining host glass. Based on our results, we develop a mechanistic model that explains the high tendency of RE3+ to form clusters in alkali aluminoborosilicate glasses.

High-frequency quantitative ultrasound to assess chemical crosslinking induced changes in the posterior sclera

Myopia alters the collagen microstructure of the sclera which changes the tissue biomechanical properties. Although most forms of myopia are mild, progression of the disease to high or pathologic myopia can lead to significant weakening of the sclera and greatly increase the risk for the formation of a posterior staphyloma, a sight-threatening condition. Potential treatments to counteract effects of myopia on the sclera include chemical crosslinking (CXL). Formaldehyde releasing agents like sodium hydroxymethylglycinate (SHMG) are an attractive option for the posterior sclera because they increase tissue stiffness and require no light activation. Previous research has demonstrated that high-frequency quantitative ultrasound (QUS) is sensitive to the microstructural changes in the sclera caused by myopia. In this study, we investigate the ability of QUS to monitor microstructural changes caused by CXL. Ex vivo pig eyes were immersed in a 26.5 mM SHMG solution and scanned by an 80MHz ultrasound transducer at regular time intervals. RF echo data were analyzed to compute QUS parameters from the backscatter coefficient, normalized power spectrum, and echo envelope statistics. Correlations between QUS parameters and immersion time were investigated to evaluate the efficacy of QUS for monitoring CXL treatment. Results of this study demonstrate the capability to high-frequency QUS to assess changes in tissue microstructure caused by CXL with SHMG.

Electron–electron double resonance detected NMR spectroscopy using ensemble NV centers at 230 GHz and 8.3 T

The nitrogen-vacancy (NV) center has enabled widespread study of nanoscale nuclear magnetic resonance (NMR) spectroscopy at low magnetic fields. NMR spectroscopy at high magnetic fields significantly improves the technique’s spectral resolution, enabling clear identification of closely related chemical species. However, NV-detected NMR is typically performed using AC sensing through electron spin echo envelope modulation, a hyperfine spectroscopic technique that is not feasible at high magnetic fields. Within this paper, we have explored an NV-detected NMR technique for applications of high field NMR. We have demonstrated optically detected magnetic resonance with the NV Larmor frequency of 230 GHz at 8.3 T, corresponding to a proton NMR frequency of 350 MHz. We also demonstrated the first measurement of electron–electron double resonance detected NMR using the NV center and successfully detected 13C nuclear bath spins. The described technique is limited by the longitudinal relaxation time (T1), not the transverse relaxation time (T2). Future applications of the method to perform nanoscale NMR of external spins at 8.3 T and even higher magnetic fields are also discussed.

Radiation-Induced Stable Radicals in Calcium Phosphates: Results of Multifrequency EPR, EDNMR, ESEEM, and ENDOR Studies

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electron-nuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for 14N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure.

Rotational Coupling in Methyl-Tunneling Electron Spin Echo Envelope Modulation

Coherence between tunnel-split states of a methyl quantum rotor can be generated and observed in stimulated and spin-locked echo experiments, if hyperfine coupling of a nearby electron spin to the methyl protons breaks C_3 symmetry and is of the same order of magnitude as the tunnel splitting. Here, we consider the case of two methyl groups bound to the same sp^{3}-hybridized atom, which is important in the context of common nitroxide spin labels. For a simple form of the rotor-rotor coupling Hamiltonian, we provide an approach that allows for density operator computations of this system with 1152 quantum states with moderate computational effort. We find that, in the regime where the ratio between rotor-rotor coupling and rotational barrier is much smaller than unity, three-pulse ESEEM and hyperfine-decoupled ESEEM depend only on the tunnel splitting, but not on this ratio. This finding may simplify the treatment of tunnel-induced electron decoherence in systems where the methyl groups are bound to sp^{3}-hybridized atoms.

Geometry and water accessibility of the inhibitor binding site of Na+-pump: Pulse- and CW-EPR study

Spin labels based on cinobufagin, a specific inhibitor of the Na,K-ATPase, have proved valuable tools to characterize the binding site of cardiotonic steroids (CTSs), which also constitutes the extracellular cation pathway. Because existing literature suggests variations in the physiological responses caused by binding of different CTSs, we extended the original set of spin-labeled inhibitors to the more potent bufalin derivatives. Positioning of the spin labels within the Na,K-ATPase site was defined and visualized by molecular docking. Although the original cinobufagin labels exhibited lower affinity, continuous-wave electron paramagnetic resonance spectra of spin-labeled bufalins and cinobufagins revealed a high degree of pairwise similarity, implying that these two types of CTS bind in the same way. Further analysis of the spectral lineshapes of bound spin labels was performed with emphasis on their structure (PROXYL vs. TEMPO), as well as length and rigidity of the linkers. For comparable structures, the dynamic flexibility increased in parallel with linker length, with the longest linker placing the spin label at the entrance to the binding site. Temperature-related changes in spectral lineshapes indicate that six-membered nitroxide rings undergo boat-chair transitions, showing that the binding-site cross section can accommodate the accompanying changes in methyl-group orientation. D2O-electron spin echo envelope modulation in pulse-electron paramagnetic resonance measurements revealed high water accessibilities and similar polarity profiles for all bound spin labels, implying that the vestibule leading to steroid-binding site and cation-binding sites is relatively wide and water-filled.

Chiral-induced spin selectivity in the formation and recombination of radical pairs: cryptochrome magnetoreception and EPR detection

That the rates and yields of reactions of organic radicals can be spin dependent is well known in the context of the radical pair mechanism (RPM). Less well known, but still well established, is the chiral-induced spin selectivity (CISS) effect in which chiral molecules act as spin filters that preferentially transmit electrons with spins polarized parallel or antiparallel to their direction of motion. Starting from the assumption that CISS can arise in electron transfer reactions of radical pairs, we propose a simple way to include CISS in conventional models of radical pair spin dynamics. We show that CISS can (a) increase the sensitivity of radical pairs to the direction of a weak external magnetic field, (b) change the dependence of the magnetic field effect on the reaction rate constants, and (c) destroy the field-inversion symmetry characteristic of the RPM. We argue that CISS polarization effects could be observable by EPR (electron paramagnetic resonance) of oriented samples either as differences in continuous wave, time-resolved spectra recorded with the spectrometer field parallel or perpendicular to the CISS quantization axis or as signals in the in-phase channel of an out-of-phase ESEEM (electron spin echo envelope modulation) experiment. Finally we assess whether CISS might be relevant to the hypothesis that the magnetic compass of migratory songbirds relies on photochemically-formed radical pairs in cryptochrome flavoproteins. Although CISS effects offer the possibility of evolving a more sensitive or precise compass, the associated lack of field-inversion symmetry has not hitherto been observed in behavioural experiments. In addition, it may no longer be safe to assume that the observation of a polar magnetic compass response in an animal can be used as evidence against a radical pair sensory mechanism.

Tylopeptin B peptide antibiotic in lipid membranes at low concentrations: Self-assembling, mutual repulsion and localization

The medium-length peptide Tylopeptin B possesses activity against Gram-positive bacteria. It binds to bacterial membranes altering their mechanical properties and increasing their permeability. This action is commonly related with peptide self-assembling, resulting in the formation of membrane channels. Here, pulsed double electron-electron resonance (DEER) data for spin-labeled Tylopeptin B in palmitoyl-oleoyl-glycero-phosphocholine (POPC) model membrane reveal that peptide self-assembling starts at concentration as low as 0.1 mol%; above 0.2 mol% it attains a saturation-like dependence with a mean number of peptides in the cluster <n> = 3.3. Using the electron spin echo envelope modulation (ESEEM) technique, Tylopeptin B molecules are found to possess a planar orientation in the membrane. In the peptide concentration range between 0.1 and 0.2 mol%, DEER data show that the peptide clusters have tendency of mutual repulsion, with a circle of inaccessibility of radius around 20 nm. It may be proposed that within this radius the peptides destabilize the membrane, providing so the peptide antimicrobial activity. Exploiting spin-labeled stearic acids as a model for free fatty acids (FFA), we found that at concentrations of 0.1–0.2 mol% the peptide promotes formation of lipid-mediated FFA clusters; further increase in peptide concentration results in dissipation of these clusters.

Ultrasonic imaging recognition of coal-rock interface based on the improved variational mode decomposition

Extracting effective ultrasonic echo signals in multilayered media is the key technology for detection of coal-rock interface, which is the basis of automatic height adjustment of the shearer drums during automatic coal mining. However, it is tough to recognize echo signals of coal-rock interface efficiently and accurately through common signal processing methods such as Fourier Transform (FT), Wavelet Transform (WT) and Empirical Mode Decomposition (EMD). Therefore, an improved Empirical Variational Mode Decomposition (EVMD) method for ultrasonic echo signal processing is proposed in this paper. Firstly, EVMD is used to preprocess the received echoes of coal-rock interface. Secondly, the optimization iterative method of weight coefficient is introduced to fit part of distorted echo envelope with asymmetric Gaussian model, which can effectively reduce the calculation error. Finally, peak focusing imaging technique is used to image the processed envelopes. The experimental results show that the proposed method can recognize the coal-rock interface efficiently.

Dynamic Nuclear Polarization with Vanadium(IV) Metal Centers

Summary Dynamic nuclear polarization (DNP) harnesses the large polarization of electron spins to dramatically increase nuclear magnetic resonance (NMR) sensitivity. This study expands the scope of DNP beyond its traditional focus on hyperpolarizing the solvent network using exogenous polarizing agents (PAs). We introduce 1H DNP with endogenous V4+ centers positioned in a set of vanadyl complexes with tunable V4+-1H distances. We traced the polarization transfer from V4+ to 1H spins, specifically differentiating between direct V4+-1Hs polarization transfer and the 1H spin-diffusion-mediated bulk solvent 1H polarization buildup and illuminated the effect of the V4+-1H distance on these processes. These results deepen our understanding of polarization pathways and expand the catalog of PAs to broad-line transition metals. This study establishes crucial first steps toward employing strategically positioned endogenous paramagnetic metal centers for DNP and the conceptual framework of hyperfine DNP spectroscopy that merges both spatial and chemical diagnosis of target nuclear spins.

Artificial neural network classification of foliage targets from spectrograms of sequential echoes using a biomimetic audible sonar

Classifying foliage targets using echolocation is important for recognizing landmarks by bats using ultrasonic emissions and blind human echolocators (BEs) using palatal clicks. Previous attempts to classify foliage used ultrasonic frequencies and single sensor (monaural) detection. Motivated by the echolocation capabilities of BEs, a biomimetic sonar emitting audible clicks acquired 5600 binaural echoes from five sequential emissions that probed two foliage targets at aspect angles separated by 18°. Echo spectrograms formed feature vector inputs to artificial neural networks (ANNs) for classifying two targets, Ficus benjamina and Schefflera arboricola, with leaf areas that differ by a factor of four. Classification performances of ANNs without and with hidden layers were analyzed using tenfold cross-validation. Performance improved with input feature size, with binaural echo classification outperforming that using monaural echoes for the same number of emissions and for the same number of echoes. Linear classification accuracy was comparable to that using nonlinear classification with both achieving fewer than 1% errors with binaural spectrogram features from five sequential emissions. This result was better by a factor of 20 compared to previous classification of these targets using only the time envelopes of the same echoes.

1H-HYSCORE Reveals Structural Details at the Fe(II) Active Site of Taurine:2-Oxoglutarate Dioxygenase.

Proton Hyperfine Sublevel Correlation (1H-HYSCORE) experiments have been used to probe the ligation structure of the Fe(II) active site of taurine:2-oxoglutarate dioxygenase (TauD), a non-heme Fe(II) hydroxylase. To facilitate Electron Paramagnetic Resonance (EPR) experiments, Fe(II) derivatives of the enzyme were studied using nitric oxide as a substitute for molecular oxygen. The addition of NO to the enzyme yields an S = 3/2 {FeNO}7 paramagnetic center characterized by nearly axial EPR spectra with g⊥ = 4 and g|| = 2. Using results from (i) an X-ray crystallographic study of TauD crystallized under anaerobic conditions in the presence of both cosubstrate 2-oxoglutarate and substrate taurine, (ii) a published theoretical description of the {FeNO}7 derivative of this form of the enzyme, and (iii) previous 2H-Electron Spin Echo Envelope Modulation (ESEEM) studies, we were able to assign the proton cross peaks detected in orientation-selected 1H-HYSCORE spectra. Discrete contributions from the protons of two coordinated histidine ligands were resolved. If substrate taurine is absent from the complex, orientation-selective HYSCORE spectra show cross peaks that are less resolved and when combined with information obtained from continuous wave EPR, support an alternate binding scheme for 2-oxoglutarate. HYSCORE studies of TauD in the absence of 2-oxoglutarate show additional 1H cross peaks that can be assigned to two distinct bound water molecules. In addition, 1H and 14N cross peaks that arise from the coordinated histidine side chains show a change in NO coordination for this species. For all of the TauD species, 1H hyperfine couplings and their orientations are sensitive to the detailed electronic structure of the {FeNO}7 center.

Characterisation of the Fe(III):H+ Defect Centre in Natural Amethyst

A natural single crystal of amethyst was investigated by means of continuous-wave and pulsed Electron Paramagnetic Resonance (EPR), with the aim of structurally characterizing the substitutional S2 Fe(III):H+ centre. In this centre, Fe(III) replaces Si(IV) in the tetrahedral site, whereas H+ is coupled to Fe(III) to maintain the charge balance. The spectroscopic investigations, mainly the interpretation of the Electron Spin Echo Envelope Modulation, allowed a detailed localisation of the proton to be obtained. H+ occurs in the channels crossing the crystal parallel to the crystallographic c axis, in a largely eccentric position. The Fe(III)-H+ distance, evaluated in 2.70 Å, is found associated with a non-negligible isotropic hyperfine coupling, which can be linked to the relative stability of the S2 centre in natural amethyst.

Cryogenically frozen PEGylated liposomes and micelles: Water penetration and polarity profiles

Poly(ethylene glycol) (PEG)-grafted lipid dispersions are widely investigated in fundamental and biotechnological research for their successful use in drug-delivery. Here, we consider mixtures of the bilayer-forming lipid dipalmitoylphosphatidylcholine (DPPC) with the micelle-forming lipid PEG:2000-phosphatidilethanolamine (PEG:2000-DPPE) fully hydrated in D2O and measured at 77 K. Electron Spin Echo Envelope Modulation and continuous wave Electron Paramagnetic Resonance of chain-labelled lipids are employed to detect the extent of solvent permeation and the environmental polarity, respectively, across the hydrocarbon regions of the lipid assemblies. Sigmoidal water penetration and polarity profiles are described in sterically stabilized liposomes (SSL) formed at submicellar content of PEG:2000-DPPE incorporated in DPPC. Compared to DPPC bilayers, SSL show increased hydrophobicity at both the polar/apolar interface and the chain termini, and a broader transition that is shifted toward the interface. Solvent exposure and polarity decrease on going down the chain in PEG:2000-DPPE micelles. However, compared to SSL, polymer-lipid micelles show higher solvent permeation at any chain segment and the chain termini are accessible to water. In any sample, heterogeneity is found in H-bond formation between the spin-label nitroxide groups and the solvent molecules. The results at cryogenic temperature add new insights into the biophysico-chemical characterization of PEGylated lipid dispersions.

Open and Closed Radicals: Local Geometry around Unpaired Electrons Governs Magic-Angle Spinning Dynamic Nuclear Polarization Performance

The development of magic-angle spinning dynamic nuclear polarization (MAS DNP) has allowed atomic-level characterization of materials for which conventional solid-state NMR is impractical due to the lack of sensitivity. The rapid progress of MAS DNP has been largely enabled through the understanding of rational design concepts for more efficient polarizing agents (PAs). Here, we identify a new design principle which has so far been overlooked. We find that the local geometry around the unpaired electron can change the DNP enhancement by an order of magnitude for two otherwise identical conformers. We present a set of 13 new stable mono- and dinitroxide PAs for MAS DNP NMR where this principle is demonstrated. The radicals are divided into two groups of isomers, named open (O-) and closed (C-), based on the ring conformations in the vicinity of the N-O bond. In all cases, the open conformers exhibit dramatically improved DNP performance as compared to the closed counterparts. In particular, a new urea-based biradical named HydrOPol and a mononitroxide O-MbPyTol yield enhancements of 330 ± 60 and 119 ± 25, respectively, at 9.4 T and 100 K, which are the highest enhancements reported so far in the aqueous solvents used here. We find that while the conformational changes do not significantly affect electron spin-spin distances, they do affect the distribution of the exchange couplings in these biradicals. Electron spin echo envelope modulation (ESEEM) experiments suggest that the improved performance of the open conformers is correlated with higher solvent accessibility.