Water 1H relaxation dispersion analysis on a nitroxide radical provides information on the maximal signal enhancement in Overhauser dynamic nuclear polarization experiments

Bennati, Marina; Luchinat, Claudio; Parigi, Giacomo; Türke, Maria-Teresa

doi:10.1039/c002304n

Cited by 81 publications

(143 citation statements)

References 39 publications

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“…The coupling factor calculated from the experiments at the three different concentrations amounts to 0.33 AE 0.02, and is independent of concentration as expected. This value is in agreement with the coupling factors derived by nuclear magnetic relaxation dispersion (0.33-0.35) 21 and with molecular dynamics simulations (0.30). 22 We conclude that pulsed ELDOR provides the suitable approach to evaluate the saturation factors for DNP in liquid solution.…”

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confidence: 90%

“…Using these values for e, the effective saturation factors s eff and the leakage factors f from ref. 21, we have evaluated the coupling factor x from the Overhauser equation (1), as summarized in Table 1. The coupling factor calculated from the experiments at the three different concentrations amounts to 0.33 AE 0.02, and is independent of concentration as expected.…”

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confidence: 99%

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Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Türke

Bennati

2011

Phys. Chem. Chem. Phys.

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We propose the use of the pulse electron double resonance (ELDOR) method to determine the effective saturation factor of nitroxide radicals for dynamic nuclear polarization (DNP) experiments in liquids. The obtained values for the nitroxide radical 15 N at different concentrations are rationalized in terms of spin relaxation and are shown to fulfil the Overhauser theory.Dynamic nuclear polarization (DNP) in aqueous solution at ambient conditions is a major topic of current efforts to enhance the sensitivity of high resolution NMR and magnetic resonance imaging. [1][2][3] In the liquid state, DNP is governed by the Overhauser mechanism, 4,5 in which polarization is transferred from paramagnetic centers to coupled nuclear spins by microwave pumping of the EPR line. The enhancements e depend on four factors, i.e. the ratio of the gyromagnetic constants of the electron spin g s and the target nucleus g I , the coupling factor x, the leakage factor f and the effective saturation factor s eff of the EPR line: 6In recent studies, nitroxide radicals have been favoured as polarizing agents for DNP since they are soluble in water, well compatible with biological systems, non-toxic and have been found to account for large DNP enhancements at magnetic fields up to 9 T. 7,8 However, the determination of the saturation factor for this class of polarizers has emerged as one of the major difficulties in rationalizing the observed DNP enhancements in terms of the Overhauser equation (1). [8][9][10][11][12] Due to the strong hyperfine coupling between the electron and the nitrogen nucleus, which splits the EPR spectrum into two ( 15 N) or three ( 14 N) separate lines, the saturation factor, defined as s eff = (S B À hS z i)/S B with S B the Boltzmann polarization of the electron spin, cannot be simply extracted from the saturation behaviour of the pumped line according to the Bloch equation, as formerly suggested for trityl radicals. 13 Moreover, the expectation value of the electron spin polarization hS z i depends on the population of all energy levels involved. In recent years, this issue has been debated in the literature and several theoretical models were proposed to calculate the saturation factor for nitroxides 9-11 but to date no direct experimental approach has been provided. In this paper, we show that the effective saturation factor in Overhauser DNP can be determined by a pulsed electron double resonance (ELDOR) experiment, 14 which measures the intensity of a hyperfine line when pumping a coupled hyperfine line. This approach is demonstrated with a 15 N containing nitroxide because this gives the maximum enhancement for DNP and is relevant for future DNP applications. However, it is fully applicable to radicals incorporating a 14 N nucleus as well. Our experiments were performed at X-band (9.7 GHz electron Larmor frequency) in a commercial dielectric EPR/ENDOR resonator (Bruker EN4118X-MD4), as used for DNP, 12 that is overcoupled to permit irradiation at two frequencies with Dn up to 100 MHz. The saturation factors...

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Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Türke

Bennati

2011

Phys. Chem. Chem. Phys.

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“…The range of τ accessible by ODNP at ω e = 9.8 GHz covers the dynamic range for hydration water surrounding solvent-exposed sites, as well as to some extent buried sites of proteins and other biomacromolecules (33). To qualitatively compare the hydration dynamics in different biological environments, we define the retardation factor of hydration dynamics, ρ t = 〈τ〉/τ bulk , which is the average translational correlation time of the local hydration water around a tethered spin label divided by that of bulk water (i.e., τ bulk = 33 ps, measured by ODNP at 0.35 T using a small nitroxide radical dissolved in water) (30,46). As illustrated in Fig.…”

Section: Approach To Quantify Local Hydration Dynamics At Biomolecularmentioning

confidence: 99%

Hydration dynamics as an intrinsic ruler for refining protein structure at lipid membrane interfaces

Cheng

Varkey

Ambroso

et al. 2013

Proc. Natl. Acad. Sci. U.S.A.

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Knowing the topology and location of protein segments at watermembrane interfaces is critical for rationalizing their functions, but their characterization is challenging under physiological conditions. Here, we debut a unique spectroscopic approach by using the hydration dynamics gradient found across the phospholipid bilayer as an intrinsic ruler for determining the topology, immersion depth, and orientation of protein segments in lipid membranes, particularly at water-membrane interfaces. This is achieved through the sitespecific quantification of translational diffusion of hydration water using an emerging tool, 1 H Overhauser dynamic nuclear polarization (ODNP)-enhanced NMR relaxometry. ODNP confirms that the membrane-bound region of α-synuclein (αS), an amyloid protein known to insert an amphipathic α-helix into negatively charged phospholipid membranes, forms an extended α-helix parallel to the membrane surface. We extend the current knowledge by showing that residues 90-96 of bound αS, which is a transition segment that links the α-helix and the C terminus, adopt a larger loop than an idealized α-helix. The unstructured C terminus gradually threads through the surface hydration layers of lipid membranes, with the beginning portion residing within 5-15 Å above the phosphate level, and only the very end of C terminus surveying bulk water. Remarkably, the intrinsic hydration dynamics gradient along the bilayer normal extends to 20-30 Å above the phosphate level, as demonstrated with a peripheral membrane protein, annexin B12. ODNP offers the opportunity to reveal previously unresolvable structure and location of protein segments well above the lipid phosphate, whose structure and dynamics critically contribute to the understanding of functional versatility of membrane proteins.electron paramagnetic resonance | site-directed spin labeling | protein-lipid interaction | liposomes | Parkinson disease

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“…Equation (2) describes the major effect of temperature on the DNP enhancement: with increasing temperature, both D S and D R will increase, therefore shortening s t and increasing the DNP enhancement e. Additionally, the temperature dependence of the relaxation rates of the electron and nuclear spin might change the saturation factor s and leakage factor f. In this work, we measured the DNP enhancements of water protons with Fremy's Salt as radicals for temperatures between 15 and 65°C at a magnetic field of 9.2 T. So far, such temperature measurements were done and discussed only up to magnetic fields of B = 3.4 T [14,15]. The high enhancements obtained are compared with predictions based on classical Overhauser theory for liquids, taking translational correlation times extracted from low-field DNP and NMRD experiments [16][17][18][19][20], and calculations of the coupling factor from MD simulations [21,22].…”

Section: Introductionmentioning

confidence: 92%

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2 T

et al. 2012

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The temperature dependence of the water-proton dynamic nuclear polarization (DNP) enhancement from Fremy's salt nitroxide radicals was measured in a magnetic field of 9.2 T (corresponding to 260 GHz microwave (MW) and 392 MHz NMR frequencies) in the temperature range of 15-65°C. The temperature could be determined directly from the proton NMR line shift of the sample. Very high DNP enhancements of -38 (signal integral) or -81 (peak intensity) could be achieved with a high-power gyrotron MW source. The experimental findings are compared with classical Overhauser theory for liquids, which is based on the translational and rotational motion of the molecules and with molecular dynamics calculations of the coupling factor.

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Water 1H relaxation dispersion analysis on a nitroxide radical provides information on the maximal signal enhancement in Overhauser dynamic nuclear polarization experiments

Cited by 81 publications

References 39 publications

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments

Hydration dynamics as an intrinsic ruler for refining protein structure at lipid membrane interfaces

Temperature Dependence of the Proton Overhauser DNP Enhancements on Aqueous Solutions of Fremy’s Salt Measured in a Magnetic Field of 9.2 T

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