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

Abstract

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...

Quantitative cw Overhauser effect dynamic nuclear polarization for the analysis of local water dynamics

“…Türke and Bennati have since used pulsed ESR on 15 N nitroxides to directly interrogate the non-resonant contribution to s max [3, 30]. They also turned the attention of the ODNP community toward the earlier work of Hyde, Chien, and Freed, who employed a double resonance cavity to partially saturate one hyperfine transition while observing the reduction, R , in a different hyperfine transition.…”

“…[31]. Türke and Bennati [30] employed modern pulsed ELDOR measurements to quantify both w h and w e for 15 N nitroxides and found a value of b ″ = C × 198.7 M −1 , where C is the spin probe concentration. They also placed this measurement in context by using it to determine the value of s max in Eq.…”

First determination of the spin relaxation properties of a nitronyl nitroxide in solution by electron spin echoes at X-band: A comparison with Tempone

“…This is due to the relevance of the persistence of spin polarization, a condition which is required for applications in different fields, e.g. for stable radicals either in liquid solutions as polarizing agents for Dynamic Nuclear Polarization (DNP) [1][2][3], or in solids as memory units in spintronics applications [4][5][6][7][8]. Also some pulse EPR methods based on electron spin echo (ESE), as the today very popular Double Electron-Electron Resonance (DEER) [9], are feasible only for sufficiently slow transverse electron spin relaxation or echo dephasing [10].…”

Nonlinear Scaling of Surface Water Diffusion with Bulk Water Viscosity of Crowded Solutions

“…The value of τ c,w , which picks out the diffusivity of the relatively un-perturbed 21 bulk water (2.3×10 −9 m 2 s −1 ) around the small spin label, is 33.3 ps. 22 We calculate translational retardation factors, e.g. τ c /τ c,w , which can be approximated as the ratio between bulk water diffusion dynamics and local surface water.…”