Saturation factor of nitroxide radicals in liquid DNP by pulsed ELDOR experiments
Physical Chemistry, Chemical Physics - PCCP
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
... fforts to enhance the sensitivity of high resolution NMR and magnetic resonance imaging. 1-3 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: 6 e = 1 À s eff fx|g s |/g I In 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) .      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 were measured for 15 N-and 2 H-labelled TEMPONE (4-Oxo-TEMPO) dissolved in water in concentrations ranging from 0.5 to 25 mM. These values were compared to theoretical predictions based on the analytical solution of the rate equations for the electron spin coupled to the 15 N nucleus. Finally, the saturation factors allow extraction of the coupling factors for DNP according to eqn (1). To discuss the physical parameters that determine the saturation factor, we illustrate in Fig. 1 the four energy levels of an electron spin (S = 1 2 ) strongly coupled to a nucleus with spin I = 1 2 in a static magnetic field as for a 15 N labelled nitroxide radical. The expectation value of the electron spin Fig. 1 Relaxation scheme for an S = 1 2 and I = 1 2 coupled system containing the spin lattice relaxation rates w e , w n and the spin exchange rate K x . Continuous microwave irradiation on one of the EPR transitions as applied during the DNP experiment is indicated.