Relevance of electron spin dissipative processes to dynamic nuclear polarization via thermal mixing

The available theoretical approaches aiming at describing Dynamic Nuclear spin Polarization (DNP) in solutions containing molecules of biomedical interest and paramagnetic centers are not able to model the behaviour observed upon varying the concentration of trityl radicals or the polarization enhancement caused by moderate addition of gadolinium complexes. In this manuscript, we first show experimentally that the nuclear steady state polarization reached in solutions of pyruvic acid with 15 mM trityl radicals is substantially independent from the average internuclear distance. This evidences a leading role of electron (over nuclear) spin relaxation processes in determining the ultimate performances of DNP. Accordingly, we have devised a variant of the Thermal Mixing model for inhomogenously broadened electron resonance lines which includes a relaxation term describing the exchange of magnetic anisotropy energy of the electron spin system with the lattice. Thanks to this additional term, the dependence of the nuclear polarization on the electron concentration can be properly accounted for. Moreover, the model predicts a strong increase of the final polarization on shortening the electron spin-lattice relaxation time, providing a possible explanation for the effect of gadolinium doping.Comment: 13 pages, 12 figure

Evidences of spin-temperature in Dynamic Nuclear Polarization: an exact computation of the EPR spectrum

In dynamic nuclear polarization (DNP) experiments, the compound is driven out-of-equilibrium by microwave (MW) irradiation of the radical electron spins. Their stationary state has been recently probed via electron double resonance (ELDOR) techniques showing, at low temperature, a broad depolarization of the electron paramagnetic resonance (EPR) spectrum under microwave irradiation. In this theoretical manuscript, we develop a numerical method to compute exactly the EPR spectrum in presence of dipolar interactions. Our results reproduce the observed broad depolarisation and provide a microscopic justification for spectral diffusion mechanism. We show the validity of the spin-temperature approach for typical radical concentration used in dissolution DNP protocols. In particular once the interactions are properly taken into account, the spin-temperature is consistent with the non-monotonic behavior of the EPR spectrum with a wide minimum around the irradiated frequency.Comment: 8 pages, 7 figures. Title and abstract change

Role of the glassy dynamics and thermal mixing in the dynamic nuclear polarization and relaxation mechanisms of pyruvic acid

The temperature dependence of $^1$H and $^{13}$C nuclear spin-lattice relaxation rate $1/T_1$ has been studied in the 1.6 K - 4.2 K temperature range in pure pyruvic acid and in pyruvic acid containing trityl radicals at a concentration of 15 mM. The temperature dependence of $1/T_1$ is found to follow a quadratic power law for both nuclei in the two samples. Remarkably the same temperature dependence is displayed also by the electron spin-lattice relaxation rate $1/T_{1e}$ in the sample containing radicals. These results are explained by considering the effect of the structural dynamics on the relaxation rates in pyruvic acid. Dynamic nuclear polarization experiments show that below 4 K the $^{13}$C build up rate scales with $1/T_{\text{1e}}$, in analogy to $^{13}$C $1/T_1$ and consistently with a thermal mixing scenario where all the electrons are collectively involved in the dynamic nuclear polarization process and the nuclear spin reservoir is in good thermal contact with the electron spin system.Comment: 14 pages, 13 figure

Cognitive deficits and brain myo-Inositol are early biomarkers of epileptogenesis in a rat model of epilepsy

One major unmet clinical need in epilepsy is the identification of therapies to prevent or arrest epilepsy development in patients exposed to a potential epileptogenic insult. The development of such treatments has been hampered by the lack of non-invasive biomarkers that could be used to identify the patients at-risk, thereby allowing to design affordable clinical studies. Our goal was to test the predictive value of cognitive deficits and brain astrocyte activation for the development of epilepsy following a potential epileptogenic injury. We used a model of epilepsy induced by pilocarpine-evoked status epilepticus (SE) in 21-day old rats where 60–70% of animals develop spontaneous seizures after around 70 days, although SE is similar in all rats. Learning was evaluated in the Morris water-maze at days 15 and 65 post-SE, each time followed by proton magnetic resonance spectroscopy for measuring hippocampal myo-Inositol levels, a marker of astrocyte activation. Rats were video-EEG monitored for two weeks at seven months post-SE to detect spontaneous seizures, then brain histology was done. Behavioral and imaging data were retrospectively analysed in epileptic rats and compared with non-epileptic and control animals. Rats displayed spatial learning deficits within three weeks from SE. However, only epilepsy-prone rats showed accelerated forgetting and reduced learning rate compared to both rats not developing epilepsy and controls. These deficits were associated with reduced hippocampal neurogenesis. myo-Inositol levels increased transiently in the hippocampus of SE-rats not developing epilepsy while this increase persisted until spontaneous seizures onset in epilepsy-prone rats, being associated with a local increase in S100β-positive astrocytes. Neuronal cell loss was similar in all SE-rats. Our data show that behavioral deficits, together with a non-invasive marker of astrocyte activation, predict which rats develop epilepsy after an acute injury. These measures have potential clinical relevance for identifying individuals at-risk for developing epilepsy following exposure to epileptogenic insults, and consequently, for designing adequately powered antiepileptogenesis trials

Dynamic Nuclear Polarization of β-Cyclodextrin Macromolecules

(1)H dynamic nuclear polarization and nuclear spin-lattice relaxation rates have been studied in amorphous complexes of β-cyclodextrins doped with different concentrations of the TEMPO radical. Nuclear polarization increased up to 10% in the optimal case, with a behavior of the buildup rate (1/TPOL) and of the nuclear spin-lattice relaxation rate (1/T1n) consistent with a thermal mixing regime. The temperature dependence of 1/T1n and its increase with the radical concentration indicate a relaxation process arising from the modulation of the electron-nucleus coupling by the glassy dynamics. The high-temperature relaxation is driven by molecular motions, and 1/T1n was studied at room temperature in liquid solutions for dilution levels close to the ones typically used for in vivo studies

Fluctuations and correlations in a frustrated S= 1/2 square lattice with competing ferromagnetic and antiferromagnetic interactions studied by muon-spin relaxation

Zero and longitudinal field muSR measurements in Pb2VO(PO4)2 and BaCdVO(PO4)2, two prototypes of the frustrated S=1/2 square lattice model with competing ferromagnetic and antiferromagnetic interactions, are presented. Both systems are observed to undergo a phase transition to a long-range magnetic order at TN~3.46 K, for Pb2VO(PO4)2, and at TN~0.99 K, for BaCdVO(PO4)2. In Pb2VO(PO4)2 both the temperature dependence of the order parameter and the longitudinal relaxation rate above TN are consistent with a twodimensional XY model, as it is found for Sr2CuO2Cl2. On the other hand, for BaCdVO(PO4)2, which lies very close to the magnetically disordered region of the phase diagram where a bond-nematic order was predicted, a peculiar logarithmic increase in the relaxation is observed above TN. In both systems a rather broad distribution of internal fields at the muon sites is noticed below TN. The origin of this distribution is discussed in the light of the muSR experiments already performed on S=1/2 frustrated antiferromagnets on a square lattice

Spin dynamics in the neutral rare-earth single-molecule magnets [TbPc2]0 and [DyPc2]0 from muSR and NMR spectroscopies

The spin dynamics in [TbPc2]0 and [DyPc2]0 single-molecule magnets have been investigated by means of muon and nuclear spin-lattice relaxation rate measurements. The correlation time for the spin fluctuations was found to be close to 0.1 ms already at 50 K, about 2 orders of magnitude larger than the one previously found in other lanthanide-based single-molecule magnets. In [TbPc2]0 two different regimes for the spin fluctuations have been evidenced: a high-temperature activated one involving spin fluctuations across a barrier around 880 K separating the ground and first excited states and a low-temperature regime involving quantum fluctuations within the twofold degenerate ground state. In [DyPc2]0 a high-temperature activated spin dynamics is also evidenced which, however, cannot be explained in terms of a single spin-phonon coupling constant

Low-energy excitations in the electron-doped metal phtalocyanine LiO.5MnPc from Li and 1H NMR

NMR and magnetization measurements in Li0.5MnPc (Pc=C32H16N8), recently proposed as a strongly correlated metal, are presented. Two different low-frequency dynamics are evidenced. The first one, probed by 1H nuclei, gives rise to a slowly relaxing magnetization at low temperature and is associated with the freezing of MnPc S=3/2 spins. This dynamics is similar to the one observed in pristine beta-MnPc and originates from Li-depleted chain segments. The second one, evidenced by the 7Li spin-lattice relaxation rate, is associated with the hopping of the electrons along Li-rich chains. The characteristic correlation times for the two dynamics are derived, and the role of the disorder is briefly discussed

Strong electronic correlations in LixZnPc organic metals

Nuclear magnetic resonance, electron paramagnetic resonance and magnetization measurements show that bulk LixZnPc are strongly correlated one-dimensional metals. The temperature dependence of the nuclear spin-lattice relaxation rate 1/T1 and of the static uniform susceptibility chi_S on approaching room temperature are characteristic of a Fermi liquid. Moreover, while for x around 2 the electrons are delocalized down to low temperature, for x-> 4 a tendency towards localization is noticed upon cooling, yielding an increase both in 1/T1 and chi_s. The x dependence of the effective density of states at the Fermi level D(EF) displays a sharp enhancement for x=2, at the half filling of the ZnPc lowest unoccupied molecular orbitals. This suggests that LixZnPc is on the edge of a metal-insulator transition where enhanced superconducting fluctuations could develo

Low-energy spin excitations in Mn3Ã3 molecular nanomagnets

NMR and µSR relaxation measurements in the Mn9(C19H15N9O2)6(ClO4)6.18H2O(Mn3×3) molecular grid are presented. The temperature dependence of the relaxation rates can be conveniently described by a single correlation time for the spin fluctuations at temperatures of the order of the exchange coupling among Mn2+ ions. The inverse of the correlation time is observed to reach the MHz range at low temperature and its temperature dependence can be justified on the basis of a theoretical model where the electron spin fluctuations are driven by spin-phonon coupling. Finally, it is pointed out that the magnetic field dependence of the µSR relaxation rate allows one to derive the Zeeman splitting of the low-energy spin levels