Direct Enhancement of Nuclear Singlet Order by Dynamic Nuclear Polarization

Abstract

Hyperpolarized singlet order is available immediately after dissolution DNP, avoiding need for additional preparation steps. We demonstrate this procedure on a sample of [1,2− 13 C 2 ]pyruvic acid.T he large signal improvement provided by spin hyperpolarization methods has dramatically extended the in vivo potential of NMR spectroscopy.1 Hyperpolarized nuclear spin order permits tracing of both endogenous and nonendogenous substances as they are transported through the blood vessels and organs and participate in metabolism.2−4 A limitation of these techniques is the often short observation time scale imposed by nuclear spin relaxation. Hyperpolarized magnetization typically decays with the longitudinal relaxation time T 1 , which is typically between a fraction of a second and 1 min and can be particularly short in vivo because of the presence of paramagnetic species and high concentrations of other magnetic nuclei. Short decay times hinder the use of hyperpolarized substrates for imaging metabolism in vivo because of the relatively long times taken for substrates to reach the tissue of interest from the point of injection. For instance, while hyperpolarized 129 Xe can be used as a lung imaging agent in gas-phase magnetic resonance imaging (MRI) because its relaxation time is several hours in the gas phase, its potential as a tracer in blood is very limited because the T 1 relaxation time of 129 Xe is reduced to only a few seconds upon dissolution. 5A potential remedy for the limited hyperpolarization lifetime is the use of nuclear singlet states involving coupled spin-1 / 2 pairs. The lifetime of the nuclear singlet state (|αβ⟩ − |βα⟩)/ √2 is capable of exceeding T 1 in some circumstances, since many common relaxation mechanisms are symmetric with respect to spin exchange and cannot induce singlet−triplet transitions. 6−9 In previous work, we reported the potential of doubly 11In some cases, substances exhibiting hyperpolarized singlet order are accessible directly by chemical reactions of hydrogen enriched with the para-hydrogen spin isomer.12−14 An alternative approach is to prepare hyperpolarized magnetization using a method such as dynamic nuclear polarization (DNP) 15,16 and then to convert it into singlet order. Several conversion methods are available:• application of resonant radiofrequency pulses in a high magnetic field to excite a "precursor state", 7 which is transformed into singlet order upon adiabatic transport to a low field; 17,18• preparation of singlet order in a high magnetic field using a radiofrequency pulse sequence;19−21• application of an audio-frequency pulse sequence in a low magnetic field on a prepolarized sample; 22• exploitation of chemical symmetry-switching reactions. 23All of these rely upon manipulations in addition to spin hyperpolarization. In certain cases they may be difficult to implement, require extra hardware, or take up valuable time during an experiment.In this communication, we show that these preparations may be circumvented by the availability of sing...

Singlet NMR methodology in two-spin-1/2 systems

“…Finally, a certain degree of singlet order can be directly generated directly in any of the above regimes by spin hyperpolarisation [63]. The hyperpolarisation process redistributes populations among spin energy levels to an extent that is far from Boltzmann equilibrium.…”

“…At the same time it creates a large imbalance between the populations of the singlet state and the triplet manifold, i.e., a large singlet order. If the amount of longitudinal order (polarisation) prepared by hyperpolarisation is denoted as p, then the same process leads to an amount p 2 /3 of (hyperpolarised) singlet order [63]. This idea has been exploited, for example, as a way to build singlet order under fully equivalent conditions, in conjunction with a chemical reaction for detection [62,64].…”

“…Filtration of singlet order by rapidly fluctuating magnetic fields has been suggested [63] as an alternative to the methods discussed above when filtration has to be accomplished outside of the spectrometer, where radiofrequency pulses and pulse field gradients are not easily accessible/available. This filtering method consists in shaking the sample in the highly inhomogeneous field present inside a mu-metal chamber.…”

“…These two facts suggest a number of interesting uses of singlet order in NMR and MRI spectroscopy. A number of researchers have been indeed exploiting its long lifetime for the purposeof storage of hyperpolarisation [4,5,6,7,8,9,10,11,12]. Hyperpolarisation storage, combined with on-demand accessibility, suggests the use of singlet order as a repository of hyperpolarisation from where the signal enhancement gained by a given hyperpolarisation technique can be repeatedly accessed over a long time (tens of minutes) [12].…”

Long-lived nuclear spin states in rapidly rotating CH 2 D groups

“…The population imbalance between the singlet state and the triplet states is called singlet order and is protected against many common relaxation mechanisms [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15]. The relaxation time constant for singlet order is called T S and often exceeds the conventional longitudinal relaxation time T 1 by a large factor [16][17][18][19][20][21][22].…”