Ultrafast two-dimensional nuclear magnetic resonance spectroscopy of hyperpolarized solutions

Abstract

Two-dimensional (2D) NMR is an important tool for elucidating molecular structure and dynamics 1 . However, the method is limited by the low sensitivity inherent to NMR techniques, resulting in typical acquisition times for 2D NMR spectra ranging from minutes to hours. A number of hyperpolarization techniques have been explored to boost NMR's sensitivity, including an ex situ dynamic nuclear polarization method capable of yielding-for an array of molecules and under conventional observation conditions for liquid samplessignals that exceed those currently afforded by the highestfield spectrometers by several orders of magnitude 2 . Whereas this methodology is able to provide the sensitivity equivalent of ∼10 6 scans, it is constrained to extract its 'super-spectrum' within a single transient, making it a poor starting point for conventional 2D NMR acquisitions. Here, we show that if the ex situ dynamic nuclear polarization approach is suitably merged with spatially encoded ultrafast NMR spectroscopy 3 , 2D NMR spectra of liquid samples at submicromolar concentrations can be acquired within ∼0.1 s.The future of NMR hinges on the efforts devoted to increase the resolution and sensitivity of this branch of spectroscopy. Key among the physical concepts that define NMR's resolving power are multidimensional (nD) acquisitions, which spread and correlate the resonances arising from individual sites onto multiple frequency axes 1,4,5 . Although these experiments are intrinsically lengthier than their conventional one-dimensional (1D) counterparts, nD NMR's success is best portrayed by its nearly ubiquitous contemporary practice in chemistry and biology, as well as by its various NMR imaging (MRI) clinical derivations. Comparable improvements regarding NMR's sensitivity, a particularly pressing issue given the weak signals associated with magnetic resonance observations in bulk, have been slower in coming. Moreover, the moderate dependence that sensitivity exhibits on the magnetic field, B NMR , has led to diminishing returns despite investments on ever-larger NMR magnets. Driven by this reality, recently there has been an increased interest in devising alternatives that prepare nuclei in 'hyperpolarized' states, whose spin population differences depart from the usual ≈10 −5 Boltzmann distributions and approach unity values. Methods proposed and demonstrated for achieving such metastable spin states include chemical synthesis and parahydrogen 6,7 , optical pumping 8,9 and microwave-driven transfers of magnetization from electrons to nearby nuclei via dynamic nuclear polarization 10-13 (DNP). DNP is arguably the most generally applicable of these methods, providing relatively high enhancements reaching up to the γ electron /γ nucleus ratio between the spins' magnetogyric constants, while having the relatively modest requirement that the targeted system be mixed with a free radical to be irradiated at its Larmor frequency.A logical approach to exploit the benefits of DNP within a high-resolution liquid-state NMR setting wou...

Exploiting level anti-crossings for efficient and selective transfer of hyperpolarization in coupled nuclear spin systems

“…Although hyperpolarization techniques made numerous new NMR applications possible, 6,[9][10][11][12][13][14][15][16][17] there are still many problems to be solved in this field. In particular, it is of importance to further develop strategies for optimizing the hyperpolarization process and the transfer of polarization from the primarily polarized spins to the target nuclei of choice.…”

Improving resolution in single-scan 2D spectroscopy

“…The spatial encoding can be greatly improved by using adiabatic frequency-modulated pulses [7][8][9]. This new technique has been successfully combined with liquid chromatography [10] and with methods to increase the polarization of nuclear spins [11]. In addition, the method has been adapted for single-scan magnetic resonance imaging [12,13].…”

HyperSPASM NMR: A new approach to single-shot 2D correlations on DNP-enhanced samples

“…This hyperpolarization, however, decays with the longitudinal relaxation time T 1 , and it cannot be preserved at the level of constancy normally demanded by conventional 2D NMR acquisitions. By contrast, ultrafast 2D techniques that rely on spatio-temporal encodings to monitor the indirect-domain evolution [15][16], have been shown capable of making full use of this hyperpolarization [17][18][19]. While spatio-temporal methods can deliver 2D NMR spectra in a single scan without a priori knowledge, they face a number of technical limitations.…”