High Flow-Rate Benchtop NMR Spectroscopy Enabled by Continuous Overhauser DNP

Abstract

Analysis of a fast-flowing liquid with NMR spectroscopy is challenging because short residence times in the magnetic field of the spectrometer result in inefficient polarization buildup and thus poor signal intensity. This is particularly problematic for benchtop NMR spectrometers because of their compact design. Therefore, in the present work, different methods to counteract this prepolarization problem in benchtop NMR spectroscopy were studied experimentally. The tests were carried out with an equimolar acetonitrile + water mixture flowing through a capillary with a 0.25 mm inner diameter at flow rates up to 2.00 mL min–1, corresponding to mean velocities of up to 0.7 m s–1. Established approaches gave only poor results at high flow rates, namely, using a prepolarization magnet, using a loopy flow cell, and using a T 1 relaxation agent. To overcome this, signal enhancement by Overhauser dynamic nuclear polarization (ODNP) was used, which is based on polarization transfer from unpaired electron spins to nuclear spins and happens on very short time scales, resulting in high signal enhancements, also in fast-flowing liquids. A corresponding setup was developed and used for the studies: the line leading to the 1 T benchtop NMR spectrometer first passes through a fixed bed with a radical matrix placed in a Halbach magnet equipped with a microwave cavity to facilitate the spin transfer. With this ODNP setup, excellent results were obtained even for the highest studied flow rates. This shows that ODNP is an enabler for fast-flow benchtop NMR spectroscopy.

Functionalized Controlled Porous Glasses for Producing Radical-Free Hyperpolarized Liquids by Overhauser DNP

“…The ODNP-enhanced continuous-flow NMR set-up is shown in a simplified overview in Figure 2 . The liquid passes first a packed bed with the radical matrix that is placed in a 0.35 T Halbach magnet [ 20 ], which is equipped with an ENDOR probehead. Here, microwave irradiation and, thus, ODNP polarization build-up is accomplished.…”

“…This is a relaxation-mediated process, which is most efficient when the longitudinal relaxation time of the nuclear spin is dominated by interactions with electron spins. ODNP happens on very short time scales, which makes this technique ideally suited for the continuous hyperpolarization of flowing liquids [ 9 , 10 , 11 , 12 , 13 , 14 , 15 , 16 , 17 , 18 , 19 , 20 ].…”

“…Currently, Overhauser DNP (ODNP) is increasingly used in combination with compact NMR instruments [ 20 , 21 , 22 , 23 , 24 ] because ODNP units are rather small and add only moderately to the weight and size of the overall NMR system. Compact NMR systems are mainly based on permanent magnets and allow a very flexible use of NMR spectroscopy because a dedicated laboratory infrastructure and cryogenic coolants are not needed.…”

Toward protein NMR at physiological concentrations by hyperpolarized water—Finding and mapping uncharted conformational spaces

“…This means that the NMR signals of the water protons are enhanced in comparison to conventional NMR by pretreating it in a dedicated DNP (dynamic nuclear polarization) apparatus before being used for dissolving the target protein. Other applications of hyperpolarized water can, for example, be found in magnetic resonance imaging (18,19) or low-field (triplet or Overhauser DNP) NMR (20)(21)(22)(23)(24)(25)(26). The hyperpolarization boosts the water NMR signal intensities by more than two orders of magnitude.…”

Nuclear Overhauser spectroscopy in hyperpolarized water – chemical vs. magnetic exchange

“…Various methods, from para hydrogenation to dynamic nuclear polarization (DNP) to optical pumping, 1 enable the creation of hyperpolarized spin states for a wide array of applications. The use of hyperpolarized water to boost signal intensities in biomolecular NMR spectra has recently received ample attention [2][3][4][5] as dissolving a target biomolecule in a hyperpolarized buffer enables new approaches at substantially enhanced sensitivity: residue-and time-resolved protein NMR, 6 2D and 3D correlation spectra, 7 protein folding and folding monitoring, 8 membrane interactions, 9 structural dynamics, 10 and exchange processes. 11 However, the understanding of the hyperpolarization transfer mechanism 12 from the buffer to the targets remained incomplete; not least as data acquisition in the, most often used, dissolution DNP (dDNP) experiments is typically limited to short periods of 1-2 min that do not allow for determining the exchange pathways via complex pulse sequences.…”