Traditional nuclear magnetic resonance (NMR) spectroscopy can be enhanced using dissolution dynamic nuclear polarization (d-DNP), allowing applications such as real-time monitoring of biochemical interactions, chemical reactions, and metabolic processes.
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How does d-DNP NMR work?
In traditional NMR, the detection of molecules can be hampered by weak signal intensities, which means target molecules need to be present in concentrations higher than physiologically common in order for them to be rapidly detected.
D-DNP overcomes this using hyperpolarized nuclear spin states, which allow for boosting of signal intensities by as much as 3 to 4 orders of magnitude. D-DNP is also known as hyperpolarized NMR spectroscopy, along with PHIP and SABRE techniques.
d-DNP was first invented in 2003 by Ardenkjaer-Larsen and colleagues and has since then been explored in protein and metabolomic settings. d-DNP uses the concepts of DNP including using high polarization of electron spins, which is achieved via transferring the spin to nuclear spins using low temperatures and microwave irradiation.
The dissolution aspect of d-DNP stems from quick sample dissolution following DNP, thereby retaining the majority of the hyperpolarization. The fast jump from low temperatures during microwave irradiation to room temperature is key to increasing the signal intensities, as it can increase nuclear spin polarization.
In general, d-DNP experiments follow three steps. The first involves dynamic nuclear polarization at low temperatures. The second step involves a rapid jump from low to ambient temperatures of the sample, transforming it to the liquid state before a quick move to the NMR spectrometer.
The third step, which is the detection itself, occurs at ambient or nearly ambient temperature. The acquisition is of one-dimensional spectra, and so 2D NMR cannot be done using d-DNP methods except in certain recently discovered cases.
What are the applications of d-DNP NMR?
d-DNP has been applied to magnetic resonance imaging (MRI), but more commonly it is used in conjunction with NMR or other spectrometry methods. It shares applications with other NMR methods, such as describing the structure of proteins.
When d-DNP is used with hyperpolarized solvents as vectors to move the hyperpolarization from cold conditions to a protein in a DNP apparatus, multidimensional NMR by d-DNP becomes possible.The d-DNP aspect of NMR leads to hyperpolarization of proteins and molecules, which can assist in situations where targets are present in low concentrations, such as under physiological conditions.
In traditional NMR conditions and concentrations, the transcription factor MAX exhibits a homodimer structure that contrasts from the intrinsically disordered conformation seen at physiological concentrations. Using d-DNP NMR, such biases can be removed.
The possibilities for using d-DNP NMR to explore protein interactions is gaining some attention, partially due to its ability to remove previously described biases at physiological conditions. For example, osteopontin is an intrinsically disordered protein whose core can only be observed when it is bound to its ligand, heparin.
The binding causes conformational changes that allow residues to be affected by the hyperpolarization, where they otherwise would remain unaffected. Therefore, D-DNP NMR can be used to understand protein interactions and structural dynamics, especially within intrinsically disordered proteins.
While the aforementioned applications have pertained mainly to proteins, d-DNP is increasingly seen as a viable application for metabolomics. NMR in metabolomics has been overshadowed by mass spectrometry due to its increased sensitivity, but evidence suggests d-DNP NMR could out-do this.
Approaches include isotope-labeling, where samples are labeled with 13C prior to d-DNP, and untargeted methods, where samples are hyperpolarized at natural 13C abundance. As of yet, d-DNP has not been applied to “real” metabolomic studies as it remains imperfect for this application.
For example, the use of 13C means that detection is limited to quaternary carbons due to their longer relaxation times. Such issues can theoretically be solved, but this has not been achieved in practice yet.
The real-world effects of d-DNP NMR are still being discovered, but it has already shown the importance of the detection of cancer in vivo, drug development, monitoring of treatment responses, biomolecular analysis, and others.
- Jannin S., et al. (2019). Application and methodology of dissolution dynamic nuclear polarization in physical, chemical, and biological contexts. Journal of Magnetic Resonance. https://doi.org/10.1016/j.jmr.2019.06.001
- Kovtunov K.V., et al. (2018). Hyperpolarized NMR spectroscopy: d-DNP, PHIP, and SABRE techniques. Chemistry: An Asian Journal. https://doi.org/10.1002/asia.201800551