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HyperMaple: Maple Syrup as a Glassing Agent for DNP Sample Preparation1Oxford Centre for Clinical Magnetic Resonance Research, University of Oxford, Headington, United Kingdom, 2Department of Physiology Anatomy & Genetics, University of Oxford, Oxford, United Kingdom
Synopsis
Certain compounds require a glassing agent to be hyperpolarized for dynamic nuclear polarization experiments. We propose here the novel use of Canadian maple syrup, a delicious, hypoallergenic, and readily available concentrated sugar solution, as an alternative to conventional glassing agents such as dimethyl sulfoxide and glycerol, and have demonstrated its utility in hyperpolarizing [13C]urea.
Introduction
Dissolution dynamic nuclear polarization (DNP) can generate a signal enhancement of greater than four orders of magnitude [1], facilitating in vivo detection of small molecules. Conventional sample preparations of solid agents for DNP involve co-dissolution with radical in a glassing agent such as glycerol [1] or dimethyl sulfoxide [2]. High concentrations of sugar provide a cryoprotective effect that is exploited by some species of animals to survive winter freeze-thaw cycles [3]. This abstract explores the possibility of using a highly concentrated sugar solution such as maple syrup (68% sugar by weight) as a glassing agent in dissolution DNP.Methods
A solution of 6.0 M [13C]urea and 15 mM OX063 was prepared in Canadian maple syrup (No. 2 Grade A Amber). A 250 μL aliquot was polarized at 3.35 T using a HyperSense DNP polarizer (Oxford Instruments, Abingdon, UK) at 94.134 GHz for approximately 45 min. Two solid state polarization build-ups were performed, with and without the addition of 1.5 mM Gd-DOTA.
Rapid dissolution on the sample without Gd-DOTA was performed with 5 mL of 100 mg/L Na2EDTA solution to give a final concentration of 300 mM [13C]urea (1.8% w/w) and 3.4% w/w sugar that was injected directly into a 10 mm NMR tube placed in a 11.7 T vertical bore imaging system. Pulse/acquire spectroscopy was performed with a TR of 1 s for 5 minutes (10º tip, 34.7 kHz, 16384 complex points). Liquid state polarization was estimated by comparison to a 12 hour scan at thermal equilibrium polarization (TR = 5 min, 144 averages, 10º tip, 34.7 kHz, 16384 complex points).
Results
Urea, OX063, and Gd-DOTA were all readily soluble in maple syrup. Solid state 13C signal buildup time constants of 4069 s without Gd and 3149 s with 1.5 mM Gd-DOTA were observed, with limiting solid-state build up amplitudes of 30000 ADC units without Gd and and 55000 ADC units with 1.5 mM Gd-DOTA. A time-integrated spectrum of hyperpolarized [13C]urea is shown in Figure 1(A) with a zoomed view in (B) showing potentially polarized natural abundance 13C signal from the glassing agent. A liquid state [13C]urea polarization of 6% was obtained without Gd. This is predicted to increase to 11% with 1.5 mM Gd-DOTA based on the solid-state NMR signal. The [13C]urea signal decayed with a T1 of 21 s. The largest hyperpolarized non-urea resonance at 108 ppm exhibited a T1 of 9.4 s, which is not seen in non-Maple preparations.Discussion
As a proof of concept study, we have demonstrated the feasibility of using maple syrup, a highly concentrated sugar solution, as a glassing agent. Urea hyperpolarized excellently with use of the novel glassing agent, showing the same increase in limiting polarization seen with or without gadolinium addition as previously reported. Dissolution in 5 mL of liquid resulted in a solution of 1.8% w/w urea and 3.4% w/w sugar, which is osmotically similar to the 5% dextrose (D5W) intravenous solution routinely used in the clinic.
We note that maple syrup is a potentially more pleasant vehicle for injection compared to conventional glassing agents such as dimethyl sulfoxide. It is substantially less viscous than 6 M urea dissolved in glycerol (maple viscosity 0.154 Pa·s [4] vs 1.412 Pa·s for glycerol). The literature T1 value of [13C]urea in water at 11.7 T is 44 s [5]. The maple syrup may have contained traces of paramagnetic ions which may have decreased the apparent T1 of [13C]urea. Future work will focus on generating a sterile isotonic sugar solution that would be suitable for intravenous administration, and potentially reveal additional metabolic behaviour.
Conclusion
Canadian maple syrup was used as a model glassing agent for DNP preparation of [13C]urea. We believe this is the first time (delicious) concentrated sugar solutions have been used as a glassing matrix for DNP sample preparation of any hyperpolarized substrate.Acknowledgements
The authors would like to acknowledge the following sources of funding: NIHR Oxford Biomedical Research Centre (JYCL), Novo Nordisk Fellowship Programme (JJJJM), Medical Research Council (LAJY and TZ), Engineering and Physical Sciences Research Council (AT), and British Heart Foundation (DJT).References
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[2] Lumata L, Kovacs Z, Malloy C, Sherry AD, Merritt M. The effect of 13C enrichment in the glassing matrix on dynamic nuclear polarization of [1-13C] pyruvate. Physics in Medicine & Biology. 2011 Feb 1;56(5):N85.
[3] Storey KB, Storey JM. Freeze tolerant frogs: cryoprotectants and tissue metabolism during freeze–thaw cycles. Canadian Journal of Zoology. 1986 Jan 1;64(1):49-56.
[4] Ngadi MO, Yu LJ. Rheological properties of Canadian maple syrup. Canadian Biosystems Engineering. 2004;46:15-8.
[5] Wilson DM, Keshari KR, Larson PE, Chen AP, Hu S, Van Criekinge M, Bok R, Nelson SJ, Macdonald JM, Vigneron DB, Kurhanewicz J. Multi-compound polarization by DNP allows simultaneous assessment of multiple enzymatic activities in vivo. Journal of magnetic resonance. 2010 Jul 1;205(1):141-7.
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