Porous functionalized polymers enable generating and transporting hyperpolarized mixtures of metabolites

Abstract

Hyperpolarization by dissolution dynamic nuclear polarization (dDNP) has enabled promising applications in spectroscopy and imaging, but remains poorly widespread due to experimental complexity. Broad democratization of dDNP could be realized by remote preparation and distribution of hyperpolarized samples from dedicated facilities. Here we show the synthesis of hyperpolarizing polymers (HYPOPs) that can generate radical- and contaminant-free hyperpolarized samples within minutes with lifetimes exceeding hours in the solid state. HYPOPs feature tunable macroporous porosity, with porous volumes up to 80% and concentration of nitroxide radicals grafted in the bulk matrix up to 285 μmol g^-1. Analytes can be efficiently impregnated as aqueous/alcoholic solutions and hyperpolarized up to P(¹³C) = 25% within 8 min, through the combination of ¹H spin diffusion and ¹H → ¹³C cross polarization. Solutions of ¹³C-analytes of biological interest hyperpolarized in HYPOPs display a very long solid-state ¹³C relaxation times of 5.7 h at 3.8 K, thus prefiguring transportation over long distances.

Fig. 1. Illustration of the steps from production to extraction of transportable hyperpolarization.

a Scanning electron microscopy of the HYPOP material used in this study. b Photograph of the impregnation step of powdered HYPOP. c Schematic representation of the porous polymer (yellow) with its PAs (black dots) impregnated with a ¹³C-labelled molecule (red dots) in aqueous solution (blue). ¹H spins (grey dots) are abundant both in the HYPOP material and aqueous solution. d Schematic representation of the polarization transfer from the electrons of the PAs located within the HYPOP material to the ¹H nuclei located within the HYPOP material, followed by ¹H ↔ ¹H spin diffusion across the material interface towards the aqueous frozen solution impregnated in the pores, and finally ended by a cross-polarization (CP) transferring the ¹H polarization to the ¹³C spins of the target molecules. e Schematic representation of the slow ¹³C nuclear spin-lattice relaxation in solid state, mostly free from any paramagnetic relaxation since the ¹³C spins are physically well isolated from the PAs. f Carbonyl region of the ¹³C-NMR spectrum measured on a 80 MHz Bruker BioSpin Fourier 80 benchtop spectrometer (1 scan, 2 s), of a sample containing 1 M [1-¹³C]sodium acetate, 1 M [1-¹³C]sodium formate and 1 M [1-¹³C]glycine, hyperpolarized with HYPOP materials, displaying a liquid-state polarization enhancement exceeding 5000.

Fig. 2. Synthesis of HYPOP materials.

a Synthesis of TEMPO-functional and structured epoxy resins from diglycidyl ether of bisphenol A (DGEBA), isophorone diamine (IPDA) and 4-amino-TEMPO in the presence of various amounts of non-reactive polypropylene glycol (PPG). b The various morphologies obtained in the absence of 4-amino-TEMPO, when varying the fraction and the average molar mass in number (M̅n) of PPGs are reported in a pseudo-phase diagram: liquids (slurry and suspensions) in dark blue, solids with closed porosity in red, heterogeneous mixtures of solids and liquids in grey and solids with open porosity in light blue. The highlighted data (85 wt% of PPG-400 g mol⁻¹) corresponds to the HYPOP-I series (c) that was functionalized with 4-amino-TEMPO and is reported in the remainder of the manuscript.

Fig. 3. ¹H DNP performances of the HYPOP-I series.

Samples were compared before (blue open circles) and after (red filled circles) impregnation with a solution of ethanol-d₆/H₂O/D₂O (1/1/8 v/v/v), polarized at 1.4 K and 7.05 T, with a microwave frequency of f_uw = 197.648 GHz using a triangular frequency modulation of width Δf_uw = 160 MHz and a modulation rate f_mod = 500 Hz. a Steady-state ¹H polarization levels (in case of incomplete build-up, asymptotic polarization values have been extrapolated from the fits in Sections 8.2 and 8.3 in the SI) and b corresponding polarization build-up rate R_DNP(¹H) = 1/T_DNP(¹H) values. ^*,▪,Δ Please see details in Sections 8.2 and 8.3 in the SI.

Fig. 4. ¹³C hyperpolarization and solid-state relaxation of target solution in HYPOP-I_A.

a ¹H DNP build-up of HYPOP-I_A sample impregnated with a solution (see in the text) measured at 1.4 K and 7.05 T, with a microwave frequency of f_uw = 197.648 GHz with a triangular frequency modulation of width Δf_uw = 160 MHz and rate f_mod = 500 Hz. b Subsequent multi-CP transfer of polarization to ¹³C of target molecules. c Subsequent ¹³C relaxation after warming the sample to 3.8 K, showing a characteristic decay constant T₁(¹³C) = 5.7 ± 0.1 h.

Fig. 5. NMR spectra of extracted hyperpolarized metabolites.

a Full and detailed view of the first ¹³C spectrum. b Time series of the ¹³C hyperpolarized spectra measured every 5 s with a 5° nutation angle pulse in the liquid state after hyperpolarization of a 100 μL solution of 0.9 M [1-¹³C]glycine at, 0.9 M [1-¹³C]sodium formate, 0.9 M [1-¹³C]sodium acetate, in 90% D₂O and 10% ethanol-d₆ in HYPOP-I and dissolution with 7 mL of D₂O, filtration transfer and injection into a Bruker Fourier 80 benchtop NMR spectrometer (80 MHz ¹H frequency).