Triplet dynamic nuclear polarization of pyruvate via supramolecular chemistry

Abstract

Dynamic nuclear polarization (DNP) significantly improves the sensitivity of magnetic resonance imaging, and its most important medical application is cancer diagnosis via hyperpolarized ¹³C-labeled pyruvate. Unlike cryogenic DNP, triplet-DNP uses photoexcited triplet electrons under mild conditions. However, triplet-DNP of pyruvate has not been observed because of incompatibility of the hydrophobic polarizing agent with hydrophilic pyruvate. This work demonstrates that supramolecular complexation with β-cyclodextrin can disperse 4,4'-(pentacene-6,13-diyl)dibenzoate (NaPDBA), a pentacene derivative with hydrophilic substituents, even in the presence of high sodium pyruvate concentrations. The polarization of photoexcited triplet electron spins in NaPDBA was transferred to the ¹³C spins of sodium pyruvate via triplet-DNP of ¹H spins in water and ¹H-to-¹³C cross-polarization. This provides an important step toward the widespread use of ultra-sensitive MRI for cancer diagnosis.

Fig. 1. Hyperpolarization of [1-¹³C, d₃] sodium pyruvate (NaPyr) by triplet dynamic nuclear polarization (DNP) and cross polarization (CP). (A) Scheme of triplet-DNP and CP. (B) Molecular structures of NaPDBA, βCD, and [1-¹³C, d₃] NaPyr. (C) NaPDBA aggregate in DNP juice in the absence of β-cyclodextrin (βCD), but the dispersibility is significantly increased by supramolecular complexation with βCD. Polarization transfer from photoexcited triplet electron spins to ¹H spins of water and then to ¹³C spins of [1-¹³C, d₃] NaPyr.

Fig. 2. Dispersion of NaPDBA in the presence of NaPyr. Absorption spectra of NaPDBA in methanol (black line), NaPDBA in DNP juice containing NaPyr (blue line), the NaPDBA–βCD complex in DNP juice (dashed green line) and the NaPDBA–βCD complex in DNP juice containing NaPyr (red line) at room temperature. The concentrations of NaPDBA, NaPyr, and βCD were 1 mM, 1.5 M and 5 mM, respectively. Photographs of each solution are shown.

Fig. 3. Molecular dynamics (MD) simulations of NaPDBA–βCD supramolecular complexes. (A and B) MD simulation snapshots of the NaPDBA–βCD complex ([NaPDBA] = 1 mM and [βCD] = 5 mM) in glycerol/H₂O (6/4, v/v) at 300 K. Glycerol and H₂O were omitted for clarity. (C and D) MD simulation snapshots of the NaPDBA–βCD complex with NaPyr ([NaPDBA] = 1 mM [βCD] = 5 mM and [NaPyr] = 1.5 M) in glycerol/H₂O (6/4, v/v) at 300 K. Glycerol and H₂O are omitted for clarity.

Fig. 4. Time-resolved electron spin resonance (ESR) spectra of NaPDBA and peak signal decays in DNP juice (glycerol/H₂O (6/4, v/v)). (A) ESR spectra of NaPDBA containing NaPyr (top), the NaPDBA–βCD complex containing NaPyr (middle), and the NaPDBA–βCD complex (bottom) after 527 nm photoexcitation at 140 K. The concentrations of NaPDBA, NaPyr and βCD were 1 mM, 1.5 M and 5 mM, respectively. Spectra were fitted with the EasySpin toolbox in MATLAB (red lines). (B) Decays of peak ESR signals (black lines). Single-exponential fits are also shown (red lines).

Fig. 5. (A) ¹H NMR signals of water in DNP juice (glycerol-d₈/D₂O/H₂O, 60/30/10, v/v/v) containing NaPDBA, βCD and [1-¹³C, d₃] NaPyr under thermal conditions (five scans every 5 min) and after triplet-DNP (integrated solid effect sequence for 2 min and 1 scan) at 100 K. (B) ¹H polarization buildup curve of DNP juice containing NaPDBA, βCD and [1-¹³C, d₃] NaPyr at 100 K. The enhancement factors were calculated by comparing the peak areas after triplet-DNP with that of thermal equilibrium. The enhancement factor relative to thermal equilibrium at room temperature is shown. (C) Sequence of triplet-DNP and ramped-amplitude cross-polarization (RAMP-CP). (D) 6.864 MHz ¹³C NMR spectra of [1-¹³C, d₃] NaPyr. The red line shows the spectra after triplet-DNP and RAMP-CP (integrated solid effect sequence for 2 min, followed by RAMP-CP, 20 scans) and the black line shows the spectra after RAMP-CP with thermal ¹H spins (20 scans).