PASADENA hyperpolarization of succinic acid for MRI and NMR spectroscopy

Abstract

We use the PASADENA (parahydrogen and synthesis allow dramatically enhanced nuclear alignment) method to achieve 13C polarization of approximately 20% in seconds in 1-13C-succinic-d2 acid. The high-field 13C multiplets are observed as a function of pH, and the line broadening of C1 is pronounced in the region of the pK values. The 2JCH, 3JCH, and 3JHH couplings needed for spin order transfer vary with pH and are best resolved at low pH leading to our use of pH approximately 3 for both the molecular addition of parahydrogen to 1-13C-fumaric acid-d2 and the subsequent transfer of spin order from the nascent protons to C1 of the succinic acid product. The methods described here may generalize to hyperpolarization of other carboxylic acids. The C1 spin-lattice relaxation time at neutral pH and 4.7 T is measured as 27 s in H2O and 56 s in D2O. Together with known rates of succinate uptake in kidneys, this allows an estimate of the prospects for the molecular spectroscopy of metabolism.

Figure 1

Cis molecular addition of parahydrogen to 1-¹³C-fumaric acid-d₂ to produce 1-¹³C-succinic acid-d₂. The catalytic reaction was carried out at 62 °C in H₂O or D₂O with substrate concentrations of 1–5 mM.

Figure 2

¹³C NMR multiplets (experimental in black and simulated fits in red) of natural abundance succinic acid at various pH. All spectra acquired at 14 T with 128–512 scans using Varian triple resonance H/C/N probe without ¹H decoupling.

Figure 3

Typical ¹³C spectrum of 3.5 mM 1-¹³C-succinate-d₂ hyperpo-larized at pH = 2.9. The chart on the left shows the typical reproducibility achieved; a series of three experiments was conducted within 30 min yielding 17.6 ± 1.4% polarization. Natural abundance ethanol with 207 mM of ¹³C per carbon site is used as a reference. Spectra are acquired at 4.7 T with a Bruker Avance console and home-built double resonant probe.